EFFECT OF HYPOVIRULENT RHIZOCTONIA-SOLANI ON RHIZOCTONIA DISEASE, GROWTH, AND DEVELOPMENT OF POTATO PLANTS

Field experiments were conducted to determine whether a hypovirulent, anastomosis group (AG) 3 isolate (Rhs 1A1) of R. solani could control rhizoctonia disease of potato caused by a virulent AG-3 isolate (Rhs 27). A uniform amount of Rhs 1A1, Rhs 27, or both was applied to formaldehyde-disinfected potato seed tubers at planting. In plots inoculated with Rhs 1A1 and Rhs 27, the area of infected stem tissue was significantly (P = 0.05) reduced by 56% as compared with that of plots inoculated with Rhs 27 alone. The number of stolons per plant was decreased by isolate Rhs 27 to one-half that of the uninoculated control, but remained unchanged in plots treated with both Rhs 1A1 and Rhs 27. The type of inoculum did not affect the percentage of infected stems or stolons. Plants inoculated with Rhs 1A1 alone exhibited a significantly (P = 0.05) greater growth response, expressed as a 4-fold increase in the dry weight of stolons, a 1.7-fold increase in the dry weight of stems, and reached full bloom approximately one week earlier than their respective control. Tuber yield increases associated with Rhs 1Al-treated plots were not statistically significant. When untreated seed tubers bearing natural inoculum (sclerotia) of R. solani were used, Rhs 1A1 failed to reduce disease severity or sig-nificantly stimulate plant growth. © 1990 Springer.