Effect of temperature on virulence of Rhizoctonia solani on soybean leaves and seedlings

Abstract

Isolates representing 11 anastomosis groups (AGs) of Rhizoctonia solani from various geographic locations and host plants were tested for virulence on soybean leaves at 15, 20, 25, 30, and 35°C, and on soybean seedlings at 20, 25, and 30°C. Numbers of infection cushions formed on soybean leaves were determined using light microscopy. Isolates of AG‐1 IA, AG‐1 IB and AG‐5 were more virulent on soybean leaves at 20, 25, and 30°C than isolates of AG‐1 IC and AG‐4. Maximum numbers of infection cushions were formed on soybean leaves by AG‐1 (IA, IB, and IC), AG‐4, and AG‐5 at 25 and 30°C. The other AGs tested did not form infection cushions on soybean leaves although some caused minimal disease severity. Isolates of AG‐1 IA formed significantly more infection cushions and caused greater disease severity than AG‐1 IB and other isolates at 35°C. Maximum seedling infection, based on per cent area of hypocotyl region covered by lesions occurred at 25 C for AG‐1 (IA, IB, and IC) and AG‐4. Isolates of AG‐5 caused greater seedling infection at 20°C than at 25 and 30°C. The other AGs tested caused only minimal damage to the seedlings. Isolates of AG‐4 and AG‐5 are not known to cause Rhizoctonia foliar blights of soybean in Louisiana, but their potential to be destructive foliar pathogens is confirmed.