Anastomosis Group Identity and Virulence of Rhizoctonia solani Isolates Collected from Potato Plants in Alberta, Canada

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