Soil sterilization would, of course, destroy the sporangia, but no practical method of treatment in the field is known.
Fortunately, varieties of potato are known which are immune, i.e., completely resistant to wart disease. As a result of extensive trials in England, over two hundred varieties have been shown to be immune and rather more susceptible. Among the resistant English varieties are Early:—Di Vernon, Snowdrop, Immune Ashleaf, Boston Comrade; Second Early:—Arran Comrade, Great Scott, Ally; Late:—Tinwald Perfection, Rhoderick Dhu, Majestic. The disease is a notifiable one in Great Britain and is the subject of an order (Wart Disease of Potatoes Order, 1923) by the Ministry of Agriculture and Fisheries.
This disease, also known as Corky Scab, is due to a minute parasite known as Spongospora subterranea. Generally, the disease appears as small, rough scabs on the skin and doing little damage, but in severe cases the flesh of the tuber may be seriously damaged. The scabs contain masses of spores arranged in a peculiar manner and known as spore balls; these escape into the soil and are the source of fresh infections. Contaminated soil can retain its infecting power for a number of years. Treating the infected tubers with a weak solution of f ormalin and treating the soil with flowers of sulphur have been found useful ; liming the soil only increases the disease.
This disease, often known as common scab of potato, is an entirely different disease from Powdery Scab. It is caused by various species of Actinomyces. The disease is par ticularly troublesome on sandy and gravelly soils and on alkaline soils. Humus is important in preventing serious attacks of scab and the addition of grass mowings has been shown in England to have a striking effect.
This disease attacks the tubers during storage. It is due to a fungus, Fusarium caeruleum. The fungus can penetrate through an unbroken skin, but the disease usually starts from a wound, so that a badly handled crop is particularly liable to suffer in store.
This disease is not confined to the potato, but attacks a number of other plants. It is due to a fungus belonging to the group Ascomycetes (see FUNGI) and is known as Sclerotinia Sclerotiorum. It is most troublesome under damp conditions, and in Ireland, next to blight, it is the most serious disease of the potato. The disease usually appears as patches of white fungus threads on the stem, often near the ground. Under damp conditions these threads develop rapidly and white cushions are produced, which later turn hard and black; these are the sclerotia. They are a resting stage of the fungus and remain dormant in the soil until the next spring. The stem generally is invaded and sclerotia are formed in the hollow of the stem, as well as externally. In the early summer these sclerotia sprout and produce small stalked cups which, when ripe, shoot out spores into the air. These spores, scattered by the wind, can infect the older leaves of the plant ; infection, however, ap pears also to be by mycelium present in the soil. If diseased plants are collected and burnt, the spread of the disease is prevented and the sclerotia do not get into the soil. Soil which has become badly infected may be sterilized, but this is only possible in greenhouse work or on a small scale ; otherwise a susceptible crop should not be grown on such a soil for at least three years. In Ireland late planting has been found to be helpful in keeping the disease in check. Another disease of definite parasite nature may be mentioned here, viz., Black-leg of potatoes, which is due to a bacterium, Bacillus phytophthorans, and characterized by a rotting and blackening of the base of the stem. The chief pre
ventive is the use of healthy seed.
For the nature of these dis eases see article PLANTS : Pathology of. They are infectious diseases in which no actual parasite has yet been observed, the usually accepted explanation being that the parasite is so small (ultramicroscopic) as to escape observation under the microscope. In some virus diseases infection can be brought about by the simple transference, as by a needle prick, of the sap of a diseased plant to a healthy one; in many other cases infection is not so simple and can only be secured by grafting infected tissue on to healthy tissue or by an appropriate insect biting first the dis eased and then the healthy plant. The two chief virus diseases of potatoes are Mosaic and Leaf-roll. Mosaic has probably been troublesome for many years, but it has only recently been rec ognized as a distinct disease. It is found all over Europe and in North America. The usual symptom is the mottling of the foliage ; the leaves, instead of being uniformly green, show areas of different colour (different shades of green or slight yellowness), i.e., a mosaic ; the leaves may also show some crinkling. In cases of severe attack the plants may be dwarfed, and the yield markedly reduced. Under hot and dry conditions the mosaic
is not so marked and may disappear altogether. The disease is carried from season to season in the tuber. Infection from plant to plant takes place by means of aphides (green fly), feeding first on infected plants and then on healthy ones. There is considerable difference among varieties insusceptible to attack. In controlling this disease, the healthiness of the "seed" tubers is of the first importance, as infected plants not only give re duced yields but also, through aphides, are the cause of infection of healthy plants. Early weeding out (roguing) of diseased plants may be helpful, but is usually not economically practicable.
This disease is very common in Britain and in potato-growing countries generally. It is characterized by the rolling up over the upper surface of the edges of the leaflets, those of the lower leaves generally showing this symptom first; later the rest of the foliage may or may not show the "rolled" effect. Another symptom is that the leaves are thicker and drier than is normal. The rolled leaves are found to be much fuller of starch than the normal leaves; this is, no doubt, related to the fact that the channels in the stem (i.e., the phloem), which normally carry away the elaborated products from the leaves become disorganized sooner or later. In severe attacks of the disease the plant may be badly stunted. As in the case of the mosaic and the other virus diseases described above, the disease can be transmitted by aphides (green fly) and possibly by other insects as well.
Insect transmission is the common method of infection in nature, but infection can also be produced by grafting together of healthy and diseased tubers or other portions of the plant. The control methods are similar to those to be employed against mosaic. The "seed" should be free from the disease and no tubers should be saved from affected plants. Roguing as soon as diseased plants can be distinguished and during the growth of the crop can be employed with advantage where the plants are grown especially for seed.
See W. W. Robbins, Botany of Crop Plants (Philadelphia, 1924) ; A. W. Gilbert, The Potato (1917) ; R. N. Salaman, Potato Varieties (1926) ; "Cultivation and Diseases of Potato," Sectional Booklet 3o, Ministry of Agriculture and Fisheries (1928). (V. H. B.)