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Rhizoctonia Decline: Studies on Hypovirulence and Potential Use in Biological Control. Brad Castanho, Graduate Research Assistant, Department of Plant Pathology, University of California, Davis, CA 95616, Present address of senior author: Monsanto Agricultural Products Co. TIJ, 800 N. Lindbergh Blvd., St. Louis, MO 63166; E. E. Butler, Professor, Department of Plant Pathology, University of California, Davis, CA 95616. Phytopathology 68:1511-1514. Accepted for publication 1 May 1978. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1511.

Virulence of a severely diseased isolate, 189a, of Rhizoctonia solani and its healthy counterpart, isolate 189HT5, was examined. In petri-plate tests using cabbage seedlings 189a killed only 12% of all seedlings whereas 189HT5 killed 100% of the seedlings. Coinoculation of plates with 189a and 189HT5 resulted in the killing of 38% of the seedlings. Pathogenicity trials in which soil was infested with R. solani showed 189a to be nonpathogenic to sugar beet seedlings; in contrast, 189HT5 caused 23% pre-emergence damping-off and 79% postemergence damping-off. Plants grown in soil infested with 189a and 189HT5 showed no significant amount of damping-off, which indicated complete biological control of the healthy, highly virulent 189HT5. The addition of viable, 189a mycelium to the seed furrow of soil previously infested with 189HT5, as blended or air-dried mycelium, resulted in a fivefold reduction in postemergence damping-off. However, introducing cultures of 189a into soil previously infested with 189HT5 as a broadcast treatment did not reduce damping-off. The survival of 189a and 189HT5 was tested under sterile and nonsterile soil conditions; 189a survived less than 1 mo, whereas 189HT5 survived 2 yr.