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Characterization and Pathogenicity of Rhizoctonia from Soybean. Berlin Nelson, Department of Plant Pathology, North Dakota State University, Fargo 58105. Ted Helms, Tracy Christianson and Ilhan Kural. Plant Dis. 80:74-80. Accepted for publication 9 October 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0074.

Of 102 isolates of Rhizoctonia recovered from roots and stems of soybean, 98 were R. solani and were identified to four anastomosis groups (AG): AG-2-2 (3.1%), AG-3 (2.0%), AG-4 (45.9%), and AG-5 (37.8%); 10.2% of the isolates did not consistently anastomose with any of the tester isolates (AG-1 to 9 and AG-BI). Four isolates from roots were binucleate Rhizoctonia. AG-2-2, AG-4, and AG-5 were virulent on soybean seedlings and adult plants, whereas AG-3 caused small lesions only on tap roots of adult plants. The binucleate Rhizoctonia were not pathogenic on soybean. AG-5 was generally less virulent on soybean than AG-2-2 and AG-4,, but when inoculum was placed in direct contact with seeds, AG-5 caused high levels of pre-and postemergence damping-off. AG-5 also caused high disease severity ratings on adult soybean when the inoculum level was increased. Sugar beet seedlings were highly susceptible to AG-2-2 and AG-4, but only slightly susceptible to AG-5. Dry bean, mustard, and flax seedlings were susceptible to AG-2-2 and AG-4, and dry bean and flax were slightly susceptible to AG-5. AG-4 and AG-2-2 caused moderate reductions in emergence of sunflower, and AG-2-2 caused a root rot on corn seedlings. These results indicate that AG-5 could be an important soybean pathogen and that other rotational crops are hosts to R. solani recovered from soybean.