Submitted to: Australasian Plant Pathlogy Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2002
Publication Date: 6/20/2003
Citation: BOTHA, A., DENMAN, S., LAMPRECHT, S.C., MAZZOLA, M., CROUS, P.W. CHARACTERIZATION AND PATHOGENICITY OF RHIZOCTONIA ISOLATES ASSOCIATED WITH BLACK ROOT ROT OF STRAWBERRIES IN THE WESTERN CAPE PROVINCE, SOUTH AFRICA. AUSTRALASIAN PLANT PATHLOGY SOCIETY. 2003. v. 32(2). p. 195-201. Interpretive Summary: Black root rot is an important disease of strawberry on a global basis. Although the disease is incited by a fungal complex that can vary among geographic regions, species belonging to the genus Rhizoctonia have consistently been implicated in the phenomenon. Studies were conducted to determine the species of Rhizoctonia associated with black root rot and the relative importance of these fungi in development of this disease. The population of Rhizoctonia recovered from strawberry included both R. solani and multiple anastomosis groups of R. fragariae. Both groups of fungi were shown to cause disease of strawberry but the effect of the individual species on growth was variable with R. solani causing the greatest level of damage to plants. This is the first species confirmation and AG tpe identification of Rhizoctonia taxa causing root rot of strawberries in South Africa.
Technical Abstract: 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 were determined, and their pathogenicity and relative virulence assessed. 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 and DNA sequence analysis of the internal transcribed spacer (ITS) regions were used to further characterize relationships among the isolates of Rhizoctonia. The molecular results correlated with those obtained from the conventional methods. 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%, 6%, respectively. Pathogenicity trials were conducted on eight-week-old cv. Tiobelle plants. All Rhizoctonia isolates were pathogenic to strawberry but R. solani (AG 6A) 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. This is the first species confirmation and AG type identification of Rhizoctonia taxa causing root rot of strawberries in South Africa.