Genetic and pathogenic variability of Rhizoctonia solani causing crown and root rot on sugar beet in France

Rhizoctonia crown and root rot causes severe damage on sugar beet. We collected twenty-seven isolates of Rhizoctonia solani from diseased sugar beets in France and Belgium over two years. All of them were pathogenic on sugar beet but displayed variable aggressiveness levels. Based on their internal transcribed spacer (ITS) sequences, we identified them as belonging to the anastomosis group (AG) 2-2. All isolates were further classified within the subgroup AG-2-2 IIIB based on their ability to grow at 35 degrees C. As we observed genetic diversity within and among isolates, we clarified heterogeneity within isolates by cloning their ITS region and sequencing 129 clones. The genetic variability levels among clones of a same isolate and among isolates were similar, suggesting that the 27 isolates were closely related. Reference isolates of R. solani AG-2-2 IIIB were genetically more diverse than the isolates collected in our study. The collection of R. solani established in this study provides resources for future investigations on the diversity and evolution of pathogenic populations and for breeding programs.