Powdery scab

Powdery scab - a potato disease caused by a mucous membrane (pseudo-fungus) plasmodiomycete Spongospora subterranea (Wallr.) Lagerh . It does not have a developed mycelium, in a vegetative state it is a lump of protoplasm without a shell in the form of an amoeboid that can move. The structure of the fungus determines the form of existence - intracellular parasitization in the cells of higher plants . Amoeboids can merge with each other and form plasmodium . When adverse conditions occur, the amoeboid is covered with a membrane and turns into a resting cyst . In conditions of high humidity, the shell mucilages, and the amoeboid receives the ability to move. This feature determines the confinement of the development of powdery scab only to conditions of high humidity. Moving in waterlogged soil, amoeboids or plasmodia reach the roots, potato tubers, penetrate the cells and begin to parasitize.

Content

Maliciousness

Powdered scab is a widespread tuber disease that can cause very significant damage, including a decrease in the commercial quality of tubers and their shelf life during storage, because pathogens of various rot - fungi and bacteria - penetrate ulcers caused by scab. In addition, the causative agent of powdery scab is a carrier of the dangerous mop-top virus.

Powdery scab affects all the underground organs of plants, but unlike ordinary scab, it infects roots, which leads to premature wilting and death of plants. Therefore, it can cause a significant decrease in yield.

The disease develops more on waterlogged and heavy soils in granulometric composition in cool weather.

Symptoms and life cycle of the pathogen

The causative agent of powdery scab infects the roots, stolons and tubers of potatoes through lentils, eyes and wound sites. On the roots of plants galls appear - growths of irregular shape of white color, which subsequently turn brown, and on the surface of tubers light pustules in the form of warts form. When the tubers ripen, the spots round off, become embossed, dry out, their peel crack, and ulcers form, filled with a brown dusty mass consisting of fungal spores and destroyed tuber tissues. The remains of the tuber peel are stored for a long time along the edges of the pustules and give them a stellate shape. On storage of tubers affected by powdery scab, dry rot and late blight often develop during storage.

The source of infection is the remains of plants (roots, stolons) and tubers, in which resting spores of the pathogen remain. Spores are stable in the environment and can remain viable for up to 3-4 years. When spores germinate in a humid environment, zoospores are formed first, and then amoeboids. They penetrate into the cells of the tuber, root, and stolons mainly through lentils and develop into a multinucleated plasmodium. Plasmodium soon breaks up into small lumps, which are covered with a dense shell and turn into resting spores, glued together in a spherical dark glomeruli of irregular shape.

Control Measures

Compliance with crop rotation. In infected areas, potato cultivation is excluded for at least 3-4 years, and with a high degree of soil infection by the pathogen, up to 7 years.
Use healthy planting material.
Preplanting treatment of seed tubers with Maxim fungicide (400 g / t).
It is advisable to harvest in dry weather conditions.

Literature

Peresypkin V.F. Agricultural phytopathology. M .: Agropromizdat, 1989 .-- 480 p. ISBN 5-10-000292-1