Identification and virulence of anastomosis groups in Rhizoctonia solani Kuhn associated with potato in Merida, Venezuela

In Venezuela potato (Solanum tuberosum) has economic relevance particularly bt the Andean stares (Merida, Tachira and Trujillo), whose contribution to the national production is approximately 80%. In the last five years the R hizoctonia disease, caused by Rhizoctonia solani, has become an important p roduction limiting factor of potato production. The present study was carri ed out to determine the identity and virulence of R. solani anastomosis gro ups associated with potato in 12 localities of Merida Stare and 1 locality in Trujillo State. One hundred and seventy six isolates whose hyphae and se pta showed typical morphology of the fungi included in the Rhizoctonia Comp lex, were obtained from roars, stolons, petioles, and mainly from sclerotia on tubers. Of these isolates, 173 were multinucleate and 3 were binucleate . Multinucleate isolates were recognized as R. solani. One hundred sixty th ree R. solani strains were AG-3 and 10 belonged to AG-2-1. AG-3 and AG-2-1 isolates averaged 9.5 and 8.3 nuclei / vegetative cell, respectively. AG-3 strains were found in 12 out of 13 evaluated localities, and AG-2-1 strains in. 3 localities. Only AG-2-1 strains were detected in one locality (El Va lle). AG-3 and AG-2-1 strains were recovered from The same field in Bailado res and Mucuchies. As a group, AG-3 strains were more virulent than AG-2-1 strains. The wide dissemination and high virulence of AG-3 strains indicate that members of this group are the. main cause of Rhizoctonia disease on t he potato grown in Merida and in Tuname (Trujillo State).