Characterization and pathogenicity of Rhizoctonia spp. isolated from winter squash in the Black Sea region of Turkey

A total of 27 isolates of Rhizoctonia spp. were recovered from diseased winter squash (Cucurbita maxima) plants, collected from Amasya, Ordu, Samsun and Sinop provinces in the Black Sea region, Turkey. The isolates of Rhizoctonia spp. associated with root and crown rot of winter squash were examined for their cultural characteristics, anastomosis groups and subgroups using sequence analyses of the rDNA-ITS and pathogenicity. Based on ITS1–5.8S-ITS2 rDNA sequencing, Rhizoctonia solani anastomosis group (AG) 4 was the pathogen most frequently recovered (42.8 %) in all fields surveyed. Twenty-four isolates of R. solani AG 4 were further divided into three subgroups, AG 4 HG-I (13), AG 4 HG-II (6) and AG 4 HG-III (5). In addition, three isolates recovered from different fields were identified as binucleate (BN) Rhizoctonia AG A (7.4 %) and AG K (3.7 %). All isolates grew at temperatures of 10, 15, 20, 25, 30 and 35 °C, and they were completely inhibited at 5 and 40 °C. The results of pathogenicity tests on winter squash seedlings showed statistically significant differences (P < 0.001) among the mean disease severity index (DSI) of three subgroup isolates of R. solani AG 4. The DSI of the isolates of R. solani AG 4 ranged from 2.0 to 5.0. Isolates of AG 4 HG-I and HG-III had the highest virulence levels (DSI ranged from 3.8 to 5.0). The virulence level of isolates of BN Rhizoctonia AG A and AG K were low (DSI ranged from 2.0 to 2.3). To our knowledge, this is the first report of R. solani AG 4 HG-I, HG-II, and HG-III, and BN Rhizoctonia AG A and AG K occurring on winter squash, one of the most important cucurbit species in Turkey.