potato powdery scab

Spongospora subterranea (Wallr)Lagerh.

[Host crops] Potato, Solanum nigrum and Nicotiana tabacum.

[Disease diagnosis] mainly infects underground tubers and roots, and aboveground stems can also be infected. Tubers infected, the initial production of pinhead brown spots on the epidermis, the periphery has a translucent halo, gradually expanded, raised, forming a diameter of 3~5 mm rash "blister spot." The epidermis of the early disease spot is not broken, which is the stage of "closed blister"; later the disease expands, a cork ring is formed at the junction of the disease and healthy tissues, and then the epidermis breaks and rolls back, the subcutaneous tissue turns orange, and a large number of dark brown powder is scattered. The blister sinks to expose the cavity, showing a crater-shaped powdery scab, which is the stage of "open blister". The potato pieces that have formed powdery scab can be dried and shriveled due to epidermal rupture and water loss during storage, or infected by other pathogens. Roots damaged, bean-sized, solitary or aggregated nodules growing on one side of roots, young plants wilting and dying in severe cases. The aboveground stems are infected, and the disease spots are characterized by damaged tubers. The pathogen was potato powdery scab. The brown powder scattered from the infected spots is the resting sporangia of the pathogen. Sporangia vary in size and consist of many subspherical yellow-brown resting sporangia. They are hollow and cavernous in appearance. Orifice structures can be seen under the microscope. A single resting sporangium is spherical to polygonal, thin-walled, smooth, and germinates to produce zoospores. Zoospores are subspherical, without cell walls, terminate in two flagella of unequal length, then rest to form morphosome, invading host root hairs.

The pathogen overwinters in seed tubers as resting sporangia or leaves in soil with diseased bodies, becoming the initial invasion source of the following year. The long-distance transmission of disease depends on seed potato transportation, while the field transmission depends on disease soil, disease fertilizer and irrigation water. The pathogen invades the host through root hairs, dermatomes and wounds. Infected tissue is easy to disintegrate, so that the bacteria fall into the soil and spread through the winter. Soil moisture 90%, temperature 18~20℃, pH 4.7 ~5.4 partial acid conditions, suitable for the development of bacteria, conducive to disease. In years and areas with more rainfall and cooler summer, diseases occur seriously. The severity of the disease mainly depends on the number of initial infection, and the effect of reinfection in the field is not very important.

(1) Strict quarantine system. Block the ward, keep the quarantine gate, and prohibit the entry and exit of diseased seed potatoes. (2) Select disease-free seed potatoes. Establish disease-free farming; control harvest, storage, seeding, eliminate diseased potatoes. (3) Do a good job of seed potato disinfection. Soak seeds with 2% hydrochloric acid solution for 5 minutes, or soak seeds with 40% formalin 200 times solution for 5 minutes or soak seeds for 2 hours, dry and sow seeds. (4) Strengthen cultivation management. The disease area shall be rotated for more than 5 years; base fertilizer and phosphorus and potassium fertilizer shall be applied; lime and plant ash shall be applied to improve soil pH; high ridge cultivation shall be adopted to avoid flood irrigation and prevent the spread of germs.