Anastomosis Group Identity and Virulence of Rhizoctonia solani Isolates Collected from Potato Plants in Alberta, Canada. P. S. BAINS, Crop Diversification Centre, North, Alberta Agriculture, Food and Rural Development, R.R. #6, Edmonton, Alberta, T5B 4K3, Canada. V. S. BISHT, Crop Diversification Centre, North, Alberta Agriculture, Food and Rural Development, R.R. #6, Edmonton, Alberta, T5B 4K3, Canada. Plant Dis. 79:241-242. Accepted for publication 4 November 1994 . Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0241.

Of 64 isolates of Rhizoctonia solani collected from potato plants grown in central and southern Alberta, 49 belonged to AG-3, and seven each to AG-4 and AG-5. One isolate did not anastomose with any of the tester cultures (AG-2-1, AG-3, AG-4, AG-5, and AG-9) used. In vitro radial growth rate of AG-3 isolates, as a group, was significantly slower than that of AG-4 and AG-5 isolates. Growth rates of AG-4 and AG-5 were similar. Members of AG-3, AG-4, and AG-5 were not restricted to any particular geographical region of the province, and occurred on infected plants growing in the same field. As a group, AG-3 isolates were significantly more virulent than AG-4 and AG-5 isolates, whereas virulence of AG-4 and AG-5 isolates was similar. The high incidence and virulence of AG-3 isolates indicate that AG-3 is the major cause of Rhizoctonia disease of potato in Alberta, whereas AG-4 and AG-5 are of minor importance.