In this study, three aspects of plant-pathogenic Sclerotium fungi were investigated: overwinter survival of Sclerotium rolfsii and S. rolfsii var. delphinii; development of a rapid method for detecting resistance of hosta cultivars to petiole rot disease; and clarification of phylogenetic relationship among pathogenic Sclerotium species. To test the hypothesis that differential tolerance to climate extremes affects the geographic distribution of S. rolfsii and S. rolfsii var. delphinii, overwinter survival of these fungi was investigated in the northern and southeastern U.S. At each of four locations, nylon screen bags containing sclerotia were placed on the soil surface and buried at 20-cm depth. Sclerotia were recovered six times from November 2005 to July 2006 in North Dakota and Iowa, and from December 2005 to August 2006 in North Carolina and Georgia. Survival was estimated by quantifying percentage of sclerotium survival on carrot agar. Sclerotia of S. rolfsii var. delphinii survived until at least late July in all four states. In contrast, no S. rolfsii sclerotia survived until June in North Dakota or Iowa, whereas 18.5% survived until August in North Carolina and 10.3% survived in Georgia. The results suggest that inability to tolerate low temperature extremes limits the northern range of S. rolfsii. In the second study, a rapid assay was developed to assess hosta cultivars for resistance to petiole rot caused by S. rolfsii var. delphinii. The leaf-petiole junction of excised leaves of greenhouse-grown hosta (Hosta kikutii and H. spp. cultivars Lemon Lime, Munchkin, Tardiflora, Pearl Lake, Zounds, Honeybells, Gold Drop, and Halcyon) were treated with 20 &mul of oxalic acid (50 mM) on a cotton swab, then incubated at 100% relative humidity and 27 yC. After 4 days, incidence of leaves with lesions was evaluated. Cultivar resistance rankings were generally consistent with those of field and greenhouse cultivar screening tests in which whole plants were inoculated with the pathogen. In the third study, genetic evidence and morphological features were used to determine the taxonomic placement of eight plant-pathogenic Sclerotium species and transfer one Ceratorhiza species to the genus level. Sequences of rDNA large subunit (LSU) and internal transcribed spacer (ITS) regions were generated for isolates of each species. Parsimony analysis grouped two species, S. denigrans Pape and S. perniciosum Slogt. et K.S. Thomas, within the Ascomycota. Mycelium morphology generally matched with results of the parsimony analysis. S. hydrophilum Sacc and S. rhizodes Auersw. were transferred to Ceratorhiza hydrophilum (Sacc.) Xu, Harrington, Gleason, et Batzer, comb. nov. and Ceratorhiza rhizodes (Auersw.) Xu, Harrington, Gleason, et Batzer, comb. nov., respectively.