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Rhizoctonia is a genus of anamorphic fungi in the order Cantharellales. Species do not produce spores, but are composed of hyphae and sclerotia (hyphal propagules) and are asexual states of fungi in the genus Thanatephorus. The symptoms of this rhizoctonia disease appear on the tuber in the form of small brownish-black spots closely resemble and are sometimes mistaken for soil particles. These spots adhere closely to the tuber and can’t be removed by washing. Excepting for appearance, the tuber itself is not damaged. However, when infected tubers are planted, or if clean seed is planted in infected soil, the rhizoctonia disease attacks and injures the young shoots, roots, rhizomes, and tubers of the new crop. Often the sprouts are attacked and destroyed before they reach the soil surface. This may result in the development of new sprouts, and they in turn may be destroyed. Brown cankers develop and at times completely encircle the stem of the young sprouts. If plants are infected (a) they may have an erect appearance, (b) their leaves often become yellow or reddish-yellow in color and have a tendency to roll, (c) swellings develop in the region of the nodes, and (d) many small tubers may form just above or below the soil surface. The organism that causes this disease is able to live for many years in the soil. It develops most rapidly during cool, moist weather.
How to control rhizoctonia disease of potato?
Practice that aid in reducing injury caused by the rhizoctonia organism include (a) the use of disease-free seed or seed disinfection to destroy the organisms on the seed, (b) crop rotation, (c) shallow planting. The same seed disinfectants that controlthe common scab fungus are also effective for this organism. Shallow planting results in more rapid germination and emergence. Plants established in this manner are injured less by the rhizoctonia organism than that emerge more slowly because of deeper planting.