Rhizoctonia zeae is a common pathogen of tall fescue and other turfgrasses in various regions of the United States. Disease caused by R. zeae typically occurs during hot, humid summer months when conditions are ideal for growth of the fungus but less than ideal for growth of tall fescue. While R. zeae has been reported on turfgrasses in several Mid-Atlantic states, there are no records of this pathogen being present in Virginia. Rhizoctonia isolates were obtained from samples of various turfgrasses exhibiting typical Rhizoctonia disease symptoms in Virginia. Additional isolates were obtained from several other states. All were characterized with respect to species characteristics as well as pathogenicity and virulence on two common cultivars of tall fescue. All isolates were pathogenic on tall fescue but there was some variability in virulence. There was consistently slightly less disease present on Crossfire II than on Kentucky 31. Experiments were also conducted to determine the impact of prior exposure to high air temperatures on the severity of disease. Preliminary data showed that one week of exposure to higher air temperatures caused an initial increase in overall turf quality, but as length of exposure increased the quality of turfgrass declined. Tall fescue plants were subjected to 0, 7, and 35 days of heat stress prior to inoculation with several isolates of R. zeae. No relationship was found between predisposing heat stress and disease severity. / Master of Science

Rhizoctonia blight (sensu lato) is a common and serious disease of many turfgrass species. The most widespread causal agent R. solani consists of several genetically different anastomosis groups (AGs) and subgroups. Though anastomosis or hyphal fusion reactions have been used to group Rhizoctonia species, they are time consuming and sometimes difficult to interpret. Anastomosis reactions are incapable of identifying isolates belonging to different AG subgroups within an AG. This study evaluated molecular techniques in comparison with traditional anastomosis grouping (AG) to identify and group isolates of Rhizoctonia. More than 400 Rhizoctonia isolates were collected from diseased turfgrass leaves from eight geographic areas in Virginia and Maryland. A random sample of 86 isolates was selected and initially characterized by colony morphology, nuclei staining and anastomosis grouping. Molecular identification was performed by analysis of rDNA-ITS region and DNA fingerprinting techniques universally primed PCR (UP-PCR) and amplified fragment length polymorphism (AFLP). The cladistic analysis of ITS sequences and UP-PCR fragments supported seven clusters. Isolates of R. solani AG 1-IB (n=18), AG 2-2IIIB (n=30) and AG 5 (n=1) clustered separately. Waitea circinata var. zeae (n=11), and var. circinata (n=4) grouped separately. A cluster of six isolates (UWC) did not fall into any known Waitea group. Most of the binucleate Rhizoctonia-like fungi (BNR) (n=16) grouped separately. AFLP grouping also largely agreed with the above results. However, UWC isolates clustered into two groups. Molecular analyses corresponded well with traditional anastomosis grouping by clustering isolates within an AG or AG subgroup together. UP-PCR cross-hybridization could distinguish closely related Rhizoctonia isolates to their infraspecies level. Genetically related isolates belonging to the same AG subgroups cross-hybridized strongly, while isolates of different AGs did not cross-hybridize or did so weakly. Sequence-characterized amplified region (SCAR) markers were generated from UP-PCR products to identify isolates of major pathogenic groups AG 1-IB and AG 2-2IIIB. Specific primer pairs successfully distinguished isolates of AG 1-IB and AG 2-2IIIB from isolates of other AGs. Sensitivity of Rhizoctonia species and AGs was tested in vitro to commercial formulations of iprodione, triticonazole and pyraclostrobin. W. circinata isolates were moderately sensitive to iprodione while isolates of R. solani and BNR were extremely sensitive. Isolates of AG 2-2IIIB showed less sensitivity to triticonazole than other Rhizoctonia isolates. W. circinata var. zeae isolates were moderately sensitive to pyraclostrobin while most of the other isolates were extremely sensitive. / Ph. D.