The Diseases of Crops
For nineteen years, 1905-23, I was engaged in a study of the wheat crop of India, which included work on the creation of new varieties. The records of the work on Indian wheat carried out at Pusa will be found in Wheat in India, published in 1908, and in a series of thirty-four papers issued by the Agricultural Research Institute, Pusa. A list of these papers will be found in The Application of Science to Crop Production, Oxford University Press, 1929, and a summary in Bulletin 171 of the Agricultural Research Institute, Pusa, 1928.
Pusa is situated near the eastern extremity of the area under this crop, where the wheat and rice tracts are intermingled and where there is more rice than wheat. As would be expected, both the soil and atmospheric conditions are distinctly on the damp side for wheat. All three of the common rust fungi -- brown, yellow, and black rust -- were much in evidence. In one respect this was an advantage in plant breeding. It was easy to arrange for abundant infecting material for testing the reaction of the various cultures to these parasites. I did nothing to destroy these rusts; I did everything possible to have them always at hand. The result was that my ideas as to the cause of fungous diseases were constantly being verified. If a variety of wheat is resistant to one or more of these rusts, it makes no difference at all how much infecting material rains upon it or how much diseased stubble is ploughed into the land. Nothing happens even in wet seasons which always favour infection.
In the course of this work some interesting observations on immunity were made. Among the types of wheat in the submontane tracts of North Bihar a number were found which were very seldom or never attacked by rust. They were, to all intents and purposes, immune. Unfortunately they all possessed weak straw and poor yielding power, and were only useful as plant breeding material. Should, in the future, any wheat breeder need such types, they could either be collected at harvest time or selected from the crop raised from bazaar samples of wheat from this tract.
Another wheat which was immune to all three rusts was the primitive species known as einkorn (Triticum monococcum). But this wheat never flowered at Pusa, remaining in the vegetative condition till harvest time. One year some of these dense tufts were allowed to remain in the ground till the rains broke in June. This species was not killed by the intense hot weather of April and May, but as the hot season developed it began to show signs of infection by some parasite. This proved to be black rust -- an interesting example of the destruction of immunity by adverse weather conditions, and a very striking confirmation of Mr. J. E. R. McDonagh's views on the limits of immunity set by extreme climatic conditions (Chapter 11).
The most interesting case of wheat disease I met with in my tours was in an area of low-lying land in the Harnai valley in the mountains of the Western Frontier. Here I found wheat growing in wet soil, in which the aeration was poor and the general soil conditions more suitable for rice than for wheat. It appeared this area was always affected by eelworm, which, however, never spread to the adjoining wheat areas which continued almost without a break for at least 1,000 miles to the east. Through this valley there was a constant stream of all kinds of traffic both ways -- towards Afghanistan to the west and towards the great cities of the plains in the east. Nothing was done to check the infection of the neighbouring wheat areas by preventing the cysts of the eelworm being carried by the feet of animals or men or by wheeled traffic. Infection both ways must have been going on without interruption for hundreds of years. But nothing had happened. Obviously the eelworm is not the cause of the trouble or no power on earth could have stopped the whole of the wheat areas of a sub-continent becoming infected. Before infection is possible the soil conditions must be favourable.
A similar case of eelworm on rice occurred in the deep-water rice areas of Bengal, where the disease is known as ufra. Again we have a heavily infected area in close contact with one of the greatest rice areas of the world. No precautions are taken to isolate the area and protect the surrounding rice from infection. There has been no spread of the trouble outside the small deep-water areas which favour the eelworm.
These two outstanding cases, I think, dispose of the eelworm bogey, which threatens to raise its head in this country in connection with the eelworm diseases of potato and sugar beet. The experts propose measures to control the potato crop so as to prohibit the movement of tubers from and into certain areas. They also recommend that infested areas should give up growing these crops for some years till the eelworm dies out naturally. Before these suggestions are accepted by the authorities consideration might be given to the significance of the two cases -- wheat and rice -- cited above, and also to the elimination of eelworm on farms and gardens in Southern Rhodesia by dressings of freshly prepared compost (see below, Potato).
Intimately bound up with the resistance of the growing wheat plant to disease is the way wheat straw can stand up to the processes of decay when used as thatch. Is there any connection between the life of a thatched roof and the manurial treatment of the land which produced the wheat straw? There is. Farmyard manure results in good thatch, artificials in bad thatch. This will be evident from the following extracts from an article entitled "Artificial Manures Destroy Quality", which appeared in the News-Letter on Compost, No. 4, October 1942, p. 30:
"In the case of the wheat crop raised on Viscount Lymington's estate in Hampshire, careful records have been kept of the life of wheat straw when used for thatching. Wheat straw from fields manured with organic matter, partly of animal origin, lasts ten years as thatch; straw from similar land manured with artificials lasts five years."
Interesting confirmation of this view on the life of wheat straw in thatch has been supplied in a recent letter dated 10th September 1942 from a correspondent (Mr. J. G. D. Hamilton, Jordans, Buckinghamshire), who writes:
"About five years ago, while visiting craftsmen in Wiltshire, I was told by two old thatchers in different parts of the county that the straw they had to work with now was not nearly so good as that which they had had in years gone by. Both gave as the reason the modern use of artificials in place of farmyard manure."
Anyone owning a thatched building, who wishes to compare the virtues of compost with the harm done by chemical manures, can easily make use of the above experiences when the time comes to renew the roof. Alternate strips of the two kinds of straw will soon show interesting differences and will suggest a further trial -- a comparison of the whole wheat bread made from the two samples of wheat.
One of the oldest crops in the world is the vine. Its original home is said to be in Central Asia whence it has spread everywhere. Even when outdoor conditions have made its cultivation impossible, it has been successfully grown under glass often, as in Holland, on a commercial scale. Such an ancient branch of crop production might, therefore, have much to teach us about disease and its prevention.
During some thirty years, from 1910 to 1939, I came in close contact with this crop, which I soon began to regard as one of my ablest teachers. The instruction I received falls naturally into three independent courses which can best be dealt with in order.
From 1910 to 1918, the summers of which were spent in the Quetta valley on the Western Frontier of India, I saw a good deal of grape growing in desert areas, as it had been successfully practiced for many centuries. The tribesmen of Baluchistan select the well-drained slopes of the valleys for their vineyards, where the subsoil is sufficiently well aerated for healthy root development. The vines are grown in deep, narrow trenches, the excavated soil being piled on the undisturbed surface between to form ridges a few feet high, which break the force of the dry, hot winds which often sweep down these valleys. The floors of these trenches are well manured with farmyard manure, irrigated by flow when the vines are planted, after which they are supported by the steep earthen walls of the ditches. As the natural rainfall during the growth period is almost nil and as the trenches are naturally well drained, there is no danger of waterlogging. The amount of irrigation water needed is not excessive, as the trench system checks evaporation. The annual rainfall is mostly received in the form of snow, so that watering does not begin till after the buds break in the spring. These partly buried vineyards are invisible at a distance, as the vines are never allowed to grow above the ground level.
At first sight all the conditions necessary for fungous and insect diseases seemed to have been provided -- a damp atmosphere round the vines and restricted air movement in the trenches. Nevertheless, there was no disease of any kind -- at least I never found even the beginnings of such trouble. On the contrary, both the foliage and the wood exhibited every sign of robust health and well-being. The yield of grapes was heavy, the quality and keeping power excellent. Moreover, the varieties grown had been in cultivation for centuries. Nowhere did I hear of the activities of plant breeders in producing new types: no cases of the introduction of varieties from areas outside Central Asia came to my notice. Another characteristic of this cultivation, the significance of which was not fully appreciated till later, was never to cover the whole of the available area with vineyards. The tribesmen seemed to be content with a modest fraction of their land under grapes, leaving the remainder unused or devoted to crops like wheat. This enabled them to go in for mixed farming and to produce sufficient farmyard manure for their vines and other fruit. I saw no areas like many of the vine-growing regions of Europe, where every square foot of suitable land is devoted to grapes, leaving none to produce muck.
Under this system of cultivation the vine obviously flourished under semi- desert conditions; the crop possessed ample powers of disease resistance; the varieties to all intents and purposes were eternal; the fungicides, insecticides, spraying machines, and artificial manures of the West were unknown.
There was, however, one problem which needed investigation in Baluchistan. The grapes were not reaching the vast market provided by the cities of India, in spite of the fact that a direct broad-gauge railway line extended from the Afghan frontier at Chaman to all parts of the subcontinent This was due primarily to the primitive methods of packing in vogue. There was much waste of space in the railway fruit vans from the miscellaneous nature of the packages used, which were of all shapes, sizes, and weights. This naturally increased the freight rates. I was called upon to solve these problems and, although a Government official' obtained permission to trade in fruit so that l could discover at first hand the obstacles which had to be overcome. Two improvements were made: (l) the design and introduction of suitable crates, each containing twenty- four 2 lb. punnets of grapes, and (2) the unification of the rules of the many separate railway companies which handled the fruit, so that in view of the use of standard crates (by which the traffic could be easily handled and by which the revenue earned by each van could be increased) the empties were returned free of charge. The non-returnable and returnable crates adopted for grapes and tomatoes, are illustrated in Fig. 3.
Fig. 3. Returnable and non-returnable crates for tomatoes
The problem then was to find the cheapest source of wood. This proved to be Norway. The Norwegian timber was cut up into suitable sections or made into punnets at Glasgow, packed, and shipped to Karachi for the final rail journey to Quetta, where the crates were assembled and sold to the dealers. The difficulty was not to sell the crates, but to make them up fast enough to keep an adequate reserve stock during the fruit season.
At the beginning of this work an interesting thing happened. After the crates had been designed and successfully used for my own consignments, the local traders without exception refused to adopt them. They only saw one side of this question: they did not see how much better and further my grapes travelled than theirs and how this increased the demand by bringing in distant places, which had only heard of the grapes of Afghanistan and Baluchistan. But the fruit dealers all over India soon insisted on their consignments being packed exactly as mine were. The demand for the improved crates then went up by leaps and bounds. It is safe to say that had this work been confined to the design of packages only and had it not included actual trading, by which the whole subject could be explored, no reform of the frontier fruit trade would ever have taken place.
But the most difficult obstacle of all was to persuade the Indian railways to unify their rules and to agree to return the empty fruit crates free of charge in return for the increased revenue which resulted from standardization. My proposals every year were duly placed before the Railway Conference Association and were invariably rejected. Then suddenly, to my great astonishment, they were accepted in full.
This experience shows how necessary it is for the innovator in agricultural matters to have complete freedom for working out his ideas and ample time to get them adopted. It shows, also, how important it is for the scientist to keep his attention directed to every practical aspect of the problem before him, to neglect no detail, however humble. Nevertheless, these fruit-packing results would not have been possible, had not the grapes themselves been well grown. The length of the life of the grape after harvest is a short one unless a suitable variety is grown and the details of the actual growing are correct. This principle applies to most fruit and to most produce. Keeping power, like disease resistance, depends on the kind grown and on correct methods of agriculture.
But the most useful lesson in grape growing I learnt in Baluchistan must be mentioned last of all. I realized what a healthy vine should look like at all stages of its growth and how eloquent are the leaves, the buds, and the old wood about the soil conditions needed for ideal root development. How essential this item of my education has been will be evident from what follows.
My next lesson in the cultivation of the vine was in Africa -- in Cape Colony in the spring of 1933 and in Algeria and Morocco in 1936. Generally speaking, all the vineyards I saw were only moderately affected by disease. But nowhere were vines to be seen with quite the same health and vigour as those on the Western Frontier of India. I put this down at the time to a want of balance between the vines and the livestock. Everywhere were large areas under vineyards, but there did not seem to be anything like enough farmyard manure. But a change is now taking place in the Western Province of South Africa. Even in 1939 the vine growers were beginning to take up the Indore Process. One such example on the main road between Somerset West and Stellenbosch was referred to by Nicholson in the South African Farmer's Weekly of 23rd August 1939 in the following words:
"Motorists travelling along this road cannot help noticing how healthy this farmer's vineyards look and how orderly is the whole farm. Early this winter I visited it in time to see the huge stacks of manure -- beautiful, finely rotted bush, which had been helped to reach that state by being placed in the kraal under the animals. Pigs had played their part too. During the wine-pressing season all the skins of the grapes are fed to the pigs and later returned to the vineyards in the form of manure."
Since these words were written South Africa has become compost-minded and I am informed that much more attention is now being paid to livestock as a factor in successful grape growing and to the systematic conversion of all available vegetable and animal wastes into humus.
In Algeria and Morocco every available acre seemed to have been planted in vines, but the supplies of farmyard manure seemed to me to be quite inadequate. The methods of grape growing, the prevention of disease, and the manufacture of wine closely followed those in the south of France, which I was soon to study in some detail.
My last course of instruction in the raising of grapes took place during the summers of 1937, 1938, and 1939 in the Midi, where in the course of many memorable tours in the company of the late Mr. George Clarke, C.I.E., a former colleague in India, I saw many thousands of acres under the vine and learnt a good deal about the way this crop is cultivated in the south of France. What struck me most, besides the shortage of farmyard manure, was the vast sums of money spent on artificial manures to grow the crop and on poison sprays to keep the various fungous diseases at bay. In spite of all this, the crop did not seem at home. The foliage in particular looked wrong. Almost everywhere in the areas given up to vineyards there seemed to be far too little farmyard manure. In one large group of vineyards near the mouth of the Rhône, where tractors had almost entirely replaced the horse and artificials were relied on for growth, I never saw the spraying machine and the poison spray so much in evidence. One interesting result of all this was that the grapes produced in these vineyards could no longer be used to make wine, but were devoted to the production of alcohol for diluting the petrol needed for motor-cars. No one, however, seemed to realize the significance of all this -- the complete failure of artificials to maintain health in the vines and quality in the produce.
A sharp look-out was kept during these tours for vineyards in which the appearance of the foliage and of the old wood should tally in all respects with those of Central Asia, namely, well-grown plants looking thoroughly at home and in which the wood, the foliage, and the young grapes possessed the bloom of health. At last, near the village of Jouques in the Department of Bouches du Rhône, such vines were found. They caught my eye on the left-hand side of the road, as our car slowly descended by a winding roadway from the high ground above to the valley below. We halted and made discreet inquiries. These vines had never received any artificials, only animal manure; the vineyard had a local reputation for the quality of its wine. Arrangements were then made with the proprietress to have the active roots examined. As was expected, they exhibited the mycorrhizal association. The vine proved to be a mycorrhiza former. The perfect nutrition, the high quality, and good keeping power of the grapes, the long life of the variety, and the absence of disease in Central Asia were at once explained. It was equally obvious that the general degeneration of the vineyards of the Midi and the need for poison sprays to keep fungous diseases in check, as well as the necessity for the plant breeder to produce an endless supply of new varieties, could all be traced to failure to realize the vital importance of livestock and of real humus for this ancient crop.
Obviously, at some period in her history, France took the wrong turning in the cultivation of the vine and failed to realize the need of balance between livestock and crops. It is more than likely this change began with the increased demand for wine which followed the Industrial Revolution and the growth of the urban areas. In all probability the Phylloxera epidemic, which overwhelmed the vineyards towards the end of the nineteenth century, was the first of Nature's warnings and the beginning of the writing on the wall. More will come.
Looking at the cultivation of the vine from all possible angles and bearing in mind the lessons of the Orient, there can be little doubt that the faithful adoption of the law of return will speedily put an end to most of the diseases of this crop and, at the same time, establish a new base line for the investigations of the future. In the training of the investigators of to-morrow it seems essential that our future instructors should widen their experience and take into consideration the lessons the Orient has to teach us about the stability of the variety and its resistance to disease once the manuring follows the lead of Nature.
My active interest in the problems of fruit growing and the reaction of the fruit tree to disease began in the West Indies in 1899 and has continued ever since. From 1903 to 1905 a good deal of attention was paid to these matters while on the staff of the South Eastern Agricultural College at Wye. At Pusa I had a large fruit plantation under my charge for nineteen years and spent a good deal of time in the study of the problems underlying fruit production. This included an investigation of the factors concerned in the effect of grass on fruit trees. The work involved the detailed examination of the root systems of a number of different species throughout the year and the way the trees and the soil came into gear. The results of ten years' work were summarized in Chapter IX of An Agricultural Testament. At Quetta on the Western Frontier I was provided with a small experiment station from 1910 to 1918, where fruit was the main interest. On retirement in 1931 I continued my studies of fruit problems in my small garden at Blackheath. My experience of fruit and its diseases has, therefore, extended over a period of forty-five years.
During this period a few very interesting cases both of loss of quality and of active disease have been investigated, the results of which are now set forth in chronological order.
The first of these problems was met with at Pusa in the case of the peach. Quite by chance one of the peach plots happened to be planted on a well-drained, permeable soil, in which the growth was far above the average of the locality. The yield and quality of the peaches were outstanding. It was quite easy to remove the skin of any of these ripe peaches in one piece -- a quality test as good as any. On several occasions towards the end of the crop the weather changed -- the dry, hot, westerly winds, usual during the ripening period, gave place to the damp, easterly winds which always precede the south-west monsoon. With this change in the humidity two things always happened: (1) the peaches lost their quality and became tasteless; (2) they were then attacked by the fruit-fly. Now these fruit-fly attacks never occurred while the air was dry and the fruit retained its taste and quality. No sooner had the damp winds destroyed the flavour than the fruit-fly appeared and its maggots proceeded to devour the crop. Even if it had been possible to keep the fruit-flies in check, nothing would have been gained for the simple reason that when the quality is lost peaches are hardly worth saving.
Another interesting thing happened at Pusa in connection with the peach. The raising of quality crops depended on an ample supply of irrigation water after the fruit had set, because during this period little or no rain was received, the upper soil was dry, and the extensive surface root system of this crop remained dormant unless kept moist by irrigation. With no irrigation the peach managed to survive the hot season and to ripen a small crop, but with this difference -- the peaches were small, hard, and quite devoid of quality. The explanation appears to be this. The peach tree, like the other fruit trees under study at Pusa, has two root systems -- a well-developed, surface system, which comes into action during the growth period provided the surface soil is moist enough; if the peach is irrigated during the hot season, these surface roots begin to function when the buds open in the spring and continue in action during the rains, till the leaves fall; if, however, the trees are not watered, the surface roots remain dormant till the south-west monsoon in June. The function of the deep root system is to maintain the water supply during the hot season, and for this purpose new absorbing roots are produced every hot weather in the deep, moist layers of soil down to twenty feet from the surface. Obviously the two different methods of supplying the peach with water lead to very different results as regards the quality of fruit. These two methods also affect the leaves as well. Under irrigation, large, well formed leaves of the right colour were produced throughout the season: there was no difference between hot weather and rains leaves. But when the trees relied for water on the deep roots only, the hot weather leaves were small and pale green, changing suddenly into large, dark green leaves when the monsoon in June brought the surface roots into action. Unfortunately I did not have these leaf differences recorded in drawings in the case of the peach, but only in the custard apple, where the results were closely similar (Fig. 4).
Fig. 4. Hot weather (below a a) and monsoon foliage (above a a) of the custard apple
These facts suggest a promising direction for the study of quality in fruit. The development of quality depends entirely on surface roots and on the food materials these roots collect. As the peach is a mycorrhiza former and as this relationship occurs only in the surface roots, we have in this species and the other fruit trees cultivated in India, all of which possess two roots systems and all of which are mycorrhiza formers, perfect instruments for breaking new ground in nutrition and in the detailed study of the fungus-root partnership.
Another very good example of a tropical fruit, in which the mycorrhizal association affects the upper of two root systems, superficial and deep, and thus plays an important part in the development of quality and in disease resistance is the guava; this fruit is easily grown and cultivated. This root development is shown in Plate IV. Further details of the investigations made at Pusa on this crop will be found in An Agricultural Testament, Chapter IX.
Plate IV. Guava (Psidium Guyava, L.) No. 1 -- Superficial and deep roots (November 23, 1921). No. 2 -- The influence of soil texture on the formation of the rootlets (March 29, 1921). No. 3 -- The root-system under grass (April 21, 1921). No. 4 -- Superficial rootlet growing to the surface (August 28, 1921). No. 5 -- Formation of new rootlets in fine sand following the fall of the ground water (November 20, 1921). No. 6 -- Reduction in the size of leaves after twenty months under grass (right).
Another of the crops I grew at Pusa was the banana. When manured with farmyard manure, the response to this treatment as regards yield and quality was amazing. So it is when leaf-mould from the forest is used, as I once observed in the Botanical Station at St. Vincent in the West Indies about 1900, when some suckers of various varieties imported from India were tried out. The effect of leaf-mould was to confer on the fruit flavour and quality otherwise unknown. Further, both at Pusa and St. Vincent there was not the slightest trace of disease.
How very different are the plantation results in the West Indies and Central America, where large areas of steep hillsides under forest have been converted into banana fields. As long as the original humus made by the trees lasts, all goes well, but the moment this is exhausted one fungous disease after another makes its appearance and does great mischief. It appears that in these modern plantations little or no provision has been made for livestock and the preparation of large quantities of compost for maintaining the soil in a fertile condition. That this is needed is suggested by the fact that the banana is a mycorrhiza former.
That properly made humus will always be essential in banana cultivation is suggested by the following extract from a letter dated 27th February 1944 from a correspondent in Southern Rhodesia, Mr. A. D. Wilson, Burnside, Bindura, who has been trying out the effect of humus on various fruit trees. As regards the effect of humus on the banana, he writes:
"Bananas. The effect of compost on these has been perhaps the most marked of anything I have done. Bananas are not considered a commercial proposition in Southern Rhodesia and for four years I worked away without using compost. Then I began to apply it -- the change was remarkable. Year by year the plants grew larger, the bunches increased their yield till to-day I can expect bunches that carry 200 large bananas and more, and have a flavour better, so the Chief Horticulturist says, than any imported article."
Another interesting example of the effect of organic manures on the orange has just come from the Mazoe valley in Southern Rhodesia. In a letter dated 9th June 1944 Captain Moubray writes:
"I have been watching an orange grove belonging to one of the large companies. It is about twenty years old and has been fed all its life on little but artificials containing a large percentage of sulphate of ammonia. It is just about finished -- the trees are full of dead wood and the crops it bears are now unprofitable -- the soil is practically dead. Opposed to it is another grove further down the Mazoe valley, which has to a large degree been fed on organic wastes -- it is still healthy and bears good crops. Again another one, which was chemically fed till a few years ago -- the trees were cut off about four feet high and the treatment changed to organics -- the trees are now coming away strong and healthy. I think I shall write a short article about it and call it 'Two Orange Groves'. (This article appeared in the issue of The Fertilizer, Feeding Stuffs and Farm Supplies Journal of 15th September 1944.) From all information I get the same thing has happened with tea."
As already stated, fruit was my principal preoccupation during the nine seasons, 1910-18, which I spent at Quetta in Baluchistan. Many further observations were made, some of considerable interest. The way in which green-fly attacks could be induced or checked at will on the peach and the almond is described in An Agricultural Testament, p. 164. Green-fly was unknown in the area under my charge until over-irrigation produced a heavy attack which was completely checked by restoring the aeration of the soil. This has been one of the neatest examples which has come under my observation of the effect of soil aeration on the health of a crop: the results were so well marked and so definite, two quite distinct foliages being produced, one fly-infected at the base, and one quite normal and free from infection further along the shoots. It was particularly noticeable that the fly did not spread from the infected leaves to the normal. The original purpose of the extra irrigation had been to try to store the precious irrigation water during the winter in the soil itself instead of allowing it to run to waste. Evidently Nature did not agree to this suggestion and showed her refusal in the usual way.
Among my most successful attempts to grow fruit at Quetta must be mentioned outdoor tomato growing; this had also been carried on at Pusa. Each plant was allowed to produce two stems which were tied to an ordinary wire fence of the right height, the tomatoes making a wall of foliage and fruit without any loss of space. The only manure used was cattle manure, but great trouble was taken to raise really strong seedlings for transplanting. Not only were the yield and quality far above the average, but the carrying power of the fruit was amazing. It was possible to send tomatoes from Quetta to the distant Calcutta market during August and September in ordinary railway vans, first through the terrific heat of the Sind desert, followed in the Gangetic plain by the moist, hot conditions of the Indian monsoon. The tomatoes arrived without damage or loss of quality, a fact I attributed to the care expended in their growth. Besides their keeping power and good quality, not the slightest sign of any insect, fungous, or virus disease appeared in these large-scale trials.
With this experience in retrospect, I was naturally intensely interested in a letter I received some years ago from Mr. A. R. Wills of the Tadburn Nursery, Romsey, in Hampshire. Mr. Wills asked my advice about the disposal of a considerable quantity of tomato haulm which had been attacked by the common wilt disease. I advised composting and returning the compost to the same houses for the next crop. This suggestion was somewhat violently opposed by one of the experts connected with the Ministry of Agriculture, who foretold dire results if my unorthodox proposals were accepted. Mr. Wills, however, decided to adopt them. The result was a fine crop, free from disease. Mr. Wills then proceeded to install the Indore Process at Tadburn and in this work was enthusiastically backed up by the foreman in charge. The result is that since those days Tadburn has never looked back and has gone from strength to strength.
Since 1934 in my small garden at Blackheath I have conducted an experiment to ascertain the effect of a fertile soil on the incidence of fruit diseases. When the garden was taken over in 1934 the acid, sandy soil was completely worn out and the fruit trees -- apples, pears, cherries, and plums -- were literally smothered by insect and fungous pests. They were the kind of trees that most people would have consigned to the bonfire. But instead they were carefully preserved and steps were taken to convert all the available wastes of the garden into humus. Some of this was given to the trees and the reaction of the pests to the new manurial treatment noted. Nothing very much happened the first year. The next year infection was noticeably less. The third year most of the pests had disappeared of their own accord, except in one case -- a rather delicate apple tree, badly infested with American blight. During the fourth year this infection disappeared, but the tree is nothing like so robust as the others and again (1944) after a three years' abstinence from annual dressings of compost shows a distinct tendency to welcome a leaf disease -- in this case due to a fungus. It may be that the stock on which this apple is grafted does not suit the sandy soil or that the combination of stock and scion is not a happy one. But, with this interesting exception, all the fruit trees have thrown off their pests and produced fruit of really exceptional size, quality, and keeping power. A small and rather old pear tree, which in 1934 was literally alive with green-fly and plant lice, armies of the latter being observed climbing up the stem, a really disgusting sight, has been restored to health: the tiny, hard, uneatable pears of 1934 have developed into fruit of remarkable size and quality. The twigs and leaves are now healthy and quite free from pests. No fungicides or insecticides were at any period used in this work.
Perhaps the most interesting experiment in this Blackheath garden concerns a common virus disease of strawberries. This arose out of a visit to the strawberry area round Botley, near Southampton, which, as is well known, has fallen upon evil days. The crop is grown by smallholders, but no provision was made for livestock and the production of animal manure. Substitutes, mostly composed of artificials, were used instead. As long as the original stores of humus in the soil lasted, all went well and a prosperous industry developed. Trouble then began. The soils lost their texture and permeability, and the strawberry plants began to be affected by virus and other diseases and then to go on strike. The area under crop dwindled. During the same visit I saw a large, well conducted strawberry farm near Southampton, on which farmyard manure was always applied. The crops were excellent and no soil troubles or pests were to be seen. I secured samples of the roots of these thriving strawberry plants and asked Dr. Rayner to examine them. As I expected, the strawberry is a mycorrhiza former and therefore likely to respond to properly made humus.
At this point I began to wonder what would happen to virus-infected strawberries, if they were grown in compost. Would the affected plants recover? If virus-free and virus-infected plants were grown in compost side by side, would any infection take place? What would be the result of starting a new plantation in heavily composted soil from runners, half of which came from the virus-infected plants and half from healthy plants? Accordingly such a plantation was made. Two samples of Royal Sovereign strawberries were secured -- one from an experiment station, certified to be attacked by virus, the other from the best commercial strawberry farm I knew of in England, where no virus had occurred. The plots were arranged side by side on land well manured with compost. The results were interesting. No infection of the healthy strawberries occurred: the virus- afflicted plants recovered: the new plot from equal numbers of runners from the original plantings was free from any trace of disease and, moreover, has yielded good crops of fine quality. The virus disease of strawberries appears, therefore, to be a mare's nest and to result from methods of farming which are inadmissible. The remedy is to combine livestock with strawberry growing and to convert all the vegetable and animal wastes into humus.
It occurred to me in the course of this work that the Southampton strawberry industry could be assisted or perhaps salvaged outright if use could be made of the large quantities of unused humus in the controlled tips near the city. I visited one of these controlled tips near Bitterne and found, as I expected, that it was a veritable humus mine. All that was needed was to separate, by simple screening, the refractory material and to place the resulting humus at the disposal of the strawberry growers. But all my efforts to get this done failed to overcome the inertia of departmentalism. The municipal authorities concerned with the tips and the county authorities anxious to help the strawberry industry were widely separated and independent bodies. I could not, in the brief time at my disposal, discover the secret by which the various bodies concerned could be brought into fruitful co-operation. In the meantime, the strawberry industry continues to decline. This episode reminded me of the anecdote recounted in Thackeray's Book of Snobs, where the King of Spain was burnt to death because no Director of Etiquette was available to set the machinery of the Court into harmonious and effective action, so that one of the footmen on duty could pour a nearby bucket of water on the unfortunate monarch.
While in Westmorland (1940-3) I saw an excellent example of recovery from virus disease, by means of compost, in raspberries at Levens Hall. Twelve years ago Mr. F. C. King, the head gardener, decided to put to a crucial test the current views on the running out of varieties and to discover whether this is due to improper methods of soil management or to a real breakdown in constitution. For this purpose he started in fertile soil a new raspberry plot from the most virus-infected stock of Lloyd George he could find. The plants soon made a complete recovery from virus. I saw them in 1943 and found them free from disease and still producing heavy crops of fine fruit, quite up to exhibition standard. This was one of the best examples of the retreat of virus before soil fertility I have so far seen.
In all these adventures in fruit growing I never had occasion to use a spraying machine for destroying a parasite, or any fungicides, insecticides, or germicides. Disease resistance was left to the plant. The only damage from parasites that could be regarded as at all serious were the attacks of peach fly at Pusa towards the end of the crop in those seasons when the moist currents which heralded the south-west monsoon caught the crop and destroyed its quality and made it attractive to the pest. Against accidents of this kind there can be no remedy -- they must be accepted as inevitable. This long experience of the power conferred on the fruit tree by proper methods of manuring and soil management has helped to confirm my earlier ideas that bad farming and gardening are at the root of disease and that the appearance of a pest should be regarded as a warning from Mother Earth to put our house in order.
There is a further point to consider. If fruit trees need to be drenched with poison sprays before they can produce a crop, what is the effect of such fruit on the health and well-being of the people who have to consume it? We know these practices kill the bees and also the earthworms.
One of the crops under study at Pusa between the years 1905 and 1923 was tobacco grown for leaf and also for seed. Only one disease, which resulted in malformed dwarf plants, was met with during these nineteen years. This trouble has since been proved to be due to virus. Such affected plants were quite common in the various cultures for the first two years, then they became fewer and by 1910 had disappeared altogether. Similar diseased plants occurred in the neighbourhood in the fields of the cultivators from whom a portion of the labour force was obtained. At no period were any steps taken to control this disease or to regulate the movements of the labourers. Nevertheless, no infection was spread or was carried once correct methods of growing tobacco were adopted. These consisted in raising the seed on humus-filled soil, careful attention to the surface drainage, and organic manuring of the nurseries, the production of well-grown material for transplanting, and the growth of the leaf tobacco on soil fertilized by various organic manures including farmyard manure. At no period in these nineteen years was the soil of the tobacco nurseries sterilized nor were artificials or spraying machines used. My tobacco cultures, which always earned the respect of all who saw them, were examples of organic farming pure and simple. Once the details of tobacco growing were mastered there was no disease of any kind: the plants protected themselves against every form of parasite as well as virus.
Captain Moubray informs me that similar results are now being obtained in Southern Rhodesia, where tobacco is an important commercial crop. The replacement of artificials by freshly prepared compost in the nurseries and in the tobacco fields was at once followed by a very marked diminution of virus trouble.
That the other tobacco diseases which of late years have begun to trouble the farmers in Rhodesia are due to an impoverished soil is suggested by the appearance of eelworm in this crop. This disease and its prevention are referred to in the Rhodesia Herald of 4th September 1942 as follows:
"At Darwendale, Mr. O. C. Rawson has applied five tons of compost per acre to infested tobacco land. In the first year there was a reduction of eelworm, and in the second year, without a further application, the eelworm disappeared. Other tobacco farmers began to report similar experiences. The compost, of course, was applied for its fertilizing value and the consequences on the eelworm population were a surprise."
Tobacco has not proved to be an exception to the long list of crops which are mycorrhiza formers. Samples of the surface roots of Rhodesia tobacco, taken from plants grown by means of freshly prepared humus, exhibit, as was expected, this very significant symbiosis. It is more than probable that quality in this crop will be found to depend, among other factors, on the efficiency of the mycorrhizal association. If this proves to be the case, the restoration of high quality in the cured product in places like Cuba will not be a very difficult matter once properly made humus replaces artificial manures.
The leguminous crop as a rule is very sensitive to soil conditions and in particular to poor soil aeration and its consequences. In the course of the current work at Pusa and Indore some interesting cases of the relation between soil conditions and disease occurred in these crops.
Perhaps the most interesting was one which was repeated year after year at Pusa in the case of a vetch -- Lathyrus sativus, L. -- known as khesari. The various unit species of this crop, collected from all parts of India, were grown in pure culture in small oblong plots about fifteen feet by six feet. Infection by green-fly occurred every year on a number of these cultures, but the trouble never spread to the remainder. The plots could be divided as regards infection into three classes: plots immune to green-fly; plots lightly affected; plots heavily attacked. Careful note of this infection was made and the cultures were repeated year after year. The same results were invariably obtained. On looking up the history of these cultures, it was found that the immune types came from the Indo- Gangetic alluvium, the heavily infected unit species from the black cotton soils of Peninsular India, the moderately infected types from the region near the Jumna, where the transition soils between the black cotton soil area and the alluvial tracts occur. The root system of these three sets of types was then explored. It was found that the immune cultures had superficial roots; those heavily infected had very deep roots; the slightly infected types had root systems intermediate between the two. These observations suggest that defective soil aeration, particularly affecting the deep-rooted varieties, was at the root of this green-fly infection, a view which has frequently been confirmed since these observations on khesari were made.
Another interesting case of disease in a leguminous crop occurred at Indore in a small field of gram (Cicer arietinum) about two-thirds of which was flooded one day in July due to the temporary stoppage of one of the drainage canals which took storm water from an adjacent area through the estate. A map of the flooded area was made at the time. In October, about a month after sowing, the 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 fifty 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.
Perhaps the most interesting case of the relation between soil conditions and disease which I observed occurred at Indore in the case of a field of san hemp (Crotalaria juncea, L.) intended for green-manuring; this, however, was not ploughed in, but was kept for seed as the growth seemed so promising. But 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 I had to copy the methods of the cultivators who always manure this crop with farmyard manure when seed is required. Instead of farmyard manure, I used compost the next year. No infection with mildew took place and an excellent crop of seed was obtained.
It is more than probable that this observation applies to leguminous crops generally. Whenever they are grown for seed, the best results are likely to be obtained with compost or farmyard manure. In olden days it used to be the custom to muck leguminous crops like clover, but the practice was given up after the role of the root nodule in fixing atmospheric nitrogen was discovered. But the root nodule is only a device to save these crops from nitrogen starvation. Nodules by themselves are not sufficient for the rapid growth and maturation involved in producing a full crop of seed.
Confirmation of the view that humus is needed by the leguminous plant if heavy crops of seed are to be obtained is coming to hand. In this country, in the case of clover, Mr. R. G. Hawkins, Lightwaters, Panfield, Braintree, Essex, in a letter dated 30th May 1942, reported:
"For some years now I have inspected crops of Essex red clover and I have noted that the yield of seed is invariably higher on those farms which keep stock, so that the land receives a periodic dressing of dung. The difference is most pronounced in those years when clover seed is generally a poor crop." (News-Letter on Compost, No. 6, 1943, p. 56.)
In Southern Rhodesia Captain Moubray in a letter dated 1st June 1942 commented on the outstanding yields of san hemp seed he had obtained on composted land in a bad season. He obtained no less than three times the average yield of his neighbourhood (News-Letter on Compost, No. 4, 1942, p. 37). In a recent letter to the South African Farmer's Weekly of 7th June 1944 he writes:
"I remember, years ago, Sir Albert Howard telling me that the virtue of properly made compost lay not only in its contribution of humus, but also in its work as an inoculant. He suggested that its application in comparatively small quantities, before planting a legume, would considerably increase the seed yield of such a crop. I have found this to be so."
Here is a subject which urgently needs detailed study. We know that the large group of leguminous plants are mycorrhiza formers. It may well be that the efficiency of this association is one of the chief factors in seed formation. But whatever the explanation may be, it is clear that our fathers and grandfathers were right when they mucked the leguminous crop and that the agricultural colleges are wrong in telling the farmers that the root nodules will look after the nitrogenous manuring of these crops.
My study of the potato crop only began at Quetta during the war of 1914-18 in connection with the drying of vegetables for the troops on active service. At first a supply of potatoes was purchased from the neighbouring tribesmen, but these proved unsuitable as the slices turned black in the drying process. This appeared to me to be due to the excessive quantities of irrigation water used and to the subsequent caking of the soil round the tubers. An area of potatoes was then grown at the Quetta Experiment Station, taking care to use the minimum amount of water applied to the roots only, leaving the earth of the ridges where the tubers were formed quite dry. The result was that no more blackening of the slices occurred. Soil aeration is obviously a factor in successful potato growing.
My second contact with this crop occurred in the Holland Division of Lincolnshire, where for some three years (1935-8) I was provided with ample facilities for study by the late Mr. George Caudwell on his farms near Spalding in connection with an investigation on green-manuring. On these farms the supply of farmyard manure was quite insufficient for the large area -- some 1,500 acres -- under potatoes. Heavy dressings of a complete artificial were then the rule.
Two common potato diseases were observed and studied in South Lincolnshire -- blight and eelworm. In damp, close weather potato blight always occurred and had to be kept at bay by repeated dustings with finely divided copper salts. This disease was much more prevalent on the popular King Edward variety than on Majestic. I was asked why this was so. It appeared to me that the answer would be found if the root systems of these two varieties were compared. King Edward has a much deeper root system than Majestic and would, therefore, the more readily suffer from poor soil aeration, particularly during a spell of damp, close weather which would make the surface soil run together into a crust. Not only was the root system of Majestic markedly superficial, but the roots showed well defined aerotropism and invariably left the soil and grew on the surface under the fallen potato leaves.
I then went into the history of the celebrated potato area south of the Wash and found that some sixty years ago it was under grass. When first ploughed up for potatoes, the land was so rich in humus that crops sometimes as high as twenty-five tons to the acre were obtained. At first potato blight was unknown. But as the humus in the soil became worn out, dressings of superphosphate were first needed to keep up the yield, then the potato blight made its appearance, followed by the spraying machine, the poison spray, and the use of artificial manures, the annual applications of which gradually increased till they have reached fifteen hundredweight to the acre or even more.
These facts suggest that the real cause of potato disease is not, as is supposed, the potato blight assisted by hot and damp still air, but wornout soil. This view could easily be tested by bringing up, by means of compost, one or two farms in South Lincolnshire to a fertile condition, comparable with what they were some sixty years ago when the pastures were first brought under potatoes. Would the potato on such fields be attacked by blight even if it had no assistance from poison sprays? Judging from what happens in our best walled gardens, in which good old fashioned muck is the rule and in which artificials are never used, I think the answer would be in the negative.
That potato blight is of no consequence if ample farmyard manure is used to raise the crop and the plants are not grown too close together is proved by the experience of Mr. John Tarves at Heversham in South Westmorland. In 1943 I visited this garden and was shown a large potato plot on well- drained land facing south and protected from wind on the east and west, which was kept in good condition by farmyard manure and on which potatoes had been grown continuously for forty-five years. The rainfall at Heversham is very high and well distributed, the amount of sunshine is much below that of South Lincolnshire, and at first sight one would expect that here ideal conditions for potato blight had been provided. Nevertheless, on this garden this disease had caused no trouble and preventive spraying was unknown.
I spent some time in the Spalding area in the study of the eelworm disease of potatoes. This is caused by the invasion of the roots by a species of eelworm which dwarfs the plant and prevents the formation of even a small crop. Eelworm is a comparatively recent disease in this area and, as a rule, first appears on the high, light land. In such cases a remarkable change in the flora and in the soil structure precedes the outbreak. The weeds are those of semi-waterlogged and badly aerated soils and include the mare's tail, a species of Equisetum, known locally as toad-pike. The soils have lost their texture, the compound particles their cement, and the blue and red markings characteristic of heavy, clay subsoils have made their appearance. This condition is the result of continuous dressings of stimulating manures which lead to the destruction of the humic cement needed to maintain the compound soil particles. The appearance of eelworm in these potato soils is the writing on the wall and marks the complete failure of the present manurial practice -- the replacement of farmyard manure by artificial manures. No further potato crops are possible till the filth and fertility of the soils have been re-created.
As is usual in such cases, the experts were busy at the wrong end of the problem. The life history and activities of the eelworm were being studied and all kinds of methods except the right one were being tried to destroy the parasite and to stimulate the crop to ward off the disease. The result has been a complete failure. No one has seemed to grasp the fact that eelworm is one of the frequent consequences of poor soil aeration and that the cause of the trouble must be sought in a critical examination of farming practice. This pest is one of the results of upsetting the balance between arable and livestock and trying to find, by means of chemistry, a substitute for good old-fashioned muck. In all these eelworm outbreaks the soil's capital has been transferred to the profit and loss account. What will the reverse process cost before these lands are fully restored? Is the process a reversible one? If so, who is to meet the cost?
Confirmation of the views set out above that eelworm in potatoes is due to an impoverished soil comes from Southern Rhodesia. The first results are summed up in the Rhodesia Herald of 4th September 1942 as follows:
"Some years ago Mr. S. D. Timson, Assistant Agriculturist, noticed a garden where the vegetables were strong and healthy and the flowers bright and vigorous. He was surprised to learn that three years earlier cultivation had been almost abandoned because of the heavy infestation of eelworm. The excellent conditions he saw followed a good dressing of compost.
"He immediately began to observe the results of compost in regard to eelworm, to make practical tests, and induce farmers to experiment. Once the inquiry was begun evidence began to pour in."
That compost will prevent eelworm attacks on potatoes and other vegetables has again been demonstrated on a large scale at Salisbury in Southern Rhodesia by Mr. E. C. Holmes who, in the issue of the South African Farmer's Weekly of 14th June 1944, writes:
"Since I started using compost I have eradicated eelworm from my gardens, and I have no less than sixteen vegetable gardens spread all over my farm of 2,333 acres."
This eelworm story is being continued in Rhodesia. In the Rhodesia Herald of 7th July 1944 the following article appeared:
Satisfactory Expansion in the Making of Compost
Tobacco Growers Report Excellent Progress from its Use
"The expansion in the making and use of compost continued during 1943, states Mr. S. D. Timson, Government Agriculturist, in the course of his annual report. Tobacco growers, he states, gave compost much increased attention, and they continue to report excellent results from its use, and in particular that it gives better quality and greater freedom from disease. It also allows the rate of application of fertilizers to be much reduced without reduction of yield. Its use, as was to be expected, was not usually beneficial on virgin soil.
"Further reports were received from farmers that applications of compost to soil infested with eelworm resulted not only in good yields of tobacco and vegetables, despite the infestation, but also in the disappearance of the pest from the soil the year after the compost was applied.
"A striking example was in the vegetable garden of the Witchweed Demonstration Farm, where an extremely severe infestation was completely cleared up following an application of compost. On the same farm further evidence was recorded supporting Mr. Timson's previous reports of the beneficial effects of compost in controlling witchweed.
"In 1940 there were 674 farmers making compost; in 1943 the number had increased to 1,217. In the same years the amounts of compost made were 148,959 and 328,591 cubic yards (2 cubic yards = 1 ton).
"The largest producers had made from 4,000 to 9,700 cubic yards a year. The largest producers of fat cattle were now making compost instead of collecting kraal manure. They reported they were well satisfied with the change particularly in respect of the elimination of weed seeds and the reduction of the fly nuisance."
There is another potato trouble in South Lincolnshire which is not caused by insects or fungi, namely, the loss of the power of reproduction. After two or three years the potatoes of one crop cannot be used to raise the next. The yield then becomes unremunerative and fresh seed has to be imported at great expense from outside areas like Scotland, Northern Ireland, or North Wales. As this loss of reproductive power develops, the cause is considered to be due to virus. Again the research workers are starting at the wrong end and are trying to find varieties immune to virus. The results so far obtained, as far as practice is concerned, are not impressive. Indeed, it would seem that this trouble is getting worse, as the efficiency of Scotch seed is said to be falling off. If this should continue, the Lincolnshire potato industry will find itself in difficulties. The fresh start every two or three years will no longer be possible unless some alternative supply of new seed can be found.
That these frequent changes of seed of any particular variety and indeed of the production of new varieties of the potato by plant breeding methods are both unnecessary, provided proper attention is paid to the maintenance of soil fertility by organic manuring, is proved by the experience of the islanders of Tristan da Cunha, that lonely settlement in the South Atlantic rarely visited by ships. Here changes of seed are out of the question on account of the inaccessibility of the island. In a letter, dated 15th March 1945, Major Irving B. Gane, the Secretary of the Tristan da Cunha Fund writes:
"As you rightly surmise, the islanders use seaweed. A belt of thick kelp extends round the island some 400-500 yards from the shore, and rough seas wash large quantities on to the beaches. This is collected by the islanders and used for their potato patches.
"I am satisfied that the islanders have no means of changing the variety of the potatoes grown, and it would be safe to assume that the seed has been retained from year to year during the hundred years or so of the island's occupation.
"I, and my father before me, organized the despatch of stores to the island, and although we have sometimes included supplies of vegetable seeds, we have certainly never sent out any seed potatoes."
The situation in Great Britain, though alarming, is not really serious. All that is necessary in areas like South Lincolnshire is to revise the current method of potato growing by a drastic reduction in the area under potatoes, so that the head of livestock -- cattle and pigs in particular -- can be increased, and large areas put under temporary leys and cereals. In this way the raw materials for systematic compost making will be available on the spot. As these reforms proceed, the amount of artificial manures can be reduced. When the stage is reached when artificials and poison sprays are no longer necessary, the restoration of these wonderful soils will have been achieved. After this the experience of the past can be made use of to test current practices. If these soils begin to respond to artificials, attention should be paid to the humus supply. If potato blight appears, the aeration of the soil needs attention.
In the course of these potato studies a number of root samples were examined for the mycorrhizal association. All the results were negative. I understand from Dr. Rayner that the ordinary cultivated crop does not show this relationship, but that it has been observed on potatoes in the hilly regions of France near the Spanish border. Has the potato in the course of years lost something, or was its original introduction imperfect? Do the wild forms of this crop in its mountain home in South America show the mycorrhizal association, or does this crop manage to absorb, by means of its very extensive root system, the digestion products of the proteins during the early stages in the mineralization of the bodies of the soil organisms? In due course answers to these questions will no doubt be provided. They are likely to have an important bearing on disease resistance in this crop and also on the power of the plant to reproduce itself.
Some Parasitic Flowering Plants
A few cases of disease in which the active agents are fiowering parasites must be recounted.
The first of these occurred on four meadows on the farm near Bishop's Castle in Shropshire where I was born and where I spent my early boyhood. The parasite was the well-known yellow rattle (Rhinanthus Crista-Galli), which invariably attacked the grasses and considerably reduced the hay crop. I noticed at the time that a pasture alongside, on which cattle and sheep grazed, never had any of this parasite, but my studies at this period did not embrace this common example of a semi-parasitic flowering plant and its haustoria, which fasten on the roots of the grass. Some fifty years later, however, I discovered that some of the live wires in the farming community have found how to eradicate this pest. They turn the affected meadows into pastures for a couple of years, when the urine and dung of the cattle strengthen the grasses to such an extent that yellow rattle disappears altogether. As the grasses are mycorrhiza formers, we have here a most interesting problem awaiting investigation. Does the humus formed in the soil of pastures in the spring and early summer by the sheet-composting of the vegetable and animal wastes confer on the grasses, by virtue of this association, the power to resist the parasite? If so, is the increased resistance to disease nothing more than the efficient synthesis of protein, due to the passage into the leaves of the grasses of the digestion products of the protein of the mycelium of the mycorrhizal fungus? If, as seems likely, the answers to these two questions are in the affirmative, a great stride forward will have been made in establishing a scientific explanation of the relation between soil fertility and health.
During my Indian service I again came in contact with one of these flowering parasites of the grass family. This time a species of Striga was observed on the roots of the sugar-cane. The cultivators in India invariably got rid of this pest by manuring the affected crops with farmyard manure, after which the parasite disappears. Is the mycorrhizal association, which is known to occur in sugar-cane, involved in this matter? It would seem so.
After my retirement in 1931, in the course of the humus campaign in Southern Rhodesia I heard of the witch-weed (Striga lutea), one of the pests of maize (another mycorrhiza former) and its control by humus. This interesting discovery was made by Timson, whose results were published in the Rhodesia Agricultural Journal of October 1938. Humus made from the soiled bedding of a cattle kraal, applied at the rate of ten tons to the acre to land severely infested with witch-weed, was followed by an excellent crop of maize practically free from the parasite. The control plot alongside was a red carpet of this pest. A second crop of maize was then grown on the same land. Again it was free from witchweed. This parasite will therefore prove a valuable soil analyst for indicating whether the maize soils of Rhodesia are fertile or not. If witchweed appears, the land needs humus: if it is absent, the soil contains sufficient organic matter. Witch-weed will then be regarded not as a pest to be destroyed, but as a most useful soil assessor and land valuer -- as the friend, not the enemy, of the farmer.
Next: 9. Disease and Health in Livestock
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