First report of broccoli foot rot caused by Rhizoctonia solani AG-2-2 IV and pathogenicity comparison of the pathogen with related pathogens

Foot rot and wilt symptoms were found on mature broccoli plants in Hokkaido, Japan, in 2007. The causal fungus was identified as Rhizoctonia solani anastomosis group (AG)-2-2 IV based on a hyphal fusion test, cultural characteristics and DNA analysis. We compared the isolate with two reference isolates, the pathogen of broccoli damping-off, belonging to AG-1 IC and AG-2-2 IIIB, in terms of differences in virulence to broccoli plants at different growth stages under various conditions. All the isolates caused damping-off on all three broccoli cultivars tested. However, only the present isolate incited foot rot with wilt symptoms on mature plants, when temperature was optimum for plant growth (20–25 °C), indicating that the foot rot pathogen may be discriminated from the damping-off pathogen in terms of virulence to mature broccoli. We also compared the isolate with 10 reference isolates from various crops, belonging to AG-2-2 IV, AG-2-2 IIIB and AG-1 IC, in terms of virulence to broccoli. All the isolates caused damping-off on broccoli seedlings with one exception; however, only the present isolate and one of four AG-2-2 IV isolates tested caused foot rot with wilt symptoms on mature broccoli in all the replicates. These results suggest that a group of isolates among AG-2-2 IV population is capable of causing foot rot on mature broccoli plants. This is the first report of broccoli foot rot caused by R. solani AG-2-2 IV.