Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

Johnnie P. Snow


Infection cushions were formed in a similar manner on soybean leaves by Rhizoctonia solani AG-1 IA (aerial blight) and AG-1 IB (web blight) isolates, however, AG-1 IB produced more mucilagenous material. Leaf topography did not induce infection cushion formation. More infection cushions were formed by isolates of AG-1 IA and IB on collodion membranes placed over leaves of susceptible compared to resistant cultivars. Glucose and 3-O-methylglucose repressed disease severity caused by AG-1 IA and IB isolates to the same extent. However, when the solutions were applied 40 h or longer after inoculation, no repression was observed. Disease severity and number of infection cushions were significantly correlated. Isolates of AG-1 IA formed more infection cushions and caused greater disease severity than AG-1 IB isolates at 35 C. There were no significant differences at 20, 25, and 30 C. Only isolates of R. solani AG-1 IA, IB, IC, AG-4, and AG-5 formed infection cushions, while other AG's did not. More seedling infections occurred at 20-25 C, whereas, more leaf infections were observed at 25-30 C by both AG-1 IA and IB indicating that infection and colonization of different host parts are affected differently by temperature. Double-stranded RNA (dsRNA) was detected in 63% of R. solani AG-1 IA and IB isolates. Most AG-1 IA isolates had seven dsRNA components with molecular sizes ranging from 1.3-9.3 kb. Most AG-1 IB isolates had a large 12 kb dsRNA and these components appeared to be different from dsRNA of other AG's. DsRNA components were stable through successive subculturing and were located in the cytoplasm. DsRNA was not detected in isolates of AG-1 IB (2 tested) and one isolate of AG-1 IA (2 tested) after 1 wk of growth at 35 C. The presence or absence of dsRNA in AG-1 IA or IB isolates did not correlate with virulence or mycelial growth. Disease severity determined with a detached leaf inoculation technique was correlated with Rhizoctonia foliar blight ratings of soybean cultivars in an inoculated nursery. The potential use of these two techniques for evaluation of resistance is discussed.