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A Disease Of Leaves, Shoots, And Tubers

( Originally Published 1915 )

The Late-Blight disease of potatoes. Symptoms and means of spreading. Diseased tubers and the wintering of the fungus. Treatment of the disease, spraying and the use of resistant varieties.

The Late-Blight of potatoes caused by Phytophthora infestans is so common and wide-spread that it is generally spoken of as the Potato Disease. Historically, it is certainly the oldest of the potato diseases, and it is probably responsible for more loss to potato growers than all the other diseases of the potato put together. The malady first made its appearance in Europe about 1840, and the Irish Potato Famine of 1845 was the result of a very severe attack of Phytoththora. Whilst its ravages are seldom so severe nowadays as they were in that year, the disease is never altogether absent. In most seasons it makes a first appearance about the middle of July, and from that date to the end of the potato season it often difficult to find a field of potatoes wholly free from attack.

Farmers and growers generally, however, do not seem to be seriously concerned so long as, the tubers are not badly diseased. It is an important fact, however, that wherever the disease is present atall there is a risk of a proportion of the tubers becoming deseased further the attack of the fungus on the leaves invartably results in a dimmnished crop.

When a field has been badly attacked by Late-Blight the tubers almost invariably become infected. This either takes place by spores being washed down into the soil and directly infecting tubers near the surface, or by the growth of the fun us down the tissues of the dying stalks into the tubers. Potato tubers infected by this disease can readily be recognised by the purplish discoloration and rather sunken appearance of the skin in diseased parts. These features are caused by the brown colour of the cells immediately under the skin,. Living filaments of the fungus infest the discoloured cells, and if such tubers are stored the fungus slowly invades healthy cells,. often producing the so-called dry rot of the tuber. In addition to this, the presence of the fungus in the potatoes renders them extremely susceptible to, the attacks of secondary rotting fungi and also, of bacteria. In some such cases the potato shows the characters of the winter rot disease caused by Fusarum solani. The tuber gradually shrivels and at a late stage small white tufts of that fungus appear over the sunken parts In other eases the potato becomes changed into a moist, putrid mass infested by bacteria and mites. These diseases are extremely likely to appear in stored tubers that are to any degree infected by Phytophthora. Much, however, can be done to prevent healthy tubers developing these rots by careful attention to the method of storage. It is well known that the favourable conditions for the growth of most fungi are a moist, warm atmosphere and absence of light. If, therefore, the storage clamp is not carefully made, it is possible that very favourable conditions for the growth of fungi will be provided. If the tubers are stored moist, or if they are too closely covered, and if no provision is made for adequate access of air to all parts of the "pie," the temperature will rise and harmful fungi will become rampant. To avoid these dangers then, potatoes should be stored in a dry, well-ventilated shed ; or, failing that, the " pie " or clamp should be in a dry situation and well ventilated. By such pre-cautions the conditions are rendered unfavourable for the growth of rotting organisms ; and even though a few of the tubers are diseased the trouble is then unlikely to spread to the healthy tubers. Too frequently, however, it is found on opening a clamp that the whole store is a putrifying mass, simply owing to neglect of the essential facts of ventilation and dryness.

We have seen that the Phytophthora spreads throughout the summer months with amazing rapidity by air-borne conidia. If, however, these are to produce infection they must germinate immediately, for being only provided with a thin wall they cannot resist drying up or frost.

They, therefore, never serve for carrying over the fungus from one season to, the next.

The question then arises as to whether this fungus, like Pythium, produces resting spores that are able to survive the winter in the soil. The most diligent search of many investigators has failed to, show that such resistant resting spores are ever produced in Nature. On the other hand, Dr. Pethybridge, in Dublin recently, has found it possible to obtain resting spores of this fungus by growing it artificially on a medium consisting of cooked Quaker Oats stiffened with a gelatinous substance called agar.

The fungus forms a dense, white growth on the surface of this, and after some months produces the resting spores within the medium. These are quite characteristic resting spores and possess a thick resistant wall. Interesting as these artificially-produced resting spores are from the scientific point of view, the fact that their presence has never been demonstrated in Nature, renders it necessary to consider other means by which the fungus may survive the winter. It. has been repeatedly shown that diseased tubers kept over winter in the open may give rise to a growth of Phytophthora bearing spores in the warm, moist days of early summer. During the colder months filaments of the fungus in diseased tubers grow very slowly indeed, especially if the tubers are kept dry. If, however, such tubers are kept warm and moist the fungus rapidly extends through the whole tuber and even produces a network of threads bearing powdery conidia on the outside. Such conidia, carried in the air, infect leaves of potato plants in the vicinity, and in this way start an epidemic. It has frequently been noticed that portions of a potato field, near to, old potato pits or refuse heaps, have been the starting points for the disease. This mode of initial infection is, however, scarcely enough to account for the very wide-spread frequency of the disease.

Recent investigations, especially in Ireland and America, have done much to make it clear how most epidemics of "Late-Blight" originate. We have seen that the potato tubers very frequently become infected by the fungus. If badly diseased tubers are used for seed it has been found that they either wholly decay in the ground or occasionally send up a few perfectly healthy shoots. Recently, however, very careful field experiments have shown that if only slightly diseased tubers are planted a much larger proportion of them send up shoots, and under certain weather conditions some of these become diseased. It has also been proved that the disease arises in such young shoots by the growth into them of the fungus from the slightly diseased tuber. Dr. Melhus has confirmed De Bary's earlier observations that such diseased shoots occasionally reach the surface where conidia of the fungus are produced. These conidia are then carried to the leaves of adjoining plants, which very soon show typical disease spots. Upon these, more conidia arise and so starting with a single diseased shoot as a centre the disease spreads over the whole field in a few warm moist days.

In taking measures to prevent the Late-Blight disease, it is necessary, as in other plant diseases caused by fungi, to consider the means by which the fungus spreads during the season, and also the manner in which it is carried over to the following year. It has been shown that this disease spreads very rapidly throughout the summer, by means of air-borne conidia that infect the leaves on which they alight. If the germination of conidia can be prevented then the spread of the disease will be controlled.

This may be accomplished by spraying with a fungicide, that is, a poisonous substance which is harmful to germinating spores. It has been found by experience that one of the most powerful mixtures for preventing infection by filaments from germinating spores of fungi is Bordeaux mixture. This consists of a solution of Blue stone, that is copper sulphate, to which slaked lime or lime water is added. The lime is added to prevent the copper sulphate injuring the leaves of the plants treated, and also to make the mixture form a fine film on the surface of the leaves.

Bordeaux mixture should always be freshly prepared, and the home-made article is much better than any advertised preparations. It is a fairly simple matter to make the mixture, and full directions for its preparation and use are given by the Board of Agriculture in Leaflet 23.

Once the disease is evident in the field it is too late to attempt to control it by spraying, but the first application should be made some time before the first outbreak of disease is expected, i.e., about the end of June or beginning of July in most districts. The mixture should be well stirred before use and both surfaces of the leaves of the plants should he thoroughly sprayed. Three or four sprayings at intervals of a fortnight or ten days should generally be sufficient to prevent any serious epidemic.

The cost of such sprayings is relatively small, working out in normal times to about 25- an acre for three applications (i.e., 2d. per square rod).Apart even from the prevention of Late-Blight spraying has been proved benecial to the plant; for sprayed plants usually retain their leaves at least a fortnight longer than unsprayed. In certain districts of America where spraying is regularly adopted the increased profit is usually from 3 to 5 per acre.

The subject of the resistance of different varieties of potatoes to this disease has received the attention of experimental growers for many years. Darwin himself was for some years interested in the matter, and as a result, various species of wild potatoes growing in South America were experimented with and used for crossing. At the present time, therefore, something can be done to avoid the disastrous effects ,of the Late-Blight disease by growing varieties known to be resistant to this disease. It must be said, however, that though a variety is highly resistant in one locality it may be equally susceptible in another district where the conditions of environment differ. It is also notorious that disease-proof varieties lose their resistance after a few years, either because the plant, in time, deviates from the original type, or because the fungus becomes slightly modified so that it is able to break down the resistance. Needless to say, a variety which is resistant to Late-Blight disease will not necessarily be resistant to the Wart disease and other diseases of the potato.

Since the Late-Blight disease usually affects the crop most seriously towards the end of the season, it follows that early varieties do not suffer from the disease to the same extent as the main-crop and later varieties. By, therefore, selecting varieties proved to be resistant in recent years in the given district, and by choosing early varieties it is possible to avoid this disease to some extent.

It is even more important, however, to endeavour to prevent the first ,outbreak of the disease in a given area. All possible sources of infection such as diseased tubers and stalks should be carefully collected and burned at the end of the season. Since the disease is propagated from year to year in slightly diseased tubers, seed potatoes should only be used from a crop which never showed any sign of the disease even on the leaves.

We have seen that tubers become infected either, by the growth of fungus clown the stalks, or by spores being washed into the soil. If, therefore, the attack occurs late in the season all haulms should be removed and destroyed before the tubers are lifted ; and infection from the latter source may be prevented, to some extent, by seeing that the tubers near the surface are well covered with soil.

This should be done as soon as possible after the haulm has died down, for apart from the risks of the Phytophthora Mora disease, tubers left too long in the ground are more liable to be attacked later by rots in the store.

The tubers should be stored under dry conditions in such a way that air has free access to them, and diseased haulms should never be used for covering the " pie " or clamp.

If potato plants are sprayed three or four times at fortnightly intervals, beginning when they are about six inches high, it should usually be possible to prevent any serious epidemic in a given field or garden. Much can also be done to prevent infection by the immediate destruction of diseased tubers and haulms, and by only using seed potatoes from a perfectly healthy crop. By such precautionary measures, in addition to the use ,of varieties, known by experience to be resistant in the district, it should be possible to reduce the effects of this disease to a minimum.

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