Symptoms and Control Methods of Potato Rhizoctonia solani Disease

〔Pathogen scientific name〕Rhizoctonia solani Kühn

〔Host crops〕Solanaceae, Fabaceae, Poaceae and many other crops; hosts are widespread.

〔Disease diagnosis〕It mainly damages young sprouts, stem bases and tubers. When young sprouts are infected, those that have not emerged rot, forming sprout rot and causing missing plants; after emergence, infection begins with yellowing of the lower leaves and brown sunken lesions on the stem base, 1-6 cm in size. The lesions and their surroundings are often covered with a purple mycelial layer; sometimes on the stem base and tubers there are sclerotia of varying sizes (1-5 mm) and various shapes (block-like or sheet-like). Symptoms on lightly infected plants are often inconspicuous, while severely infected plants may form damping-off or wilting of the top, or leaves may appear curled. The pathogen is Rhizoctonia solani. The pathogen's hyphae are well developed and thick, mostly branched, with branching points mostly close to right angles, the base of the branches mostly showing constriction, and septa often present near the branching point. The hyphae are initially colorless, gradually turning brown as they mature, with cells becoming shorter, thicker and compressed. When hyphae gather and twist together in large numbers, they can form sclerotia. The sclerotia are initially light brown, later turning dark brown to blackish brown, spherical or irregular in shape, 1-5 mm in size.

〔Occurrence pattern〕The pathogen overwinters as sclerotia on diseased tubers or left in the soil. Infected seed potatoes are the main primary infection source for the following year and also the main route of long-distance spread. When diseased seed potatoes are sown or sowing is done in diseased soil, the pathogen can infect young sprouts through wounds or directly, causing disease, resulting in sprout rot or later the formation of diseased seedlings. The pathogen can spread through wind and rain, irrigation water, insects and farming operations, expanding the damage. It then spreads upward and downward, causing aboveground wilting or infection of underground tubers, producing sclerotia that overwinter again. The pathogen favors warm and humid conditions. When soil temperature is around 23°C and conditions are humid, the disease is severe. The formation of sclerotia in the later stage requires a relatively high temperature of 23-28°C. In the north, cold spring and humid conditions easily lead to disease. When sowing is early in spring and soil temperature is relatively low, the disease is severe. In clayey soil and low-lying waterlogged "returning slurry" land, it is difficult to raise soil temperature, making the disease easy to induce. In diseased areas, continuous cropping land has a relatively high incidence rate.

〔Control measures〕(1) Select disease-resistant varieties. Varieties with good agronomic traits and resistance can be selected according to local conditions. (2) Use disease-free seed potatoes. Establish disease-free seed multiplication fields and use disease-free seed potatoes for sowing. (3) Improve the cropping system. In severely affected areas, carry out appropriate crop rotation; choose higher terrain for planting; in severely affected areas, clayey plots, low-lying waterlogged areas and high-altitude cool mountainous areas, timely late sowing should especially be adopted to prevent excessively low soil temperature from inducing disease. (4) Disinfect seed potatoes. Before sowing, soak seed potatoes for 10 minutes in a 500-fold dilution of 50% carbendazim wettable powder, or a 1000-fold dilution of 50% thiram wettable powder, or a 1500-fold dilution of 5% jinggangmycin aqueous solution, or a 450-fold dilution of 15% hymexazol aqueous solution, or a 1500-fold dilution of 20% tolclofos-methyl emulsifiable concentrate.