CHARACTERIZATION AND PATHOGENICITY OF RHIZOCTONIA FROM SOYBEAN

Of 102 isolates of Rhizoctonia recovered from roots and stems of soybe an, 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 teste r isolates (AG-1 to 9 and AG-BI). Four isolates from roots were binucl eate Rhizoctonia. AG-2-2, AG-4, and AG-5 were virulent on soybean seed lings 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 an d AG4, but when inoculum was placed in direct contact with seeds, AG-5 caused high levels of pre- and postemergence damping-off. AG-5 also c aused 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, must ard, and flax seedlings were susceptible to AG-2-2 and AG-4, and dry b ean 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 ro ot 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.