Abstract:
A survey of anastomosis groups (AGs) of Rhizoctonia species associated with potato diseases was
conducted in South Africa. A total of 112 Rhizoctonia solani and 19 binucleate Rhizoctonia (BNR)
isolates were recovered from diseased potato plants, characterized for AG and pathogenicity. The
AG identity of the isolates was confirmed using phylogenetic analysis of the internal transcribed
spacer region of ribosomal DNA. Rhizoctonia solani isolates recovered belonged to AG 3-PT, AG 2-
2IIIB, AG 4HG-I, AG 4HG-III and AG 5, while BNR isolates belonged to AG A and AG R, with
frequencies of 74, 6.1, 2.3, 2.3, 0.8, 12.2 and 2.3%, respectively. Rhizoctonia solani AG 3-PT was
the most predominant AG and occurred in all the potato growing regions sampled whereas the other
AGs occurred in distinct locations. Different AGs grouped into distinct clades with high maximum
parsimony and maximum likelihood bootstrap support for both R. solani and BNR. An experiment
under greenhouse conditions with representative isolates from different AGs showed differences in
aggressiveness between and within AGs. Isolates of AG 2-2IIIB, AG 4HG-III and AG R were the most aggressive in causing stem canker while AG 3-PT, AG 5 and AG R caused black scurf. This is
the first comprehensive survey of R. solani and BNR on potatoes in South Africa using a molecularbased
approach. This is the first report of R. solani AG 2-2IIIB and AG 4 HG-I causing stem and
stolon canker and BNR AG A and AG R causing stem canker and black scurf on potatoes in South
Africa.
