CHARACTERISTICS OF RHIZOCTONIA ISOLATES ASSOCIATED WITH BOTTOM ROT OF LETTUCE IN ORGANIC SOILS IN OHIO

Rhizoctonia isolates obtained from lettuce with bottom rot symptoms-collected at two major Ohio lettuce production areas, Celeryville in 1989 and Hartville in 1990-were characterized according to anastomosis groups (AGs) and, when feasible, by intraspecific groups (ISGs). Isolates belonging in AG-2-1 were further subdivided into three new provisional ISGs (designated types a-c) on the basis of their cultural characteristics. Thirty-three of 51 Celeryville isolates and 28 of 59 Hartville isolates were characterized as AG-1-1B, the web blight ISG of AG-1. Additionally, AG-1-1C isolates were obtained from diseased lettuce at Celeryville (10 isolates) and Hartville (four isolates). Isolation of these ISGs of AG-1 represents a first report of the occurrence of any of the ISGs of AG-1 in Ohio. AG-2-1 (20 isolates) and AG-5 were obtained only from Hartville, whereas AG-2-2 was isolated only from Celeryville. Other isolates of AG-4, binucleate Rhizoctonia spp., and Laetisaria arvalis occurred infrequently at both locations. Of the major AGs collected and evaluated in greenhouse tests, AG-1-1B isolates were collectively the most virulent, isolates of AG-1-1C were intermediate, and isolates of AG-2-1 were least virulent. Of three lettuce types used in greenhouse tests of the Celeryville isolate population, romaine was susceptible to more isolates than Boston, and red leaf was susceptible to the lowest number of isolates. Hartville isolates were tested for virulence only on romaine and were found to differ significantly in virulence capabilities among isolates of the population of R. solani collected.