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SYMPTOMSRhizoctonia canker, caused by the fungus Rhizoctonia solani, is a fairly common problem on potato production in Kansas. The disease may occur as a surface blemish on the tubers or more seriously as a disease on new sprouts from seed pieces. Affected tubers may be pitted or cracked, but commonly they only show flat irregular lumps or specks (similar to small clumps of soil) on the skin which will not wash off. These specks are actually resting structures of the fungus called sclerotia. The tissue beneath the sclerotia is not affected. The most serious damage occurs in the spring as new sprouts are emerging. The new sprouts are infected by the fungus, which causes a distinct red to brown lesion. The lesion may expand rapidly, girdling and killing the young sprout. Reddish brown lesions may also occur on stolons and on the stem near the soil surface. Stem lesions can be confused with black leg; however, black leg lesions are black and slimy while those of Rhizoctonia canker are light brown and firm. Plants that are not killed will be stunted and may show symptoms of nutrient deficiency. DISEASE CYCLEThe fungus may persist indefinitely in the soil or as sclerotia on the surface of potato tubers. In the spring, sclerotia germinate and invade developing sprouts. Sclerotia formation on tubers may occur at any time, but is most prevalent near maturity of the tubers. CONTROLSeed pieces should be relatively free of sclerotia. It is difficult to control early spring infection in heavily infested fields, although seed treatment with fungicides may be helpful. See the discussion under Fusarium tuber rot for information on seed treatment. |
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Web updated 9/01/06 |