The Retreat of the Crop and the Animal before the Parasite
IN the previous chapter we have seen how Nature, by means of soil erosion, removes any area of worn-out land and recreates new soil in a fresh place. Mismanagement of the land is followed later on by a New Deal, as it were, somewhere else. A similar rule applies to crops: the diseased crop is quietly but effectively labelled prior to removal for the manufacture of humus, so that the next generation of plants may benefit.
Mother earth has provided a vast organization for indicating the inefficient crop. Where the soil is infertile, where an unsuitable variety is being grown, or where some mistake has been made in management, Nature at once registers her disapproval through her Censors' Department. One or more of the groups of parasitic insects and fungi -- the organisms which thrive on unhealthy living matter -- are told off to point out that farming has failed. In the conventional language of to-day the crop is attacked by disease. In the writings of the specialist, a case has arisen for the control of a pest: a crop must be protected.
In recent years another form of disease -- known as virus disease -- has made its appearance. There is no obvious parasite in virus diseases, but insects among other agencies are able to transmit the trouble from diseased to apparently healthy plants in the neighbourhood. When the cell contents of affected plants are examined, the proteins exhibit definite abnormalities, thereby suggesting that the work of the green leaf is not effective; the synthesis of albuminoids seems to be incomplete. With the development of special research laboratories, like that at Cambridge, more and more of these diseases are being discovered and a considerable literature on the subject has arisen.
The virus diseases do not complete the story. A certain number of maladies occur in which the apparent cause is neither a fungus, an insect, nor a virus. These are grouped under the general title -- physiological diseases: troubles arising from the collapse of the normal metabolic processes.
How has agricultural science dealt with the diseases of crops? The answer is both interesting and illuminating. The subject has been approached in a variety of ways, which can be briefly summed up under the following four heads:
- The study of the life history of the pest, including the general relation of the parasite to the crop and the influence of the environment on the struggle for supremacy between the two. The main object of these investigations has been to discover some possible weakness in the life history of the pest which can be utilized to destroy it or to protect the plant from infection. An impressive volume of specialist literature has resulted. As the number of investigators grows and as their inquiries become more exhaustive and tend to cover a rapidly increasing proportion of the earth's surface, there is a corresponding increase in the volume of print. It is now almost impossible to take up any of the periodicals dealing with agricultural research without finding at least one long illustrated article describing some new disease. So vast has the literature become that the specialists themselves are unable to cope with it. Most of it can only be read by the workers in abstract, for which again new agencies have been created in the British Empire -- the Imperial Bureaux of Entomology and Mycology -- bodies which act as clearing-houses of information and deal with the published papers in a way reminiscent of the methods of the Banker's Clearing House in dealing with cheques.
- The study of the natural parasites of insect pests, the breeding of these animals, and their actual introduction whenever this procedure promises success. A separate institution for this purpose has been founded at Farnham Royal in Buckinghamshire.
- The protection of the crop from the inroads of the parasite. As a rule this takes two forms: (1) the discovery of insecticides and fungicides and the design of the necessary machinery for covering the crop with a thin film of poison which will destroy the parasite in the resting stage or before it can gain entry to the host; (2) the destruction of the parasite by burning, by the use of corrosive liquids like strong sulphuric acid, or by germicides added to the soil so that the amount of infecting material will be negligible.
- The framing and conduct of regulations to protect an area from some foreign pest which has not yet made its appearance. These follow the usual methods of quarantine. Importation of plants and seeds is prohibited altogether, introduction is permitted under licence, or the plant material is inspected and fumigated at the port of entry. The principle in all cases is the same -- the crops must be protected from chance infection by some foreign parasite which might cause untold damage. As traffic by land, sea, and air grows in volume and becomes speeded up, it will be increasingly difficult to enforce these regulations. It is impossible even now to inspect all luggage and all merchandise and to prevent the smuggling of small packets of seed or cuttings of living plants. Indeed, if an investigation were to be made of the personal effects of the coolies passing backwards and forwards between India and Burma, India and the Federated Malay States and Ceylon, it would be seen what an extraordinary collection of articles these men and women carry about and how frequently plants and seeds are included. Enthusiasts in gardening often collect plants on their travels which interest them. The population, live stock, and factories of Great Britain are partly supplied with seeds from all over the world. By one or other of these agencies a few new pests are almost certain from time to time to enter the country. These quarantine methods therefore can never succeed.
More than fifty years have passed since the modern work on the diseases of plants first began. What has been the general result of all this study of vegetable pathology? Has it provided anything of permanent value to agriculture? Is the game worth the candle? Must agricultural science go on discovering more and more new pests and devising more and more poison sprays to destroy them or is there any alternative method of dealing with the situation? Why is there so much of this disease? Can the growing tale of the pests of Western agriculture be accounted for by some subtle change in practice? Can the cultivators of the East, for example, teach us anything about diseases and their control?
In this chapter an attempt will be made to answer these interesting questions.
It is a well-understood principle in business that any organization like agricultural research, which has grown by accretion rather than by the development of a considered plan, stands in need of a periodical critical examination to ascertain whether the results obtained correspond with the cost and whether any modifications are needed in the light of new knowledge and experience. I began such an investigation of the plant and animal disease section of agricultural science in 1899 and have steadily pursued it since. After forty years' work I feel sufficiently confident of my general conclusions to place them on record, and to ask for them to be considered on their merits.
I took up research in agriculture as a mycologist in the West Indies in 1899, where I specialized in the diseases of sugar-cane and cacao and became interested in tropical agriculture. Almost at once I discerned a fundamental weakness in the research organization: the mycologist had no land on which he could take his own advice about remedies before asking planters to adopt them.
My next post was botanist at Wye College in Kent, where I was in charge of the experiments on hops and had ample opportunities for studying the insect and fungous diseases of this interesting crop. But again I had no land on which I could try out certain ideas that were fermenting in my mind about the prevention of hop diseases. I observed one interesting thing: the increase in the resisting power to infection of the young hop flower which resulted from pollination. This observation has since brought about a change in the local practice: the male hop is now cultivated and ample pollination of the female flowers -- the hops of commerce -- occurs.
In 1905 I was appointed Imperial Economic Botanist to the Government of India. At the Pusa Agricultural Research Institute, largely through the support of the Director, the late Mr. Bernard Coventry, I had for the first time all the essentials for work -- interesting problems, money, freedom, and, last but nit least, 75 acres of land on which I could grow crops in my own way and study their reaction to insect and fungous pests and other things. My real training in agricultural research then began -- six years after leaving the University and obtaining all the paper qualifications and academic experience then needed by an investigator.
At the beginning of this second and intensive phase of my training, I resolved to break new ground and try out an idea (which first occurred to me in the West Indies), namely, to observe what happened when insect and fungous diseases were left alone and allowed to develop unchecked, and where indirect methods only, such as improved cultivation and more efficient varieties, were employed to prevent attack. This point of view derived considerable impetus from a preliminary study of Indian agriculture. The crops grown by the cultivators in the neighbourhood of Pusa were remarkably free from pests of all kinds; such things as insecticides and fungicides found no place in this ancient system of agriculture. I decided that I could not do better than watch the operations of these peasants, and acquire their traditional knowledge as rapidly as possible. For the time being, therefore, I regarded them as my professors of agriculture. Another group of instructors were obviously the insects and fungi themselves. The methods of the cultivators if followed would result in crops practically free from disease; the insects and fungi would be useful for pointing out unsuitable varieties and methods of farming inappropriate to the locality.
It was possible for me to approach the subject of plant diseases in this unorthodox manner for two reasons. In the first place the Agricultural Research Institute at Pusa was little more than a name when I arrived in India in 1905. Everything was fluid; there was nothing in the nature of an organized system of research in existence. In the second place, my duties, fortunately for me, had not been clearly defined. I was therefore able to break new ground, to widen the scope of economic botany until it became crop production, to base my investigations on a first-hand knowledge of Indian agriculture, and to take my own advice before offering it to other people. In this way I escaped the fate of the majority of agricultural investigators -- the life of a laboratory hermit devoted to the service of an obsolete research organization. Instead, I spent my first five years in India ascertaining by practical experience the principles underlying health in crops.
In order to give my crops every chance of being attacked by parasites, nothing was done in the way of prevention; no insecticides and fungicides were used; no diseased material was ever destroyed. As my understanding of Indian agriculture progressed, and as my practice improved, a marked diminution of disease occurred. At the end of five years' tuition under my new professors -- the peasants and the pests -- the attacks of insects and fungi on all crops, whose root systems were suitable to the local soil conditions, became negligible. By 1910 I had learnt how to grow healthy crops, practically free from disease, without the slightest help from mycologists, entomologists, bacteriologists, agricultural chemists, statisticians, clearing-houses of information, artificial manures, spraying machines, insecticides, fungicides, germicides, and all the other expensive paraphernalia of the modern Experiment Station.
I then posed to myself the principles which appeared to underlie the diseases of plants:
- Insects and fungi are not the real cause of plant diseases but only attack unsuitable varieties or crops imperfectly grown. Their true role is that of censors for pointing out the crops that are improperly nourished and so keeping our agriculture up to the mark. In other words, the pests must be looked upon as Nature's professors of agriculture: as an integral portion of any rational system of farming.
- The policy of protecting crops from pests by means of sprays, powders, and so forth is unscientific and unsound as, even when successful, such procedure merely preserves the unfit and obscures the real problem -- how to grow healthy crops.
- The burning of diseased plants seems to be the unnecessary destruction of organic matter as no such provision as this exists in Nature, in which insects and fungi after all live and work.
This preliminary exploration of the ground suggested that the birthright of every crop is health, and that the correct method of dealing with disease at an Experiment Station is not to destroy the parasite, but to make use of it for tuning up agricultural practice.
Steps were then taken to apply these principles to oxen, the power unit in Indian agriculture. For this purpose it was necessary to have the work cattle under my own charge, to design their accommodation, and to arrange for their feeding, hygiene, and management. At first this was refused, but after persistent importunity, backed by the powerful support of the Member of the Viceroy's Council in charge of agriculture (the late Sir Robert Carlyle, K.C.S.I.), I was allowed to have charge of six pairs of oxen. I had little to learn in this matter as I belong to an old agricultural family and was brought up on a farm which had made for itself a local reputation in the management of cattle. My work animals were most carefully selected and everything was done to provide them with suitable housing and with fresh green fodder, silage, and grain, all produced from fertile land. I was naturally intensely interested in watching the reaction of these well-chosen and well-fed oxen to diseases like rinderpest, septicaemia, and foot-and-mouth disease which frequently devastated the countryside. These epidemics are the result of starvation, due to the.intense pressure of the bovine population on the limited food-supply. None of my animals were segregated; none were inoculated; they frequently came in contact with diseased stock. As my small farm-yard at Pusa was only separated by a low hedge from one of the large cattle-sheds on the Pusa estate, in which outbreaks of foot-and-mouth disease often occurred, I have several times seen my oxen rubbing noses with foot-and-mouth cases. Nothing happened. The healthy well-fed animals reacted to this disease exactly as suitable varieties of crops, when properly grown, did to insect and fungous pests -- no infection took place.
As the factors of time and place are important when testing any agricultural innovation, it now became necessary to try out the three principles referred to above over a reasonably long period and in new localities. This was done during the next twenty-one years at three centres: Pusa (1910-24), Quetta (summers of 1910-18), and Indore (1924-31).
At Pusa, during the years 1910 to 1924, outbreaks of plant diseases were rare, except on certain cultures with deep root systems which were grown chiefly to provide a supply of infecting material for testing the disease resistance of new types obtained by plant-breeding methods. Poor soil aeration always encouraged disease at Pusa. The unit species of Lathyras sativus provided perhaps the most interesting example of the connexion between soil aeration and insect attack. These unit species fell into three groups: surface-rooted types always immune to green-fly; deep-rooted types always heavily infected; types with intermediate root system always moderately infected. These sets of cultures were grown side by side year after year in small oblong plots about 10 feet wide. The green-fly infection repeated itself each year and was determined not by the presence of the parasite, but by the root development of the host. Obviously the host had to be in a certain condition before infection could take place. The insect, therefore, was not the cause but the consequence of something else.
One of the crops under study at Pusa was tobacco. At first a great many malformed plants -- since proved to be due to virus -- made their appearance in my cultures. When care was devoted to the details of growing tobacco seed, to the raising of the seedlings in the nurseries, to transplanting and general soil management, this virus disease disappeared altogether. It was very common during the first three years; it then became infrequent; between 1910 and 1924 I never saw a single case. Nothing was done in the way of prevention beyond good farming methods and the building up of a fertile soil. I dismissed it at the time as one of the many mare's nests of agricultural science -- things which have no real existence.
For a period of eight years, I was provided with a subsidiary experiment station on the loess soils of the Quetta valley for the study of the problems underlying fruit-growing and irrigation. I observed no fungous disease of any importance in the dry climate of the Quetta valley during the eight summers I spent there. In the grape gardens, run by the tribesmen on the well-drained slopes of the valley, I never observed any diseases -- insect or fungous -- on the grapes or on the vines, although they were planted on the floors of deep trenches, allowed to climb up the earth walls and were frequently irrigated. At first sight, all the conditions for mildew seemed to have been provided, yet I never saw a single speck. Three favourable factors probably accounted for this result. The climate was exceedingly dry, with considerable air movement and cloudless skies; the soil made use of by the roots of the vine was open, well drained, and exceptionally well aerated; the only manure used was farm-yard manure. Growth, yield, quality, and disease resistance left nothing to be desired.
The chief pest of fruit trees at Quetta was green-fly soon after the young leaves appeared. This could be produced or avoided at will by careful attention to cultivation and irrigation. Any interference with soil aeration brought on this trouble; anything which promoted soil aeration prevented it. I frequently produced a strong attack of green-fly on peaches and almonds by over-irrigation during the winter and spring, and then stopped it dead by deep cultivation. The young shoots were covered with the pest below, but the upper portions of the same shoots were completely healthy. The green-fly never spread from the lower to the upper leaves on the same twig. The tribesmen got over the tendency of these loess soils to pack under irrigation in a very simple way. Lucerne was always grown in the fruit orchards, and regularly top-dressed with farm-yard manure. In this way the porosity of the soil was maintained and the green-fly kept in check.
At the Institute of Plant Industry, Indore, only two cases of disease occurred during the eight years I was there. The first occurred on a small field of gram (Cicer arietinum), about two-thirds of which was flooded for a few days one July, due to the temporary stoppage of one of the drainage canals which took storm water from adjacent areas through the estate. A map of the flooded area was made at the time. In October, about a month after sowing, this plot was heavily attacked by the gram caterpillar, the insect-infected area corresponding exactly with the inundation area. The rest of the plot escaped infection and grew normally. The insect did not spread to the other 50 acres of gram grown that year alongside. Some change in the food of the caterpillar had obviously been brought about by the alteration in the soil conditions caused by the temporary flooding. The second case of disease occurred in a field of san hemp (Crotalaria juncea L.) intended for green-manuring; however, this was not ploughed in but was kept for seed. After flowering the crop was smothered by a mildew; no seed was harvested. To produce a crop of seed of san on the black soils, it is necessary to manure the land with humus or farm-yard manure, when no infection takes place and an excellent crop of seed is obtained.
One experiment with cotton unfortunately could not be arranged in spite of all my efforts. At Indore there was a remarkable absence of all insect and fungous diseases of cotton. Good soil management, combined with dressings of humus, produced crops which were practically immune to all the pests of cotton. When the question of protecting India from the various cotton boll-worms and boll-weevils from America was discussed, I offered to have these let loose among my cotton cultures at Indore in order (1) to settle the question whether these troubles in the U.S.A. were due to the insect or to the way the cotton was grown, and (2) to subject my farming methods to a crucial test. I am pretty certain the insects would have found my cotton cultures very indifferent nourishment. My proposal, however, did not find favour with the entomological advisers of the Indian Cotton Committee and the matter dropped.
At Quetta and Indore there was no case of infectious disease among the oxen. The freedom from disease observed at Pusa was again experienced in the new localities -- the Western Frontier and Central India.
It was soon discovered in the course of this work that the thing that matters most in crop production is a regular supply of well-made farm-yard manure and that the maintenance of soil fertility is the basis of health.
Humus and Disease Resistance
Even on the Experiment Stations the supply of farm-yard manure was always insufficient. The problem was how to increase it in a country where a good deal of the cattle-dung has to be burnt for fuel. The solution of this problem was suggested by the age-long practices of China. It involved the study of how best to convert the animal and vegetable wastes into humus, so that every holding in India could become self-supporting as regards manure. Such a problem did not fall within my sphere of work -- the improvement of crops. It obviously necessitated a great deal of chemical work under my personal control. The organization of research at Pusa had gradually become more rigid; the old latitude which existed in the early days became a memory. That essential freedom, without which no progress is possible, had been gradually destroyed by the growth of a research organization based on fragments of science rather than on the practical problems which needed investigation. The instrument became more important than its purpose. Such organizations can only achieve their own destruction. This was the reason why I decided to leave Pusa and found a new centre where I should be free to follow the gleam unhampered and undisturbed. After a delay of six years, 1918 to 1924, the Indore Institute was founded. In due course a simple method, known as the Indore Process, of composting vegetable and animal wastes was devised, tested, and tried out on the 300 acres of land at the disposal of the Institute of Plant Industry, Indore. In a few years production more than doubled: the crops were to all intents and purposes immune from disease.
Since 1931 steps have been taken to get the Indore Process taken up in a number of countries, especially by the plantation industries such as coffee, tea, sugar, sisal, maize, cotton, tobacco, and rubber. The results obtained have already been discussed. In all these trials the conversion of vegetable and animal wastes into humus has been followed by a definite improvement in the health of the crops and of the live stock. My personal experience in India has been repeated all over the world. At the same time a number of interesting problems have been unearthed. One example will suffice. In Rhodesia humus protects the maize crop from the attacks of the witchweed (Striga lutea). Is this infestation a consequence of malnutrition? Is immunity conferred by the establishment of the mycorrhizal association? Answers to these questions would advance our knowledge and would suggest a number of fascinating problems for investigation.
The Mycorrhizal Association and Disease
Why is humus such an important factor in the health of the crop? The mycorrhizal association provides the clue. The steps by which this conclusion was reached in the case of tea have already been stated.
This association is not confined to one particular forest crop. It occurs in most if not all our cultivated plants. During 1938 Dr. Rayner and Dr. Levisohn examined and reported on a large number of my samples -- rubber, coffee, cacao, leguminous shade trees, green-manure crops, coconuts, tung, cardamons, vine, banana, cotton, sugar-cane, hops, strawberries, bulbs, grasses and clovers and so forth. In all of these the mycorrhizal association occurs. It is probably universal. We appear to be dealing with a very remarkable example of symbiosis in which certain soil fungi directly connect the humus in the soil with the roots of the crop. This fungous tissue may contain as much as 10 per cent. of nitrogen in the form of protein, which is digested in the active roots and probably carried by the transpiration current to the seat of carbon assimilation in the green leaves. Its effective presence in the roots of the plant is associated with health; its absence is associated with diminished resistance to disease. Clearly the first step in investigating any plant disease in the future will be to see that the soil is fertile and that this fungous association is in full working order. If it is as important as is now suggested, there will be a marked improvement in the behaviour of the host once the fertility of the soil is restored. If it has no significance, a fertile soil will make no difference.
I have just obtained confirmatory results which prove how important humus is in helping a mycorrhiza- former -- the apple -- to throw off disease. In 1935 I began the restoration, by means of humus, of my own garden, the soil of which was completely worn-out when I acquired it in the summer of 1934. The apple trees were literally smothered with American blight, green-fly, and fruit-destroying caterpillars like the codlin moth. The quality of the fruit was poor. Nothing was done to control these pests beyond the gradual building up of the humus content of the soil. In three years the parasites disappeared; the trees were transformed; the foliage and the new wood now leave nothing to be desired; the quality of the fruit is first class. These trees will now be used for infection experiments in order to ascertain whether the fertility of the soil has been completely restored or not. The reaction of the trees to the various pests of the apple will answer this question. No soil analysis can tell me as much as the trees will.
The meaning of all this is clear. Nature has provided a marvellous piece of machinery for conferring disease-resistance on the crop. This machinery is only active in soil rich in humus; it is inactive or absent in infertile land and in similar soils manured with chemicals. The fuel needed to keep this machinery in motion is a regular supply of freshly prepared humus, properly made. Fertile soils then yield crops resistant to disease. Worn-out soils, even when stimulated with chemical manures, result in produce which needs the assistance of insecticides and fungicides to yield a crop at all. These in broad outline are the facts.
The complete scientific explanation of the working of this remarkable example of symbiosis remains to be provided. It would appear that in the mycorrhizal association Nature has given us a mechanism far more important and far more universal than the nodules on the roots of the clover family. It reconciles at one bound science and the age-long experience of the tillers of the soil as to the supreme importance of humus. There has always been a mental reservation on the part of the best farmers as to the value of artificial manures compared with good old-fashioned muck. The effect of the two on the soil and on the crop is never quite the same. Further, there is a growing conviction that the increase in plant and animal diseases is somehow connected with the use of artificials. In the old days of mixed farming the spraying machine was unknown, the toll taken by troubles like foot-and-mouth disease was insignificant compared with what it is now. The clue to all these differences -- the mycorrhizal association -- has been there all the time. It was not realized because the experiment stations have blindly followed the fashion set by Liebig and Rothamsted in thinking only of soil nutrients and have forgotten to look at the way the plant and the soil come into gear. An attempt has been made to apply science to a biological problem by means of one fragment of knowledge only.
The Investigations of Tomorrow
The next step in this investigation is to test the soundness of the views put forward. This has been started by composting diseased material and then using the humus to grow another crop on the same land. Diseased tomatoes have been converted into humus by one of the large growers in the south of England and the compost has been used to grow a second crop in the same greenhouses. No infection occurred.
The final proof that insects, fungi, viruses and so forth are not the cause of disease will be provided when the infection experiments of to-morrow are undertaken. Instead of conducting these trials on plants and animals grown anyhow, the experimental material will be plants and animals, properly selected, efficiently managed, and nourished by or on the produce of a fertile soil. Such plants can be sprayed with active fungous and insect material without harm. Among such herds of cattle, cases of foot-and-mouth disease can be introduced without any danger of serious infection. The afflicted animals themselves will recover. When some audacious innovator of the Hosier type, who has no interest in the maintenance of the existing research structure, conducts such experiments, the vast fabric of disease-control which has been erected in countries like Great Britain will finally collapse. Farmers will emancipate themselves from the thraldom created by the fear of the parasite. Another step forward will be taken which will not stop at farming.
My self-imposed task is approaching completion. I have examined in great detail for forty years the principles underlying the treatment of plant and animal diseases, as well as the practice based on these principles. It now remains to sum up this experience and to offer suggestions for the future.
There can be no doubt that the work in progress on disease at the Experiment Stations is a gigantic and expensive failure, that its continuance on present lines can lead us nowhere and that steps must be taken without delay to place it on sounder lines.
The cause of this failure is not far to seek. The investigations have been undertaken by specialists. The problems of disease have not been studied as a whole, but have been divorced from practice, split up, departmentalized and confined to the experts most conversant with the particular fragment of science which deals with some organism associated with the disease.
This specialist approach is bound to fail. This is obvious when we consider: (1) the real problem -- how to grow healthy crops and how to raise healthy animals, and (2) the nature of disease -- the breakdown of a complex biological system, which includes the soil in its relation to the plant and the animal. The problem must include agriculture as an art. The investigator must therefore be a farmer as well as a scientist, and must keep simultaneously in mind all the factors involved. Above all he must be on his guard to avoid wasting his life in the study of a mare's nest: in dealing with a subject which owes its existence to bad farming which will disappear the moment sound methods of husbandry are employed.
The problems presented by the retreat of the crop and of the animal before the parasite and the conventional methods of investigation of these questions are clearly out of relation. It follows therefore that a research organization which has lost direction and has permitted such a state of things to arise and to develop must itself be in need of overhaul. This task has been attempted; the existing structure of agricultural research has been subjected to a critical examination; the results are set out in Chapter 13.
Howard, A. Crop Production in India, Oxford University Press, 1924, p. 176.
-- 'The Role of Insects and Fungi in Agriculture', Empire Cotton Growing Review, xiii, 1936, p. 186.
-- 'Insects and Fungi in Agriculture', Empire Cotton Growing Review, xv, 1938, p. 215.
Timson, S. D. 'Humus and Witchweed', Rhodesia Agricultural Journal, xxxv, 1938, p. 805.
Next: 12. Soil Fertility and National Health
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