Characterization and pathogenicity of Rhizoctonia species on canola

A total of 112 Rhizoctonia isolates were collected from various canola (Bra ssica napus) growing areas of Western Australia. Pectic enzyme electrophore sis differentiated these isolates into six distinct zymogram groups: R. sol ani, 54% ZG5 (AG2-1), 8% ZG6 (AG2-1), and 1% ZG9 (AG10); binucleate Rhizoct onia, 12% CZG1 (CAG1), 4% CZG4, and 6% CZG5 (AGK); and the remainder uniden tified binucleate groups (15%). Binucleate groups were also confirmed by fl uorescent nuclear staining and hyphal morphology. One or more isolates from each of the above zymogram groups (including four unidentified binucleate groups) and an isolate of ZG1-1 (AG8) that causes bare patch in cereals and legumes were tested for their pathogenicity on canola. Isolates of ZG5 and ZG1-1 were highly pathogenic on canola, delayed seedling emergence, and ca used severe hypocotyl and root rot, respectively ZG5 also induced postemerg ence damping-off. Increasing the depth of sowing from 1 to 3 cm significant ly delayed seedling emergence and increased disease severity. Four unidenti fied binucleate isolates (WAC9316, WAC9297, WAC9307, and WAC9290) were mode rately pathogenic to canola, while two isolates (WAC9307 and WAC9316) cause d significant preemergence damping-off. Two CZG5 isolates were weakly patho genic. Isolates of ZG5 and ZG1-1 were also tested for their pathogenicity o n other rotational crops (narrow-leafed lupin, subterranean clover, wheat, oats, barley, and mustard) and two weed species (wild radish and annual rye grass). ZG5 caused a severe hypocotyl rot on mustard and mild symptoms of h ypocotyl rot on narrow-leafed lupin and clover, but failed to infect any of the cereal hosts, such as wheal, oats, barley, and ryegrass. In contrast, all crops tested were highly susceptible to ZG1-1 except mustard, which was only moderately susceptible. Results indicate that ZG5 is most pathogenic to crucifers and is a mildly virulent pathogen of the leguminous crops hut not of cereal crops tested. ZG1-1, known to cause bare patch in legumes and cereals, also can cause severe root rot in canola. This is the first repor t of hypocotyl rot and pathogenicity of ZG5 on canola in Australia.