Genetic diversity of Rhizoctonia solani AG-3 from potato and tobacco in North Carolina

Abstract

Anastomosis group 3 (AG-3) of Rhizoctonia solani (teleomorph = Thanatephorus cucumeris) is frequently associated with diseases of potato (AG-3 PT) and tobacco (AG-3 TB). Although isolates of R. solani AG-3 from these two Solanaceous hosts are somatically related based on anastomosis reaction and taxonomically related based on fatty acid, isozyme and DNA characters, considerable differences are evident in their biology, ecology, and epidemiology. However, genetic diversity among field populations of R. solani AG-3 PT and TB has not been documented. In this study, the genetic diversity of field populations of R. solani AG-3 PT and AG-3 TB in North Carolina was examined using somatic compatibility and amplified fragment length polymorphism (AFLP) criteria. A sample of 32 isolates from potato and 36 isolates from tobacco were paired in all possible combinations on PDA plus activated charcoal and examined for their resulting somatic interactions. Twenty-eight and eight distinct somatic compatibility groups (SCG) were identified in the AG-3 PT and AG-3 TB samples, respectively. AFLP analyses indicated that each of the 32 AG-3 PT isolates had a distinct AFLP phenotype, whereas 28 AFLP phenotypes were found among the 36 isolates of AG-3 TB. None of the AG-3 PT isolates were somatically compatible or shared a common AFLP phenotype with any AG-3 TB isolate. Clones (i.e., cases where two or more isolates were somatically compatible and shared the same AFLP phenotype) were identified only in the AG-3 TB population. Four clones from tobacco represented 22% of the total population. All eight SCG from tobacco were associated with more than one AFLP phenotype. Compatible somatic interactions between AG-3 PT isolates occurred only between certain isolates from the same field (two isolates in each of four different fields), and when this occurred AFLP phenotypes were similar but not identical.

• clonality
• genetic structure
• mating
• population structure
• recombination

For helping with the sampling of potato and tobacco isolates of R. solani we thank Bryan Cody and Walter Gutierrez. For his patience and use of laboratory space, we thank Steve Leath (USDA/ARS, Dept. of Plant Pathology, NCSU). Thanks to Anja Forche and Shian-ren Liou, and Daniel Snyder (Duke University, Dept. of Biology) for the advice with AFLP data collection and analysis; and Pamela Puryear (Tobacco Literature Service, NCSU) for literature searches. This research was supported in part with a research assistantship from CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico)—Brasília, DF, Brazil (200873/92–8). A leave of absence was also granted to PC by UNESP (Universidade Estadual Paulista “Júlio de Mesquita Filho”), Ilha Solteira, São Paulo, Brazil, to attend a Ph.D. program in Plant Pathology at North Carolina State University. Special thanks to Drs. Ana Maria R. Cassiolato and Marli F. Stradioto Papa for assuming the teaching responsibilities of PC at UNESP during his absence.