Diversity and Pathogenicity of Anastomosis Groups of Rhizoctonia solani Isolates Associated with Potato Diseases in Northern Sinaloa, Mexico

Potato production in Mexico is affected by Rhizoctonia solani , causing severe yield losses. The most important criterion for distinguishing R. solani isolates is differentiation by anastomosis groups (AGs). However, the limitations of traditional AG differentiation have prompted the use of DNA sequence analysis as a valuable tool for distinguishing AGs. This study aimed to determine the diversity of anastomosis groups and the aggressiveness of R. solani isolates collected from commercial potato fields in northern Sinaloa, Mexico. A total of 118 R. solani isolates were obtained from symptomatic sprouts and tubers collected from 32 commercial fields distributed in the municipalities of Ahome, Guasave and El Fuerte in northern Sinaloa, Mexico. Fifty-two representative isolates were selected for anastomosis group determination and pathogenicity tests. Phylogenetic analysis with rpb2 gene sequences clearly distinguished 30 isolates belonging to AG-3 PT and 22 isolates to AG-4 HGI. The aggressiveness of these R. solani isolates was evaluated using mini-tubers of potato cv. Fianna, and colonized rice grains were used as inoculum source. The disease severity caused by R. solani on potato plants was evaluated 40 days after emergence. Aggressiveness was estimated as the percentage of necrotic area on underground stems. The results revealed that all R. solani isolates were pathogenic and differences were observed in aggressiveness among the isolates. Of the total of 52 isolates evaluated, 7% were highly aggressive; 61% had intermediate aggressiveness; and 32% exhibited low aggressiveness. These findings regarding the diversity of anastomosis groups and the aggressiveness of R. solani isolates in northern Sinaloa are fundamental for subsequent epidemiology and fungicide resistance studies.