Characterisation of <i>Rhizoctonia solani</i> anastomosis groups and their pathogenicity to potato

The significance of different anastomosis groups (AGs) of Rhizoctonia solani in potatoes was investigated. One hundred and thirty-five isolates of R. solani were collected from potato crops in Britain and assigned to an AG. The predominant group was AG3PT but isolates of AG2-1 and AG5 were also present. Identification of AG was aided by the development of a PCR assay specific for AG3PT. AG5 was also isolated from a couch grass plant present in a potato sample. Hyphal fusion between isolates of AG2-1 and different lengths of the rDNA IGS1 region indicated diversity amongst the AG2-1 isolates. Pathogenicity tests, glasshouse and field experiments were also undertaken comparing potato disease caused by different AGs. AG3PT was associated with black scurf, root infection, severe stem and stolon infection including stem pruning. Severe stem infection was also observed with AG5, but stem pruning, black scurf and root infection were uncommon. Two types of infection were observed amongst isolates of AG2-1. One set of isolates, characterised by a shorter version of the IGS1 regions, caused frequent small lesions no longer than 5 mm, on potato stems. Isolates of AG2-1 with a longer version of the IGS1 region could cause severe stem canker and pruning. Severe root infection was observed in plants infected with AG8 but little other sign of disease. In field experiments yield losses were greatest in plants infected with AG3PT and AG8, suggesting that root infection is a key factor in determining yield losses. Isolates collected in this study, in addition to other isolates, were characterised for thiamine requirement, hyphal growth rates under different temperatures, sensitivity to different fungicides, and in vitro production of sclerotia. The rDNA ITS region of several isolates were sequenced and analysed, confirming the presence or absence of subgroups in AGs associated with potato