Characterisation and pathogenicity of Rhizoctonia isolates associated with black root rot of strawberries in the Western Cape Province, South Africa

Black root rot is an important disease of strawberry caused by a complex of fungi including species of Rhizoctonia. In this study, the Rhizoctonia species and anastomosis groups isolated from diseased strawberries in the Western Cape Province of South Africa were determined and their pathogenicity and relative virulence assessed. Both binucleate and multinucleate types were recovered from diseased roots and identified as R. fragariae and R. solani, respectively. Anastomosis grouping of the isolates was carried out on a sub-sample using the conventional method of hyphal fusion, and molecular techniques were employed to confirm results of the former. RFLP analysis of the 28S RNA gene was used to further characterise relationships among the isolates of Rhizoctonia. The molecular results correlated with those obtained from the conventional methods. In the sub-sample tested, all isolates of R. solani were members of Anastomosis Group 6, whereas three AG types were identified among isolates of R. fragariae, viz. AG-A, AG-G and AG-I at a relative occurrence of 69%, 25% and 6%, respectively. Pathogenicity trials were conducted on 8-week-old cv. Tiobelle plants. All Rhizoctonia isolates tested were pathogenic to strawberry, but R. solani (AG 6) was the most virulent causing severe stunting of plants. R. fragariae AG-A and AG-G were not as virulent as R. solani but also caused stunting. R. fragariae AG-I was the least virulent, and did not cause stunting of the plants; however, it incited small, pale, spreading lesions on infected roots. This is the first species confirmation and AG type identification of Rhizoctonia taxa causing root rot of strawberries in South Africa.