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Chapter 5
The Problem of DiseaseThe Theory of Disease Resistance
The theory of disease resistance in plant, animal, and man, which is associated with Sir Albert Howard's name, goes back to his University days, when at Cambridge he first listened to the teaching of Professor Marshall Ward, to whose presentation of the problem of plant disease he always acknowledged himself indebted. The master idea, that freedom from disease is secured by the living organism's natural capacity, if in good health, to repel attack, and its inevitable decline and defeat in the face of the bacterium, virus, fungus, or parasite if any way weakened or out of condition, began to formulate itself in his mind in the course of his first years of work in Barbados and at Wye College. (Soil and Health Memorial Number, 1948, pp. 4-5.) During his Indian career, when called on to deal with a number of cases of crop diseases, some causing great economic loss, he surrendered completely to this conception, displaying a startling indifference to the presence of any immediate agent of disaster such as a fungus or parasite; in one case it amused him to prove that a presumed parasite was a mythical creation.
Investigation of the problem of disease has almost always concentrated on a study of the organisms which succumb to, not of those which overcome, attack. This was true when Sir Albert started his work. At that time prolonged attention was being given to the deficiency diseases on which a great deal of information was being accumulated in the field of human health, while the veterinarian was much occupied with the facts of parasitic attack and the botanist began to study viruses; these fields of research seemed very worth while, nor was it realized that they had one common defect -- they dealt with defeat and not with victory.
The development which Sir Albert Howard gave to the tentative idea originally thrown out by Professor Marshall Ward was notable. It is due principally to Sir Albert that the conception of resistance has been substituted for the conception of susceptibility. Sir Albert was pretty firmly in the saddle and had covered a good part of his course when Sir Robert McCarrison's reports on the healthy Hunzakuts of Gilgit in the Himalayas began to be known. These investigations were so immediate and so fundamental a confirmation of the resistance theory, pertaining moreover to the difficult field of human health, that they are rightly regarded as an integral part of the general structure. The names of the two investigators have since then gone forward together.
In addition to turning round the whole subject, to our very great advantage, Sir Albert Howard extended the field of inquiry. By doing so he completed a hitherto incomplete formulation of the problem: he filled a scientific gap. By adding the soil to the factors to be taken into account in sorting out health and disease, he carried the chain of evidence back to its real starting point. Until the life of the soil was included in the general theory of health, that theory was a very lame dog indeed, for the best part of the facts were left out. Sir Albert's standpoint can be stated in a very few words. It is extremely well known but will bear repetition until men are convinced of its truth. Such is the theory very simply stated, founded on a lifetime of the closest observation of crops and stock.
Sir Albert believed the explanation of 'this chain of health' would be found to lie in the passage of the complicated protein molecule from the lower to the higher organism. The bio-chemical elucidation of these processes carries us into quite another department of science, where Sir Albert never dreamt of himself as competent; he pointed once or twice to apparent changes in the plant sap accompanying attacks of plant disease, and once caused some chemical analyses to be made (See below, The Spike Disease of Peach Trees); otherwise he contented himself with indicating what a huge field of enquiry lay in front of the bio-chemist in this direction. But he was convinced that the truth was sufficiently clear in outline from what was already known: that the main points of his statement would hold and were of importance, both as indicating a fresh start for all our nutritional studies, and also as pointing to an era of practical reform in human health which would greatly add to the benefits which science had conferred on mankind.
It was by the favour of Providence that Sir Albert found himself a botanist. In dealing with the vegetable kingdom he enjoyed two advantages. In the first place, the idea of resistance to disease among plants was accepted; it was wholly linked up with the choice of the right variety, and was conceived in a very restricted and limited way as resistance to some specific disease like rust, not in the least as a general plant attribute. It was thus a much narrower conception than that on which Sir Albert came to rely. In the second place, by dealing with crops Sir Albert enjoyed freedom of action. The question was originally posed by Marshall Ward: what would happen if we allowed disease to run its course? What would stop it and where would it stop? But deliberately to allow disease to run its course is not easy where animals are concerned, as, apart from capital loss, too much suffering is involved, and is obviously quite impossible where human populations are affected. The plant is the only live organism on which we can freely try out such watching experiments. Yet to watch the course of an unchecked disease was in Sir Albert's view the only way of understanding it and therefore the shortest road to victory.
It is curious that, in spite of this obvious advantage accruing to the botanist, the study of plant diseases should at this time have been lagging behind the study of animal ailments, a fact commented on at a meeting of the British Association for the Advancement of Science in 1911 by Professor Bateson. The passage was cited by Sir Albert in one of his contributions to the Agricultural Journal of India, and indeed is so pertinent that it may be given here.
'Nowhere is the need for wide views of our problems more evident than in the study of plant diseases. Hitherto, this side of agriculture and of horticulture, though full of possibilities for the introduction of scientific method, has been examined only in the crudest and most empirical fashion. To name the disease, to burn the affected plants, and to ply the crop with all the sprays and washes in succession ought not to be regarded as the utmost that science can attempt. There is at the present time hardly any comprehensive study of the morbid physiology of plants comparable with that which has been so greatly developed in application to animals. The nature of the resistance to disease characteristic of so many varieties, and the methods by which it may be ensured, offer a most attractive field for research, but it is one in which the advance must be made by the development of pure science, and those who engage in it must be prepared for a long period of labour without ostensible practical results.' (Quoted in the Agricultural Journal of India, Special Indian Science Congress Number, 1916, pp. 22-3.)
Resistance or Immunity
The 'long period of labour' foretold by the speaker was indeed to fall to the lot of the man who quoted these prophetic words. This labour was to constitute itself as a series of lessons which forced themselves on his attention. Sir Albert Howard did not start with a theory of disease, he was compelled to acquire it. At point after point Nature challenged him with puzzles, to which he had somehow to find an answer: each puzzle led to another. That was why the theory became so firmly fixed in his mind; it was won slowly and after much consideration, reflection, and work.
The first lesson was to learn that the current ideas of resistant and non-resistant varieties of crops was much too cut and dried. It had been assumed that a resistant variety would retain its accredited degree of resistance whatever the circumstances; on this principle plant breeding could proceed merrily indeed. It was a shock to find that nothing of the sort could be depended on.
'The disease-resistant variety... at the present time, is a very fashionable remedy, particularly against wheat rust. As is well known, the unit species of the wheat crop differ greatly in rust resistance. By the substitution for the general mixed crop of a rust-resistant variety we can sometimes successfully dodge the disease. If, however, we pursue this matter further we find what at first sight appeared to be a solution of a difficulty is nothing more than an extension of the problem. It was soon found that rust resistance varies according to locality and to the way the crop is grown. It is not a hard and fast character. Thus at Pusa a unit species with a fair degree of resistance, when grown (1) in flower-pots, (2) on heavy land with deep cultivation after the rains, (3) on heavy land without deep cultivation after the monsoon, shows great differences in the amount of rust. In flower-pots (good aeration was secured through the sides of the pots) and after thorough preparation, the rust attack is negligible. When sown in land badly prepared, the same variety is sometimes destroyed by rust. The amount of damage by the fungus appears to increase as the physical texture and aeration of the soil fall off. The amount of rust on wheat therefore depends first on the kind and secondly on the way the kind is grown. Our problem therefore is already ceasing to be a simple one.'
Much more startling was the discovery that a variety universally credited with complete immunity could succumb like any other. The fact has already been mentioned that Einkorn, the wheat supposedly immune to rust, fell victim to a severe attack of black rust in the hot weather of 1907. (Chapter 2; cf. Wheat in India, p. 99.) This shock to complacency was administered at an early stage and put an end to any automatic faith in the merits of varieties. There was nothing hard and fast, as Sir Albert says, about the capacity to resist rust; it was not a character that triumphed under all conditions. In later years Sir Albert laid much stress on stating that in Nature no such thing as complete immunity to a specific disease exists; what can be secured is a very high degree of disease resistance 'almost amounting to immunity'.
There was one specially interesting aspect in the Einkorn episode. The rust appeared after some five weeks of hot weather in the early part of the month of May. Experience had always pointed to damp and cloudy weather favourable to rust, but on this occasion it attacked very suddenly under opposite conditions. Sir Albert was able to point to a fact which later helped him to understand the onset and retreat of a disease or pest; the weather was certainly unfavourable to the rust, but it was unfavourable also to the wheat, which, as Sir Albert frequently stresses, is really a cold country crop; though grown in India for over two thousand years, it retains this character. The hot month of May in India can be disastrous. It was thus a thoroughly weakened crop which was sought out by the fungus. (Cf. Chapter 2, The Problem of Rust.)
The point emerged even more clearly in the following account of wheat growing in Bihar. Again, temperature is the factor which disturbs the host; in this case the temperature of the soil, which caused the thoroughly weakened plant to succumb to the attacks of an insect.
'One of the difficulties in wheat cultivation in Bihar and the eastern districts of the United Provinces is to establish the crop. If sown a few days too early, the seed germinates but the seedlings are rapidly destroyed by termites, whole fields disappearing in a few days. The trouble became of some importance a few years ago in Bihar, as it interfered with the raising of seed of the new Pusa wheats on some of the private seed farms. The disease (disease and pests, are referred to at this time under the single term 'disease') was particularly serious in years when the rains ceased early and when the last monsoon showers of early October, known locally as the Lathia, were not received. In such seasons the advent of the cold weather is always postponed and the cool westerly breezes which normally set in about the middle of October are delayed till nearly the end of the month. The sowing time for wheat in Bihar in years when there is a good Lathia is just after the middle of the month and no trouble with white ants need then be feared. When, however, these sowing rains fail, nearly all the fields are destroyed by termites. More damage occurs on low-lying damp heavy soil than on the higher and dryer areas. Examination of the root system during the attack shows extensive discoloration of the new primary roots and of the first internode. Only in rare cases is there any formation of the secondary system. Before tillering can take place the first internode is devoured by the white ants and the plants wither.
'A possible explanation of the trouble appeared to be a high soil temperature, which subsequent investigation seemed to confirm. In several seasons when the late rains failed, a comparison was made between sowings on October 15th and others twelve days later. In addition to the delay, the furrows in the second case were left open for two or three days so as to cool the soil by evaporation. The early sowings were in every case destroyed by termites, while in the later ones the damage was negligible and normal root development and growth took place. The simplest explanation of these results appeared to be a fall in the soil temperature during the second half of October. That such a fall actually does take place is proved by an examination of a set of soil temperature determinations made by Leake in Bihar in 1903-4. In the year in which Leake's readings were taken the Lathia amounted to 3 to 4 inches of rain and the daily soil temperature at 4 inches at 1-2 p.m. fell gradually from 29.5 deg. C. on October 16th to 22 deg. C. at the end of the month.
'This disease of wheat seedlings, which is very common in north-east India, is of some general interest, as the termites, although the apparent cause of the trouble, were in reality engaged in the consumption of a moribund set of seedlings which had been practically destroyed, apparently by a soil temperature above the maximum for growth... There seems to be no doubt that the conditions of the active roots profoundly affect the resistance of the plant to the attacks of parasites.'Sir Albert denied the capacity of the insect to 'cause' disease and this was perhaps the first of his statements, later made categorically, which repudiated insects and fungi as the real agents of disaster.
The Study of Root Systems
In the above instance the susceptibility of the plant to onslaught from its enemy is attributed to the state of its root system. The prolonged root studies made by Sir Albert were to a large extent the origin of his theory of disease resistance. He had established the rather remarkable fact that in many Indian crops there was a differentiation between deep-rooted and shallow-rooted varieties. These were adapted to their local habitat; but encountered disaster when transferred.
To grow a deep-rooted variety where the ground-water level was high was to drown the roots: to grow a shallow-rooted variety where the ground-water level was low was to dry them up. The result was disease, which, however, was not caused by any outside agency, but was the simple consequence of mistakes in the choice of varieties.
Wilt and green-fly were the two commonest accompaniments to this lack of gearing between root and soil, but any other type of misfortune could overtake an enfeebled crop, the cause being precisely this weakness in the host and not the arrival of the pest or virus. The facts were established for linseed, any attempt to grow the deep-rooting Peninsular varieties on the alluvium ending in attacks of wilt which destroyed the plants. Another example was the onset of, or again resistance to, wilt in fibres (Hibiscus cannabinus L., Hibiscus sabdariffa L., Crotalaria juncea L.), the shallow- rooted varieties escaping, the others succumbing. In the growing of a vetch (Lathyrus sativus L.) the proof was irresistible. The experiments were carried out at Pusa in 1921.
'In that year deep-rooted varieties [of Lathyrus sativus L.] from the black soils, shallow-rooted types from the alluvium, as well as kinds with an intermediate root system from the Allahabad district, were sown in the same plot. All the types from the black soil area were badly attacked by green-fly, all those from the alluvium were immune, while the types with intermediate root-development were only slightly affected. Although the infected and disease-free plots were often side by side, in no case did the Aphides leave the deep-rooted cultures and attack those with shallow roots. Really healthy plants seem able to keep insects and fungi at bay, and their juices seem unsuitable for the nourishment of these organisms.'In the case of wheat the facts were the same.
'The influence of soil conditions on disease resistance is seen when the wheats of the Peninsula are grown in the plains. Many of the deep-rooted varieties from the black soils, when grown at Pusa, are practically destroyed by rust. In their own locality, however, rust attacks are rare. Alterations in the environment, therefore, influence the natural resistance of a variety. On the other hand, when the shallow-rooted variety, known as Pusa 6, is grown at Pusa next to these black soil types, it always remains free from rust although surrounded by a mass of infection.'
The phenomenon of disease-resistance was thus brought back to the distinction between varieties, but on a much broader basis. It was not just a question of one wheat or another, one linseed or another, one fibre or another, selected by score-card record and accredited with inheritable resistance capacity. The point at issue, the point that mattered, was whether the variety chosen was suited to the soil chosen, was geared to the soil. Thus we arrive at the real question, the relation between plant and soil.
The Explanation of Indigo Wilt
Almost every point is illustrated in an intricate set of experiments which the Howards carried out on the indigo crop. This work was done early in their career and must have influenced them in arriving at their general conclusions on the problem of disease. It seems therefore worth while to give a fairly detailed description off these enquiries and experiments.
Indigo was not original to the country, but had been introduced by planters from the East Indies, where it is a wilding. It was much grown in Bihar, where the Pusa Research Institute was situated. It was an old crop and a valuable one, with a famous history. When the Howards arrived at Pusa in 1905 the area devoted to it had greatly shrunk, especially in Lower Bengal and the United Provinces, and it was mostly confined to Bihar.
Following their usual principles the Howards grew some indigo as a matter of course, together with all the other staple crops of the district. (The area of the Pusa Research Station itself was an old indigo and tobacco estate. This had gone bankrupt and had been acquired cheap, which accounts for the unfortunate decision to place the Institute in this spot.) This afterwards proved very useful. There was, however, an independent indigo Research Station at Sursiah, which naturally concentrated its work on the new Java indigo (I. arrecta, Hochst), introduced in 1893 as a variety superior to the old Bihar indigo (I. sumatrana, Gaertn.), though the latter still maintained its position.
Suddenly between 1910 and 1913 the industry collapsed. Wilt appeared, especially on the valuable Java indigo. The failure was so complete that the Sursiah Research Station was compelled to close down for the simple reason that their crops set no seed -- there was therefore no material with which to continue. The wilt killed the plants entirely -- whole fields.
In these unpromising circumstances the problem was handed to the Howards to hold, and it certainly was a very sickly baby. Everything had been tried. In fifteen years £54,207 had been spent on research, at that time a large sum. Yet the Imperial Entomologist could find no insect, the Imperial Mycologist no fungus, and the Imperial Bacteriologist no virus to account for the plague.
The Howards proceeded differently. Their start was to grow the crop on a field scale and in the best possible way, taking note of local methods. Their observation was directed to the whole plant, above and below ground; they followed the crop throughout its life history; they looked at all the surrounding circumstances, soil, moisture, temperature. But they looked for no virus, no fungus, and no insect.
They noted that beginning in 1910 there had been a succession of wet years following a dry cycle; the appearance of the wilt had coincided with the wet years. Could there be a connection? They were helped by their previous observations on the behaviour of other crops under conditions of excessive moisture, especially of patwa (Hibiscus cannabinus L.) and sann (Crotalaria juncea L.); they were familiar with the root systems of these plants, a long tap root and thick laterals mainly concentrated in the first ten inches of soil below the surface. They knew that a distressed condition of these plants under excessive moisture had always been accompanied by a marked dying away of feeding rootlets, sometimes 'the entire destruction of everything besides the main tap root'.
An initial examination of the indigo plant showed the same characteristic root system, and the same absence of feeding rootlets in all wilted plants.
'The root system of this plant is very characteristic. There is a long tap root and a great development of long thick laterals, mainly concentrated in the first ten inches below the surface. Below this the number of laterals on the tap root is small. The appearance of the large roots of affected plants is healthy and there are none of the symptoms of definite disease. There are, however, very few feeding rootlets to be found anywhere. The functional root system is extraordinarily limited and almost absent on the laterals near the surface. Little beyond the main root system is alive and the almost entire destruction of the finer roots is most striking. This at once explains the unhealthy appearance of the crop towards the end of the monsoon and the failure of the plants to ripen proper seed afterwards. The fall of the leaves and the wilting of whole branches represent efforts on the part of the plant to adapt itself to the gradual loss of its active root system.'
The first conclusion was that the waterlogging of the Bihar soils towards the end of the monsoon was the cause of the trouble; the pore spaces of the alluvium became flooded with water, no oxygen was available to the rootlets, which therefore died; the plant was then unable to feed itself -- the so-called wilt was simply the last stage of starvation. Many apparently contradictory facts were eventually explained on this basis, everything depending on the plant's ability to grow fresh rootlets or its failure to do so, according to time and season. The first explanation, therefore, proved the final one, though much work was still awaiting the investigators before they could explain the elusive and baffling history of this crop.
Very careful experiments in growing the crop on soil adequately drained and aerated were carried out in 1912 and 1913. Various methods were tried, sowing under cover crop, irrigation, transplanting. A few fields did well for a time, but wilt appeared even on these fields towards the end of the rains, and in general the crops could only be described as poor. The conclusion was that drainage and good cultivation were not in themselves sufficient remedies to prevent the appearance of wilt.
This was a disappointment, for so much success had followed on improvements in drainage and soil aeration in the case of other crops that it might have been hoped to solve the indigo puzzle by the same means. But it was clear that there were further factors involved.
A fresh start was made and consideration given to the above-ground life history of the plant. Here the Howards became aware of the great shortcomings in the local management of this crop. The usual practice was to sow in October and by the middle of the next July to take the first harvest of leaves, cutting the plant right back, and then to repeat this process towards the end of August; seed, if desired, was reaped from a third growth on the same plants starting in September and continuing until the following February or March; the plant stayed in the ground eighteen months. On this system two things were being done by the planters which were bound to inflict a severe strain on the plant. Twice in the course of its growth the indigo plant was cut right down; after this severe treatment it was left to grow again; flower and seed were to be reaped from this weakened plant. Was the indigo capable of withstanding such tremendous shocks?
Suspecting that the fault lay in asking too much from the plant, two innovations were tried. In the first place, one branch was left at each pruning -- the plant was not deprived of the whole of its above-ground mechanism. In the second place, the plants which were to give seed were grown for that purpose only; they were not mutilated for a leaf harvest at any time, but were allowed to live their complete natural life to the end; moreover, when destined as a seed crop, the indigo was sown about the middle of August, to come into flower in October and November and bear seed in February and March, thus avoiding, during the crucial period of reproductive effort, the worst effects of the monsoon.
These improvements, reversing the practice of the planters, proved successful. Fine crops, both of leaf and of seed, could be exhibited to the planters, who were addressed at their Association meetings in 1914 and 1915.
'As is well known, the universal method of growing Java indigo in Bihar is to sow it broadcast or by drills in lines close together. The result, in both cases, is a dense crop of unbranched plants, thus leading to loss of indigo. Further, interculture to keep down weeds is impossible after the first cut. When the crop is reaped, the indigo is cut down completely and the plant has to produce new shoots during the monsoon at a time when its roots are in very moist soil. Few plants will survive uninjured such treatment during the month of July. When an indigo plant is suddenly cut down, the passage of water and food materials from the roots goes on for a time and the stumps bleed. There are no leaves to carry on the transpiration current and the result is that the normal physiological processes in the plant are greatly upset. It is not surprising, therefore, that the new growth is formed so slowly and that it is often unhealthy. Many plants, such as peaches and flowering creepers like Ipomoea, usually die outright when cut down to the ground during the rains and hardly ever recover.
'It was decided in 1913 to try the effect of pruning the crop in July at the time of the first cut and to compare the behaviour of plants treated in this way with those cut down completely. To enable the crop to branch freely the indigo was sown in lines, two feet apart, and weeds were kept down so as not to interfere with free branching from the lowest part of the stem. One-half of the plot was cut down in the ordinary way, the rest pruned so that one branch was left at cutting time. The results of the first and second cut are given in the following:
Plants pruned Plants cut down completely maunds seers maunds seers First cut 15 23 22 39 Second cut 23 37 6 19 Total 39 8 29 1
'It will be seen that the method of pruning at the first cutting and leaving a branch to carry on the growth has resulted, as was expected, in a material increase in the total crop. There is of course some loss in the pruned plot at the first cut due to the branches left. This is, however, more than made up for in the greatly increased second cut on the pruned plot.'But what was more important was the reduction in wilt. The August-sown seed, for instance, escaped wilt and carried a fine crop of healthy seed, forming a most marked contrast to the wilted plants which had been sown in June (1912). These results were repeated the next year, and 'fine healthy plants' free from wilt and covered with good pods, were obtained. Thus the seed question was overcome; the disaster which had brought to an end the work of the Sursiah Station could be considered a thing of the past.
In pursuing this question the Howards became interested in the general physiology of the indigo crop and especially in the processes by means of which this plant accumulates in its leaves the substance known as indican, which forms a reserve store of nitrogen essential to the setting of seed. It is common knowledge that plants have an enormous history and have evolved all sorts of ways of looking after themselves. Like other legumes indigo has evolved a double system of getting hold of nitrogen. It can, like non-leguminous plants, absorb nitrates from the soil solution via the root hairs in the usual way. Poor soil will have few nitrates in solution: rich soil will have many. What happens? Indigo grown in poor soil will have to work very hard to get its nitrogen and the supply will be precarious; the plant therefore makes a great effort, producing many nodules, which store an additional reserve of nitrogen obtained not from the soil but from the atmosphere. If grown in rich soil, this mechanism is absent; there appears to be what might be called a lazy dependence on readily available soil nitrates. This is well known, and the Bihar planters had a special name for the indigo grown in poor soil (which from their point of view was the best), namely, zilla indigo.
The marked variability of the plant in its capacity for storing nitrogen was a rather clear illustration of intimate gearing between plant and soil; in this instance the one organism, the plant, placed too great a dependence on the other partner in the process, the soil. The test came with the heavy monsoon, when the soil flooded. The plants with nodules found their feeding rootlets drowned; they had omitted to guard themselves against what was literally a rainy day. This state of affairs could thus be summed up in the words: 'What is known as wilt is merely the last stage in starvation.' This amply explains why no parasite, fungus, or virus had been found to account for the disease.
The double nitrogen supplying apparatus also explained why wilt set in when the plant was too severely cut down. If there were no leaves, then the other end of the plant factory could not function. Without foliage the transpiration current could not be carried on and no food could be sent down to the nodules, which therefore perished; plants were found where the nodules were empty shells. Nitrogen was then in insufficient supply, and, as stated, starvation of the whole plant set in. The baffling and contradictory incidence of wilt was thus explained by an examination of the double feeding system and of its relation to soil conditions.
'Indigo is a leguminous plant and, like all members of this order, is characterized by a high percentage of nitrogen in the seeds. This nitrogen occurs in the form of proteids and is placed there by the plant for the sole purpose of nourishing the seedling in the first stages of existence and supplying the protoplasm of its growing cells with nitrogenous food until it can lead an independent life. When sown, the indigo seeds at first make no growth above ground beyond the two small seed leaves. All the development is subterranean in the form of roots, and, at a very early stage, and before the first real leaves are produced, swellings, known as nodules, begin to appear on the roots. Soon afterwards indican can be detected in the first real leaves but this substance has not yet been found in the seed. These root nodules are of the first importance in indigo (as in all leguminous crops) and, as will be seen later, everything in indigo cultivation and also in the separation of the indigo from the green plant in the factory, depends on the successful working of these root nodules and of the roots...
'The development and activity of the root nodules of indigo take place best when the plant is grown on somewhat poor land. On such land the soil contains little nitrate, and, accordingly, the nodule factories are working at high pressure to supply the proteids required. Large amounts of the nitrogen and oxygen of the air are used up and the leaves of the indigo become rich in indican. Every planter knows that indigo grown on rather poor land (zilla indigo) gives the best yield of finished indigo and often the best colour. Poverty of soil in nitrates is one of the conditions for the numerous nodules on the roots and incidentally of high indican storage in the leaf.
'When indigo is sown on rich land containing a high proportion of organic matter such as seeth, the number of nodules formed on the root is small and the bacteria in them do not work at any great pressure. In such soil nitrates are formed in abundance and the indigo plant then behaves like tobacco and takes up its nitrogen by way of the root hairs in the form of nitrates dissolved in the soil water. Under such circumstances the growth is rapid, but little indican is accumulated and, if such plant is steeped, it gives a small proportion of indigo and moreover of poor quality indigo. This fact is also well known to planters, and the inferiority of the crop from highly manured land, compared with zilla indigo, is understood by all.
'The activity of the root nodules reaches its maximum about the time the plant is ready to flower. At this period the leaves are also rich in indican. At this time, however, the indican in the leaves begins to be called upon by the plant and to be utilized by the flowers and developing seeds. It is said that if the indigo crop bursts into flower, the yield of finished indigo will fall off and the colour will suffer.
'The activity of the nodules depends on two main factors -- a full, continuous supply of air from the atmosphere and a supply of food from the leaves for the nodule bacteria. If either of these two things is interfered with, the nodule factory does not work. As regards the supply of air round the nodule, this is at first an easy matter in Bihar provided the surface soil is well and deeply cultivated and if crusts, formed by rain, are broken up whenever they occur. When the monsoon sets in, however, difficulties in the air supply of the nodules begin. If the monsoon is short and there are no large falls, the nodules get enough air and, if there is a succession of such years, the air supply in the soil is abundant and Java indigo reaches its maximum development. If, however, there is a heavy rainfall so that the soil is packed by rain and the air spaces destroyed and filled with water for long periods at a time, then the supply of the essential air to the nodules and to the roots generally is cut off and the activity of the whole root system, including the nodules, is stopped. At the same time, no further indican can be produced. When this happens, the whole economy of the plant is upset and it cannot manufacture any more food. A starvation period sets in and the reserves are called up. As at flowering time, there is an immediate run on the reserve indican and this is consumed. If the plant in this starving condition is cut, it will give a low yield of indigo of poor colour, just as flowering indigo does. As the starvation process proceeds, the plant begins to look unhealthy, the leaves fall and alter in colour and at last the stage known as the wilt disease appears. This is not a disease but the last phase in starvation.'The explanation was now plain of why the indigo crop should so suddenly have collapsed in the three years between 1910 and 1913; this was simply the result of several wet seasons bringing disaster to a plant grown on unsuitable methods.
'It will be clear that the present practices in Bihar in growing Java indigo are about the worst that could be devised and that, in the past, the indigo plant has never had a proper chance. There has been no attempt at proper cultivation and nothing has been done to increase the air supply by means of surface drainage. The crops obtained have been the result of chance; sometimes the colour has been good, but most frequently it has been poor and only a low price has been realized.'
The argument that wilt was the last phase in starvation was important. It explained the erratic incidence of the disease. Everything depended on keeping intact the working partnership between the foliage of the plant and the bacteria of the nodules. Thus wilt on the ordinary leaf crop during the monsoon, after the first cut in June or July, was due to the water-logging of the soil, which destroyed the nodules and fine roots; wilt on indigo sown in June on particularly well-drained soil, killing practically the whole of this crop by the middle of October, while at the same time leaving untouched the plants which had been re-sown to fill up gaps in the first week of August -- the healthy and wilted plants growing next to next with interlocking root systems -- was another effect from the same cause, the escape of the later-sown plants being quite simply due to the fact that the roots of these had not had time to reach the waterlogged level where the roots of the June-sown crop had been for some time forced to exist; wilt on old branches left at the first cut, which did not spread to the new growth coming along after the cut, was due to the fact that this new vigorous growth, which sprang on the lines of communication nearer the root system, intercepted the nourishment the roots were able to send up; finally, wilt on plants sown in August, previously perfectly healthy and grown after the monsoon when there could be no question of waterlogging of the roots, but cut down to ground-level in mid-October, was due to direct starvation; these plants had been mutilated before they had had time to build up any reserves, and the removal of stem and leaves cut short the usual supply of carbo-hydrates conveyed to the nodules; these then could not do their work and, with the fine rootlets, died in a few days, and were found on examination either empty shells or discoloured; the weak and starved shoots that were sent up soon succumbed to wilt.
An indigo plant is thus capable of starving in the midst of plenty if its working system is put out of order. The main roots have not the power of repair if the nodules are not maintained. But there may be further puzzles arising out of the alternation of the feeding mechanisms of this plant. Cases were known, which baffled the planters, and certainly seemed at first sight to contradict the general explanation, of an indigo crop grown on heavy, badly drained, waterlogged land escaping wilt while another crop grown on light, high-lying land suffered severely. The reason would seem to be that, in certain circumstances, this plant can omit using the nodules and rely on its alternate mechanism for obtaining nitrogen through its roots, in the same way as is done by non-leguminous plants; this is most likely to happen when the plant is growing in rich moist soil where the soil solution holds plenty of nitrates and other salts; actually the indigo was growing in a kind of water culture. But on the light soils there would not be enough nitrates to feed the plant, and the rains would have their usual effect in inducing wilt. This, however, was an exceptional case, and, generally speaking, the resistance of indigo to wilt would depend on a supply, in the soil, of 'the essential air' to the nodules and to the roots; in other words, the state of the soil would be the most important influence governing the health of the plant.
There remained one final mystery. It was noted above that there were two types of indigo, the original Bihar indigo, the Sumatrana, and the new and superior Java indigo introduced in 1893.
'When Java indigo was first introduced into Bihar, it did exceedingly well and was noted for its rapid growth and general robustness. After a few years it seemed to slow down in growth and wilt began to appear. The crops kept for seed yielded less and less and soon the seed problem became acute. Between 1910 and 1914 the area under Java indigo in Bihar decreased from 70,000 to about 15,000 bighas largely on account of wilt and the difficulty of obtaining seed.'
Of the two types of indigo the older Bihar indigo was shallow-rooted and flowered early, the new Java indigo varied from being shallow-rooted to being deep-rooted, the deep-rooted varieties flowering late. Unfortunately, in Bihar, early flowering types seldom set seed, owing either to the absence of bees or to the dampness of the air in September and early October. Not until the latter half of October, when the late-flowering types came into bloom, was there fertilization. These late-flowering types, being also the deep-rooted types, tended most easily to get their feeding rootlets drowned in the rising subsoil water of an unusually wet season; in India the soil gets more flooded from this subsoil water than from precipitation falling on the surface. Thus the only indigo likely to flower well in Bihar also met the worst soil conditions. The result was inevitable.
'The principal cause of the degeneration of the plant is undoubtedly to be found in the methods in vogue in Bihar in growing seed. It used to be the custom to keep the old leaf crops for seed... Every planter must have noticed that when a Java leaf crop was kept on for seed many pods were formed in September and early October which contained no seed. The early plants gave little or no seed and the main seed crop was obtained from the late-flowering types. It requires little imagination to understand the cumulative effect of this unconscious selection. The early, shallow-rooted, quick growing types would slowly become eliminated and the late, deep-rooted, slow growing types, flowering after mid-October, would tend to predominate. In the course of a few years the botanical constitution of the crop would change. The Java crop would become more and more made up of slower growing, deep-rooted types. It is just these deep-rooted forms which in an alluvial soil like that of Bihar would, during the monsoon phase, be apt to be attacked by wilt. Shallow-rooted, quick growing kinds would be much less liable. The main cause of wilt is undoubtedly the unconscious selection exercised by the old method of growing seed.'
The remedy would be, not to keep on importing new seed from Java, which was little help, as the Java crop was also altering its botanical constitution in the same way, but to revise the whole system of seed growing by sowing a separate seed crop in August to be reaped in the following February or March, all late-flowering varieties to be vigorously rogued out at least twice in the course of the season. In this way, combined always with the most careful attention to drainage so as to prevent waterlogged conditions, the Java variety might be brought back to its original strength.
In commenting later on the disappearance of the natural indigo under competition with the chemically produced synthetic dye Sir Albert remarked that years had originally been wasted by launching research from the wrong end. The battle had been lost because attention had been focused on chemical problems connected with the improvement of the dye -- the plant, 'the living machine', had been ignored. His own efforts at salvage had come too late, but even then, had his advice been generally applied, it was not impossible that this interesting and historic crop might have been saved.The Effect of Grass on Trees
The work on indigo, of which the bearing became clear in the course of 1913, 1914, and 1915, overlapped a little the start of a very comprehensive set of field experiments on fruit trees. This work was carried on at Pusa for ten years, 1914 to 1924. As it has been described in detail in a book available to all (An Agricultural Testament, pp. 117-32), only a summary will be given here.
The eight trees chosen were the plum, peach, custard apple, mango, guava, litchi, sour lime, and loquat. A beginning was made by noting the inevitable competition in wild Nature between trees and grass, in which the trees so often win and spread over the pastures to become the ultimate succession. At Pusa itself in the few years between 1918 and 1922 no fewer than 594 young wildings had established themselves on just over half an acre of sown grass and had to be uprooted to save the grass. This was a small illustration of a world-wide process.
Nevertheless there were occasions when grass was able to oust certain kinds of trees. In order to master the facts of the competition the trees chosen were treated in three ways: (a) clean cultivated, (b) completely grassed over, (c) grassed over but subsequently given aeration trenches.
Those that were completely grassed over, if young trees, died, the custard apple within two years of sowing the grass (1916), then, in order, the loquats before the end of 1919, the plums by the end of 1921, and the limes by the end of 1922. Those better established were just able to maintain themselves, the guava being by far the most resistant, though even this species was greatly checked in growth by grass. But under clean cultivation the trees flourished.
'Even a temporary removal of the grass leads to a profound effect. Whenever the roots of a tree under grass are exposed (for which purpose the grass has to be removed for a few days}, there is an immediate increase in growth, accompanied by the formation of larger and darker-coloured leaves. The effect is clearly visible on the foliage above the excavation for as long as two years, but the rest of the tree is not affected.'
The examination was pursued with great care and patience, by means of washing away the soil with a sprayer, often to a depth of 11 or 12 feet, and occasionally even to nearly 20 feet. (The technique is explained in Agric. Journ. of India, Vol. XIII, 1918, Special Indian Science Congress Number, 'Some Methods suitable for the Study of Root Development'.) Drawings were made of all root systems exposed, while above ground the corresponding foliage and wood growth were measured with exactness. (Details in the paper before the Royal Society cited at the end of this chapter. Leaves and internodes were measured individually.) All trees had a double root system, namely, a surface system and a deep soil system; this was later confirmed for fifteen species of forest trees also. Thus the effect of grass on these trees was to kill them by cutting off the air supply. Only those trees which could successfully force their surface roots upwards to penetrate the grass cover and reach the air survived: these trees flourished. Such trees included among the fruit trees none but the guava, which explains why the pastures of Grenada and St. Vincent in the West Indies are so rapidly invaded and destroyed by the wild guava, a fact observed by Sir Albert at the outset of his research career. All forest trees, on the other hand, had this faculty for conquering grass, which was the eventual explanation of the conquest of trees over grass in so many parts of the world.
The extreme sensibility of the fruit trees to the effects of soil conditions betrayed itself with dramatic clearness. As already stated, the young trees died under grass. The older trees showed smaller, or larger and healthier, foliage according to the way in which they were treated; the two different types of foliage could be seen on the same branch, the change-over to the better foliage being very sudden when soil conditions improved; where, at one point, burrowing rats had chanced to give air on one side of a tree, a lopsided effect was produced. One tree, the plum, gave the final proof; when grassed down, these trees were attacked by green-fly, but the fly never spread to the healthy trees, free of grass, adjoining (July 1922).
This most interesting observation about the green-fly, as a matter of fact, was no novelty. Already during the eight summers of work at Quetta the facts on green-fly infection had been established; attack was quite obviously the result of a weakening in the host, the tree, brought about by unfavourable soil conditions. Sir Albert was able actually to induce attack on peaches and almonds by over-irrigation, and then to stop it dead by deep cultivation; the young shoots were covered by the pest below, but the upper foliage was completely healthy; the fly never spread from the unhealthy to the healthy leaves. (An Agricultural Testament, p. 164.)
Could one ask for a clearer or more convincing proof of the thesis that the health of the plant is to be sought in the condition of the soil?The Spike Disease of Peach Trees
A short investigation, not systematically planned like the ten years' experiments on grass and trees but arising accidentally, led one step further, to an examination of the metabolism of plants. It confirmed the suspected fact that in disease the sap of the plant deteriorates. It was this deterioration which in Sir Albert's opinion invited insect attack.
He had been preceded in this idea by Mr. R. S. Hole, of the Indian Forestry Service, who had challenged the hypothesis that a parasite was responsible for a disease of the sandalwood tree and had analysed the sap changes. Sir Albert seized on this explanation for something which was affecting his young peach trees at Quetta. The first case -- leaf drop, very stunted growth, often followed by death of the whole tree -- was noticed in 1915. Until 1919 the faulty trees were destroyed, but in that year it occurred to Sir Albert that to allow this disease to take its course would provide the best, if not the only means, towards a true understanding. Thus an interesting turning point was reached in the study of plant diseases, as suggested many years ago by Professor Marshall Ward.
With the help of Mr. Jatindra Nath Sen, M.A., an analysis of leaves of two affected trees as compared with leaves from two normal trees showed far less nitrogen (1.76 as against 3.68 percent), less ash phosphorus (0.25 as against 0.51 per cent), less lime (0.94 as against 1.64 per cent), less potash (0.14 as against 0.64 per cent), but far more starch (5.15 as against 2.02 per cent). (Further figures in the original paper.) The inference was that the leaves of the diseased trees were unable to obtain from the soil a sufficient amount of crude sap and were also suffering from an undue accumulation of carbohydrates. A careful botanical examination showed that faulty budding had left a tiny gap between the ring bud and the bark of the stock; the imperfect junction of stock and scion had led to an interruption of the sap flow. To presume the existence of a parasite was merely a lazy method of evading the need for an explanation of a state of affairs which arose solely out of the condition of the plant itself. The presumed parasite was nothing but a myth; the trees had died because there had been an error in management.The Solution of the Puzzle of Lathyrism
The investigation into lathyrism is included in the present chapter as having been an episode in research. It did not perhaps contribute to the thesis of disease resistance, but it has a certain interest of its own.
Lathyrism, an incurable disease of a paralytic nature, was associated with the consumption of a pulse, Lathyrus sativus L. It was all too common in the central and northern parts of India, where the poorer classes were accustomed to rely on this vegetable, known in the vernacular as khesari, for most of their proteins; in times of food shortage it formed a chief item in their diet for months at a time, and as famine conditions approached more and more was consumed and lathyrism increased in incidence.
A good deal of attention had been given to this serious problem. English, Scotch, French, German, Italian, and American scientists had carried out experiments. These had been going on over the last forty-five years with inconclusive results.
In 1921 the Indian Research Fund Association asked two medical authorities to take up the question, and Sir Albert was associated with them for the growing of the crop. He seems almost immediately to have suspected where the trouble lay -- in the admixture of certain weed seeds with the pulse. Four weeds were associated with the lathyrus as commonly grown in India; of these, one, Vicia sativa L. var angustifolia, known to the peasant as akta, had a seed which was almost indistinguishable from the lathyrus seed. Every bazaar example of lathyrus seed included it.
To start the work thirty consignments of lathyrus seed were obtained from twenty different districts where this pulse was grown. Each was then hand-picked to get rid of the akta. Then followed three years' botanical work, with classification of types as had been carried out for so many other crops, each seed being separately sown. These careful studies bore their own fruit for the Howards in giving them additional facts on the relation between root development and disease resistance, for the lathyrus, like so many Indian crops, was found to consist of deep, shallow, and intermediate rooted types.
Meanwhile the more immediate object was being attained, a supply of seed unadulterated with akta. The chemical and feeding experiments thereupon undertaken by the two medical investigators showed beyond dispute the harmlessness of the lathyrus thus grown in pure culture.
Simultaneously an investigation of the weed known as akta was carried out. The Howards isolated this seed, grew a large area in pure culture, and supplied it to the medical authorities, who had no difficulty in proving it to contain an alkaloid poison.
The problem, which had seemed so tiresome, turned out very simple. It appears almost inconceivable that no one had previously taken the trouble to make sure that the basic material for experiment was what it professed to be, that investigators had been content to work on impure samples of seed. Possibly that could not happen to-day, when the minutiae of experimental work are so carefully scrutinized; but it certainly was the case in the 1920's. It is not impossible that the discovery of such ineptitude influenced Sir Albert in his critical view of the science of his day.
The burden of the investigations, however, were not with him but with the doctors. The investigation on lathyrism is here mentioned only in view of providing an instance of his shrewd solution of practical problems. For, with the discovery of the nature of the poisonous akta, one problem had only been exchanged for another. What was to be done to get rid of it? It was not an easy question. The pods of this plant ripen a few at a time, explode and scatter their contents, which are very hard and capable of germination after a long interval. This meant that land once contaminated with akta would bear self-sown seedlings of this weed for years to come; and the samples of seed originally obtained showed that there was not a field of lathyrus in India which was not infested with this poisonous intruder.
As the seeds were so like the lathyrus seed, it was not possible to separate out at harvesting time. The difficulty could be only overcome during the growing stage. Instead of the usual broadcast sowing, it was suggested that the lathyrus should be sown in lines a foot apart, which would allow of removal of the seedlings of the akta, easily distinguishable at this stage, during early growth. By the exercise of this very ordinary precaution the cultivator could protect himself. As always, the remedy was to be within the means of the people.Explanations and Confirmations
By this time there had come to be an accumulation of experiences justifying the formal setting out of the theory that disease in the vegetable world was a phenomenon resulting from some misadjustment or dislocation of soil conditions. This was done in the form of a contribution to a British scientific journal and also in a lecture delivered at Nagpur in 1921 (Annals of Applied Biology, Vol. VII, No. 4, 1921, p. 373: 'The Influence of Soil Factors on Disease Resistance'; Agric. Journ. of India, Vol. XVI, Part VI, Nov. 1921: 'Disease in Plants'); in the last-named paper occurs the famous dictum: 'the healthy plant protects itself'.
'Why does the natural resistance of the plant to fungi like rust and red rot or to an insect like green-fly break down? We have seen that under certain circumstances at Pusa Einkorn is unaffected by rust even when surrounded by plants heavily infected. Sugar-cane escapes red rot at Chandkhuri on the porous, well aerated bhata soils. In a similar manner some of the almond trees at Quetta resisted green-fly when separated by only a few feet from trees badly attacked. When the conditions of growth in these cases are altered, the natural resistance disappears and infection takes place. Why is this? One possible explanation is the following. Normally the protoplasm in the green cells of the leaves of healthy varieties is strong enough to resist the inroads of a fungus or of an insect. The healthy plant protects itself. If, however, soil conditions alter, then root damage is likely to occur and with this it is not difficult for the acid products of decay to enter the sap current and gradually to lower the resistance of the protoplasm of the green cells. A time comes when infection is just possible, then when it is easy, and finally when it is exceedingly rapid.'
Three years later the situation had again altered and in a very unexpected way: with transference to his own Institute at Indore the actual material for study of plant diseases gave out. (See reference in Chapter 1.) Sir Albert has left it on record that during the six years of his directorship of this Institute there were only two instances of disease in crops; they were both more or less accidental. One minor instance was useful as proving more unmistakably than ever the direct connection between soil conditions and health of the plant. By the breaking of a dam a field became flooded; a map was made of the flooded areas. When the field had dried, a crop of gram was sown, and one month after, when the crop was well away, those portions of the field which had been flooded became heavily infested with the gram caterpillar, the correspondence in areas being exact; but the caterpillar made no attempt to attack the gram on the fifty acres which had not been flooded. (Farming and Gardening for Health or Disease, p. 145.) The insect was able to demarcate, on behalf of the scientist, the delicate interlocking of the damage to the soil which still lingered, and the effect this had on the plant.
It was at Indore also that the final test came in another direction, on draught animals. Sir Albert was fond of horned stock and prided himself on his familiar knowledge of them. At Pusa he had insisted on keeping his own working oxen instead of borrowing them off the farm attached to the Research Station. These farm animals were suffering from various diseases, including foot and mouth, and there was a routine of inoculation. Against a great deal of official pressure Sir Albert categorically refused to have his own animals inoculated against anything. He then describes how he saw his oxen rubbing noses over the fence with infected animals. The extremely infectious nature of foot and mouth is common knowledge, yet not one of Sir Albert's animals took this or any other disease, thus displaying a degree of resistance which was amazing. Sir Albert did not hesitate to attribute this to the way his oxen were fed, off the area belonging to the Botanical Section which he was bringing to the highest state of fertility. (Cf. the statement that the Botanical Area had been transformed 'past belief' by drainage, etc., Chapter 3, The Supreme Importance of the Air Supply to Plants). The food he was able to provide for them had a quality lacking in that given to the farm stock.
The animals at Quetta also were throughout healthy. The test came at Indore. Here more animals were needed -- twenty pair. But, as was described in Chapter 1, the land when taken over at Indore was in a very bad state. No sufficient supply of good feed could therefore be quickly grown, and when the first hot weather came and there was no green feed, about a dozen succumbed to a slight attack of foot and mouth. They easily recovered, being treated with care and attention, and following years did not see any recurrence of the disease, as special regard was paid to making a provision of silage to carry the animals over the hot weather (which is normally a starvation period for Indian stock). Moreover, these animals, which were rather poor specimens when bought, in two years of good feeding off fertile soil were 'transformed'; they became prize exhibits, and their condition a source of the utmost pride and gratification to the Indore staff. (The most detailed account in Farming and Gardening for Health or Disease, pp. 153-7.) The history of Sir Albert Howard's draught animals in India is most illuminating.
In the last pronouncement on the problem of disease which Sir Albert made before leaving India the emphasis has changed. The question is put as to whether it is worth while to spend time speculating on the occasions of disease when it would be so much wiser to give one's energy to the pursuit of health. The lesson was drawn from the pioneer work of the Dutch scientists in Java, which Sir Albert greatly admired.'Twenty-five years ago detailed studies of the insect and fungus disease of the sugar-cane were the chief feature of the work of the sugar Experiment Stations in Java. These are now no longer considered necessary, as experience has shown that the best method of dealing with sugar-cane diseases is by the efficient cultivation of suitable varieties. To grow the right kind in the right way is found in practice to be the most important factor in the control and elimination of the pests of the sugar-cane. This experience is most significant. The Java sugar industry is perhaps the high-water mark of tropical agriculture and owes its present position to various natural advantages which have been developed to the utmost by the efforts of a succession of highly qualified scientific investigators who have explored, in the greatest detail, the various directions in which the production of sugar can be increased. It is most significant that as the investigation of sugar-cane diseases became broadened and included all the factors, direct methods were given up and attention was directed solely to the variety and to its proper cultivation. This is after all only common sense. Disease follows the breakdown of the normal physiological processes in the plant when the protoplasm of the cells loses its power of resistance to the inroads of parasites. Healthy plants, on the other hand, possess a high degree of immunity to insects and fungi. It is obviously more practical to prevent disease altogether by growing the right kind in the right way than to step in at the last moment and attempt to save a moribund crop.'
The Function of Disease
What, then, is the function of disease? In a later amusing passage Sir Albert argued that the function of disease was to keep the investigator in order.
'When I left Cambridge, having sat at the feet of the distinguished professors of agriculture there and having saturated myself with the lore of Rothamsted, I was soon placed in a position of some danger when I became a government official in India. Plenty of money and every sort of facility were given to me for the improvement of the agriculture of the country, but it occurred to me one thing was lacking: what was there to keep me in order? What keeps the cultivator of the soil in order is the fact that he has got to make a living; circumstances keep him in order. But what is there to keep the privileged investigator in order, who is in government service and who does not have to make a profit? I found when I took up land in India and learned what the people of the country know, that the diseases of plants and animals were very useful agents for keeping me in order, and for teaching me agriculture. I have learnt more from the diseases of plants and animals than I have from all the professors of Cambridge, Rothamsted and other places who gave me my preliminary training. I argued the matter in this way. If diseases attacked my crops, it was because I was doing something wrong. I therefore used diseases to teach me. In this way I really learnt agriculture -- from my father and from my relatives and from the professors I only obtained a mass of preliminary information. Diseases taught me to understand agriculture. I think if we used diseases more instead of running to sprays and killing off pests, and if we let diseases rip and then found out what is wrong and then tried to put it right, we should get much deeper into agricultural problems than we shall do by calling in all these artificial aids. After all, the destruction of a pest is the evasion of, rather than the solution of, all agricultural problems. I recommend very strongly diseases for keeping investigators in order. They are the post-graduate teachers of the investigators. Although it was a bitter experience for me, nevertheless it did me a great deal of good and I learned much.' (Answer to the discussion on his paper: 'The Restoration of Soil Fertility' at the Farmers' Club, London, 1st February 1937.)
This answer, though given some years after leaving India, may be traced back to a conversation with one of the scientists from the Java Station already mentioned, who had informed Sir Albert that an outbreak of red rot fungus on a sugar estate in that island would involve the dismissal of the manager, as showing he was not doing the right thing by the plant. (Crop Production in India, p. 178.) The idea appealed to Sir Albert's humour, who later applied it to his own special field by arguing that a single fly on the municipal compost heap should terminate office for the engineer. More seriously he held that in Experiment Station work the first need was to be able to show crops and stock in a perfect state of health; exhibitions and study of disease could follow. He never afterwards modified his firm conviction on this first duty of an agricultural scientist, who must above all be a showman of what is good.
If disease were to be studied, it must be on a far more comprehensive basis than had hitherto been attempted, must take in the whole history of soil conditions preceding some attack, tracing these back possibly for months (a sketch of the ideas which would guide the investigation of disease at the new Station at Indore, in which this condition is mentioned in The Application of Science to Crop Production, p. 20), must always pay attention to environment and circumstances, for instance, to the great importance of 'growing the right thing in the right place' (the consequences of this are very important; many 'introduced' crops are by nature susceptible, because out of their habitat; a good instance is wheat in a hot climate, as was pointed out in Chapter 1; but the argument must not be exaggerated: a large number, if not most, of our domestic crops are 'introduced', especially in Europe, yet so wide is the adaptation of Nature that most of them should be capable of being grown in perfect health in the regions where they are now cultivated), must, finally, never be content to stop with the mere identification of the relevant virus, fungus, or parasite, but must find in this only a first step as seeing in these agents 'valuable indicators' ('parasites... very valuable indicators, provided by Nature, for checking the proceedings of the agriculturalist'; Crop Production in India, p. 181, 1924) of something much more fundamental which was wrong.
In later life Sir Albert Howard often referred to his good fortune in being sent to a tropical country like India, where the effects of temperature, precipitation, sun, etc., are so much more decisive than in our milder climates, and where phenomena appear on a vaster scale. If anything had been lacking to bring home to him the facts of infection and resistance and their genesis in soil conditions, he had only to look around him and cast his eyes over the Indian landscape. For century after century eelworm had infected wheat unsuitably grown (owing to the pressing need of the populations for food) in the wet tracts of the Harnai valley in the West; yet this eelworm, in spite of a mass of unrestricted foot and wheeled traffic, never spread to the thousands of acres running in an uninterrupted wheat belt across India on drier soil. Or again, there was the area of Ufra-disease infected rice in the undrained part of Bengal, adjacent to the other better drained rice lands to which the infection never spread. (Farming and Gardening for Health or Disease, pp. 128-9.) With justice he called India a country where agriculture had had time to impress its pattern on the landscape, a country of endless 'experiments' constantly repeated and plain to the eye.
Challenged once to account for the advance of locust hordes, a plague which seemed to defy the explanations which he had put forward, he states, again out of his Indian experience, that this was not so: that it was a fact that the invasions of locusts in Central and North-west India, starting from the desert where the eggs are laid, always produced their maximum damage on irrigated crops during the hot weather, where they ate everything green in their path; as soon as the rains began and normally grown vegetation was available, the swarms rapidly disappeared: what had been a terrifying visitation was 'soon reduced by Nature to its normal insignificance'. In no case had the locusts in Rajputana, for instance, established themselves permanently on lands alongside the desert when these lands were fed by the natural monsoon. The silkworm, also quoted against him, was disposed of with greater ease, for, as Sir Albert correctly points out, the silkworm industry is a thoroughly artificial one and silkworms are, in fact, delicate creatures and have to be guarded with even more solicitude than is bestowed on the majority of infants. Were silkworms let loose in a grove of well-cultivated mulberry trees their career would be a very different story. (The Empire Cotton Growing Review, Vol. XV, No. 3, July 1938, reply to criticisms on his paper in the previous number on 'The Role of Insects and Fungi in Agriculture'. The delicacy of irrigated crops was also argued from the fact that irrigated sugar-cane was invariably attacked by the moth borer in the hot weather, but as soon as the early rains came and the cane grew normally, this insect disappeared.)
Influenced by our knowledge of the incredible rate at which such organisms as bacteria, viruses, fungi, etc., can multiply, we think of infections as something which, once begun, must spread until by some mysterious process they are exhausted. But this is no explanation. We ought to look further. Infection will spread only where it finds suitable food; unless this is offered, it ceases. The fact that an infection does spread is a sure sign that the food is there, that the host is open to attack, in some way weakened, but the phenomena rather easily baffle us, for such weakness in the host is often not apparent to our observation, and its defeat in front of the invading organism is the first sign we have that anything is wrong. It is natural for us to lay stress on the obvious movements of the invader and to forget the 'invitation' which has let such invader in.
During the twenty-six years of his career in India, to which must be added the previous years in the West Indies and at Wye College, Sir Albert had travelled far. Starting with the conventional thesis that resistance to this or that disease must be traced as a Mendelian character and that breeding of specifically resistant varieties by selection or other means would be the only answer to attack, he had even at the outset combined this work with speculations of another kind: What under all circumstances was the secret of resistance when it occurred?
Working in a vast tropical country where for generations the cultivators had grown their food, depending far more on their crops for their subsistence than on their animal products, he was subject to the gradual influence of two permanent factors. On the whole, the crops grown by the peasants were free of disease and pests; on the other hand, the slightest mismanagement of soil conditions brought disaster. It has already been pointed out that under the tropical sun and rainfall soils are vulnerable; peasant practice has attained great experience in conserving and maintaining them; their attention is constantly fixed on this task. Thus the two items, healthy crops and fertile soils, were part of the same lesson, which fortunately Sir Albert had both the wit to learn and the knowledge to reaffirm in the course of his scientific work. Most stress was laid on soil aeration. This was a major problem in the East. It was after leaving India that attention was concentrated on the biological state of the soil, the presence of microflora and microfauna, the mycelia, the earthworm. These were the problems of Europe, where the poor biological life of the soil was startlingly apparent; under peasant systems such biological soil impoverishment is unknown.
Thus not the whole of the problem was mastered in India; there was certainly a further advance in thought after leaving that country. All ideas were, nevertheless, based on the long years of work in the East, which provided him with the conception of the resistant host, of the failure of infections and plagues to overleap this barrier of health, and above all of the supreme importance of tracing health and disease back to the soil.
In the discussion of plant health and plant disease it is worth observing that, while the nutrients which the plant draws from the atmosphere are limitless, constant, and immune to interference, those which it draws from the soil are limited, subject to spoliation, and singularly variable. The soil has a history which alters from hour to hour, and the same area of soil will change rapidly under specific influences, not excluding the influences brought by man, who can intervene so violently as to render a soil 'living' or 'dead'. Is it surprising that so variable a medium should confer variability on its children, the living plants?Bibliography
(a) General
Crop Production in India, 1024, Ch. XIX: 'Disease in Plants'.
The Application of Science to Crop Production, 1929, p. 20.
Agric. Journ. . of India, Vol. XI, 1916, Special Indian Science Congress No.: 'The Application of Botanical Science to Agriculture', pp. 22- 4, 'The Treatment of Disease'.
Ibid., Vol. XVI, Part VI, Nov. 1021: 'Disease in Plants' (lecture delivered at the Victoria Institute, Nagpur)
Ibid., Vol. XVIII, Part III, May 1923: 'The Role of Plant Physiology in Agriculture' (presidential address of Mrs. G. L. C Howard to the Section of Botany, Indian Science Congress); pp 215-17. 'Incidence of Disease and the Physiological State of the Plant.'
Ibid., Vol. XXI, Part III, May 1926: 'Agriculture and Science' (presidential address to the Indian Science Congress), pp. 179-80, 'Diseases of Plants'.
Annals of Applied Biology, Vol. VII, No. 4, Feb. 1921: 'The Influence of Soil Factors on Disease Resistance'.
(b) Special Problems
Wheat in India, 1909, Section II: 'The Diseases of Wheat in India"
Agric. Journ. of India, Vol. XII, 1917, Special Indian Science Congress No.: 'The Economic Significance of the Root Development of Agricultural Crops' (root systems and disease).
Proc. of the Roy. Soc., Series B, Vol. 97, No. B 683: 'The Effect of Grass on Trees', reprinted in the Agric. Journ. of India, Vol. XX, Part IV, July 1925 (root systems and disease).
Proc. of the Bihar Planters' Association, 1914: 'The Improvement of Indigo in Bihar, with Notes on Drainage and Green-Manuring'.
Bulletin No. 54 of the Agric. Research Institute, Pusa, 1915: 'Second Report on the Improvement of Indigo in Bihar'.
Bulletin No. 67, 1916: 'Third Report on the Improvement of Indigo in Bihar'.
Memoirs of the Depart. of Agric. in India (Botanical Series), Vol. XI, No. 1, 1920: 'Some Aspects of the Indigo Industry in Bihar'.
Agric. Journ. of India, Vol. XIX, Part VI, Nov. 1924: 'The Continuous Growth of Java Indigo in Pusa Soil'.
Indian Forester, Dec. 1919: 'The Spike Disease of Peach Trees: An Example of Unbalanced Agriculture'.
Indian Journ. of Medical Research, Vol. XII, 1925 (with Dr. J. L Simonsen and Captain L. A. P. Anderson): 'Studies in Lathyrism'.
The Empire Cotton Growing Review, Vol. XIII, No. 3, July 1936: 'The Role of Insects and Fungi in Agriculture'; and ibid. , Vol. XV, No. 3, July 1938: 'Insects and Fungi in Agriculture'.
The Application of Science to Crop Production, 1929, pp. 47-9 (health of the animals at Indore).
Farming and Gardening for Health or Disease, 1945, pp. 153-6 (health of Sir Albert Howard's Indian work cattle).
Next: 6. The Work on the Indore Process
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