Diseases Of Roots And Tubers
( Originally Published 1915 )
Fingers-and-Toes of Turnips, Cabbages, etc. Habits and life history of the fungus. Remedial treatment. " Liming" of the soil, etc. Similar diseases, e.g., Black Scab or Wart Disease of Potatoes, "Spongy Scab," etc.
The disease commonly known as Fingers-and-Toes, or Club-Root disease, attacks all cruciferous crops, including Cabbages, Cauliflowers, Brussels Sprouts, Kohl Rabi, Broccoli, Turnips and Swedes. It is produced by a fungus known as Plasmodiophora Brassicae, and causes considerable damage both in the gardens and fields in many districts of England ; its ravages are even greater on the Continent, and it is also abundant in America.
The general appearance of the disease is well known.
Plants, which are attacked, show irregular warty swellings on the root, and in bad cases the whole root may simply consist of a mass of these excrescences. The disease can often be recognised in its earliest stages on pulling up young cabbages for transplanting. If such seedlings are planted they prove practically useless, for the disease simply increases at the expense of the roots, any leaves developed being feeble and unable to provide for healthy growth.
Seedling cabbages, which show by the nodular swellings that they are attacked by Club-Root, are often exposed for sale. Of course it is inadvisable to plant such seedlings, for not only is it extremely unlikely that they will produce healthy plants, but when they decay in the ground they will infect the soil. If any plants of the cabbage family are grow in the same soil within two or three years of this infection, they will almost certainly suffer from the disease to an even greater extent.
The disease with which we are dealing should not be confused with the gall-like swellings on the roots of cabbages and turnips, sometimes caused by the larva of a beetle that lives within the cavity of the swollen tissue.
Eelworms, as well as certain bacteria, are also able to, produce small swellings on the roots of many plants, including Crucifers. It is a little unfortunate that frequently any swellings on roots of cruciferous plants are spoken of as Club-Root without reference to the causal organism Since, however, in the vast majority of such cases the slime fungus, Plasmodiophora Brassicae, is the cause, I am restricting the name Club-Root or Fingers-and-Toes to the disease of which it is the cause. Whilst the life history of Pythium and of the other fungi mentioned in the last chapter is typical the Club-Root fungus differs somewhat in life story and mode of nutrition.
When one, of the diseased roots, say of turnip, is cut across and examined-under a microscope it is found that the tissues ,of the foot are altogether abnormal, many of the cells being strangely altered. In the healthy root of the turnip we can distinguish an outer band of softer tissues surrounding a central core containing a certain number of woody elements arranged like the spokes of a wheel. Between the last-named are broad wedges of softer cells which are packed with the reserve food material that is stored in the root of the turnip in the form of sugar. The root increases in thickness by the growth and division of a layer of cells near the outer part of the central core. When attacked by the fungus of which we are speaking, the whole machinery for the growth of the root is, as it were, thrown out of gear ; cells which normally would produce woody tissue simply give rise to giant thin-walled cells, and the result is an excessive production of thin-walled tissues. This also occurs in the position of the tissues which normally serve for the conveyance and storage of the food materials manufactured in the leaves. The reserve supplies are thus tapped by the fungus, with the result that the quantity of sugar stored by the growing root rapidly diminishes. In such instances many of the cells of the root are enormously enlarged and differs as regards their contents From those of a healthy root.
The healthy cells are lined by the colourless, jelly-like, living protoplasm with its nucleus and contain cell-sap rich in sugar. Diseased cells differ from these in several respects. They are generally much larger; protoplasm is present as before, but it looks frothy and in it can he soon granular masses of other slimy substance which is the protoplasm of the fungus slowly absorbing that of its victim. It is a remarkable fact that. the early effect of the parasite upon the cell is to stimulate to enlarge and even to divide, and thus obtain more food material from the adjoining cells, and this is ultimately used by the unbidden guest. Gradually the protoplasm of the fungus increases in size until the whole of the protoplasm of the cell disappears. Then the fungus undergoes certain changes and soon the cell is seen to be fi lled with a large number of tiny round bodies are the spores of the fungus, and as they only measure one fifteen thousandth of an inch in diameter they can only he seen when very highly magnified. A single diseased cell will contain at least 100,000 of these spores, and since the diseased part of a. turnip, for example, contains many thousands of such cells it is, easy to see that in one such root millions of the spores of the fungus are produced.
If a very small piece of a diseased root is broken up in water myriads of the spores are liberated from the cells. After a few hours very remarkable changes can be observed to take place in these. Each tiny spore swells somewhat and then bursts producing a small hole in one side of the colourless membrane. Gradually the living protoplasm within squeezes its way through the aperture thus formed. As soon as it is free it begins to wriggle and move about as a minute speck of living protoplasm, The protoplasm at one end is drawn out into a fine hair, and the lashing of the water by this hair causes the movement of the minute organism. After a time the movements of these specks of protoplasm become more sluggish and soon they only creep about by slow movements, first pushing-one part of the protoplasm forward and dragging the rest after it. While it is unknown how long they can move about and live in the soil, they can certainly do so for many days.
In the field and garden the spores of this fungus are liberated into the soil by the decay of the diseased tissues of the infected roots. If diseased roots are left in the ground for any length of time the decay takes place rapidly and is generally accompanied by an offensive ocour. The spores pass into the soil and there remain until conditions are favourable for their germination. If liberated during the summer or early autumn they probably germinate at once, but if later it is likely that they remain in the spore condition over the winter and germinate in the warmer spring days. The smallest quantity of water is sufficient to allow the minute specks of protoplasm which are liberated, to, swim about. If young cabbages, turnips, etc., are grown in the infected ground the organism soon gains entrance to the younger roots and sets up the disturbances described above. Whenever seeds of these cruciferous plants are sown in soil known to contain the spores of this fungus the swellings typical of the disease appear on the young roots in a few weeks.
We have seen that this organism differs from a fungus like Pythium in several important respects. At no stage in its life history does it possess filaments as do the vast majority of fungi. It passes through the vegetative stage of its life as a naked speck of protoplasm living and growing inside the protoplasm of the cells of a root. Unlike Pythium it does not kill the cells of its host plant outright, but rather stimulates them to enlarge, divide and draw food supplies to them which it then utilises. Like Pythium, however, it rests in the form of spores in the decaying roots and in the soil ; it also passes part of its life as a naked free-swimming speck of protoplasm, but in order to complete its life cycle it must enter the living cells of the root of a cruciferous plant.
In considering methods of preventing the attack of this organism it is necessary, as in all such cases of plant disease, to bear in mind the habits and life history of the parasite. As has already been shown, a single diseased turnip or cabbage, if left to rot will liberate many millions of spores into the soil. Some, of these doubtless die, but many remain as a source of infection for future crops. Most important therefore, of all the means of combatting this disease are the measures taken to prevent the spores of the fungus reaching and infecting the soil. It would seem to be obvious to anyone who has observed the damage this disease can cause, that the greatest care ought to be taken to' collect and destroy by burning all diseased roots. Again and again this disease occurs both in fields and gardens, and in almost every case diseased roots can be seen left lying about, to rot and infect the land. Quite recently the writer watched a farmer carting the least diseased portion of a crop, of swedes, scarcely a root of which had escaped attack. The most badly diseased plants, already putrid and rotten, were being left in heaps to rot on the field, and then doubtless would be ploughed into the soil. In such cases not only does the soil of the field or part of the garden where diseased plants were grown become infected, but the fungus is carried on the boots of workers, on tools, or if, as so frequently happens, diseased roots are thrown on to the rubbish heap to rot, the disease ultimately gets spread over the whole field or garden. Diseased turnips are fed to animals, and though there is no direct evidence, it is quite possible that the spores pass uninjured through the bodies of the animals and return to the soil in farmyard manure. The greatest care ought to be exercised to destroy all diseased material.
Burning is the safest plan or, failing that, it should be gathered into a heap and thoroughly mixed with quick-lime.
The Club-Root disease has been noticed to be particularly abundant on soils which are badly drained, at all sour, or deficient in lime; the disease is practically unknown in chalky or limestone soils. As direct methods of treatment therefore, the drainage of the soil should be improved, and a most drastic system of " liming " adopted.
Wherever the disease has been prevalent it is best to treat the ground immediately with freshly slacked quicklime at the rate of 1/2 to 1 cwt. per square rod, spreading it evenly over the ground and digging it in. Freshly prepared quicklime is much better for this purpose than ground lime, which varies considerably in the amount of active quicklime which it contains. Gas lime, though valuable as an insecticide, is a poor substitute for quicklime, since it contains a much smaller proportion of active lime than either of the other forms.
The use of certain acid artificial manures and, indeed, also of ordinary stable manure in excess, tends to make the soil acid, and this favours the growth of the Fingers-and--Toes fungus. In infected soil, therefore, Basic Slag, or other alkaline artificials, might be advantageously used in addition to treatment with lime.
Needless to say, even after such treatment as outlined above, ground known to be infected should not be planted with cruciferous crops for at least three years. Other vegetables, including potatoes, can of course be grown without any danger, since they are not attacked by the
fungus. The soil of beds in which seedling cabbages are raised should be partially sterilised as for the ' Damping-off " Disease, and also should contain a fair sprinkling of lime.
By strict care in disposing of rubbish, by improving the drainage of the soil, and by regularly dressing the ground with quicklime, it should be possible to do some-thing towards eradicating this pest. A recent visit to some allotments within the Manchester area afforded evidence of this possibility. Some of the plots had not grown a healthy cabbage for two or three years, while adjoining plots never show a sign of the disease. The infected plots were badly drained, the soil was sour, liming had been tried, but in too small quantities. On the other hand the adjoining plots had been thoroughly and regularly limed, were well drained, not soured by over manuring, and they had therefore always borne healthy crops. It is needless to add that on the infected plots diseased cabbages had been pulled up and the roots simply thrown aside to rot and prove a further source of infection. It night be well if allotment societies had some stringent rules for dealing with the spread of such diseases via the rubbish heap.
Brief reference must now be made to some diseases of the potato tuber that present certain features in common with the Fingers-and-Toes disease. The Wart Disease, or Black Scab of Potatoes, is caused by Synchytrium endodobioticum. It is notoriously prevalent in the districts round large towns, and the restrictions of the Board of Agriculture have rendered its symptoms well known. In the early stages small warty swellings appear in the "eyes " or buds of the potato, or they may even occur on the stem near the ground level. These warts rapidly increase in size, and several often run together till the potato becomes a mass of excrescences. Plants which are attacked often grow larger and bear green leaves for a longer time than healthy plants.
The fungus causing the disease, like the Plasmodiophora, lives in the cells of the potato buds as, minute specks of naked protoplasm. In this case the presence of the fungus causes the invaded cells to enlarge, but renders them incapable of further division. On the other hand the healthy cells around are stimulated to such active growth and division that the warts are soon produced.
Infection always occurs by motile spores from the soil, which can only penetrate the healthy cells of a potato in the young condition. Later the potato forms a skin of corky cells through which the fungus cannot penetrate. After growing at the expense of the invaded cells the fungus ultimately occupies the whole of the cell cavity, and then taking on a thick, very resistant wall, forms a resting spore. With the decay of the warty tubers these resting spores find their way into the soil and may remain there as a source of infection for many years. When they germinate after an exceptionally long period of rest the thick wall bursts and liberates large numbers of actively moving spores, each possessed of a single whip of protoplasm to propel it.
These are the spores which infect new potato tubers. So far no method of successfully treating this disease by adding chemicals to the soil has been devised, but certain varieties of potato are much less susceptible to the disease than others, and it is advisable to grow these if the disease is present. Indeed, it is now compulsory for every person growing potatoes to do this and to follow certain other stringent regulations in areas where the Wart is.
In addition to the Wart Disease there are several other scab diseases of potato tubers which, owing to a certain degree of similarity, may at times be confused with the Wart Disease. The Black Speck, or Violet Rhizoctonia Disease, caused by Rhizoctonia Violacea, can be detected by the minute size of the blackish warts which can be rubbed off the surface of the tuber without causing any evident injury. The spongy or powdery scab, caused by Spongospora solani, a fungus akin to the Wart Disease organism, is easily recognised by the light brown colour of the rather powdery warts, or cankers, it produces. Brown Scab, or Ordinary Scab, of the tuber also shows as light brown warts, but usually evenly distributed over the surface of the potato. It may be due to a variety of causes, in which mechanical irritation by gritty particles of soil and infection by definite parasitic fungi probably play an important part. None of these diseases, however, are so destructive as the Wart Disease ; further particulars of them may be obtained from the leaflets issued by the Board of Agriculture.t It is quite certain that these scab-like injuries are caused by different fungi, but it is not surprising that the resulting diseases present certain superficial similarities. In each case the presence of a parasitic fungus irritates the cells of a potato, causing them to actively divide and thus give rise to the warts. Even in the Fingers-and-Toes disease the result is similar, and in all the cases with which we have dealt the attack arises from the presence of the organism in the soil. By, therefore, adopting precautions to prevent the infection of the soil, and by appropriate treatment similar to that advised for the Club-Root disease, such diseases can, in some measure, be controlled.