Phylogenetic analyses and morphological characteristics were used to assess the taxonomic placement of eight plant-pathogenic Sclerotium species. Members of this genus produce only sclerotia and no fruiting bodies or spores, so Sclerotium species have been difficult to place taxonomically. Sequences of rDNA large subunit (LSU) and internal transcribed spacer (ITS) regions were determined for isolates of Sclerotium cepivorum, S. coffeicola, S. denigrans, S. hydrophilum, Ceratorhiza oryzae-sativae, S. perniciosum, S. rhizodes, S. rolfsii and S. rolfsii var. delphinii. Parsimony analysis grouped two species previously thought to be in the Basidiomycota, S. denigrans and S. perniciosum, within the Ascomycota; these species were found to have affinities with the teleomorph genera Sclerotinia and Stromatinia and the asexual Sclerotium cepivorum, which was known earlier to be related to Sclerotinia species. The other Sclerotium species were placed in one of two basidiomycetous groups, genera Athelia or Ceratobasidium. Based on rDNA analysis and morphology the basidiomycetous Sclerotium hydrophilum and S. rhizodes were transferred to genus Ceratorhiza, the anamorph of Ceratobasidium species. Sclerotium coffeicola was found to be close to S. rolfsii var. delphinii and S. rolfsii var. rolfsii, which was shown earlier to have an Athelia teleomorph.

Previously known only from the southern United States, hosta petiole rot recently appeared in the northern United States. Sclerotium rolfsii var. delphinii is believed to be the predominant petiole rot pathogen in the northern United States, whereas S. rolfsii is most prevalent in the southern United States. In order to test the hypothesis that different tolerance to climate extremes affects the geographic distribution of these fungi, the survival of S. rolfsii and S. rolfsii var. delphinii in the northern and southeastern United States was investigated. At each of four locations, nylon screen bags containing sclerotia were placed on the surface of bare soil and 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 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 µl of oxalic acid (50 mM) on a cotton swab, then incubated at 100% relative humidity and 27 °C. 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. vi 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. 1 CHAPTER 1. GENERAL INTRODUCTION Dissertation Organization This dissertation consists of an abstract and five chapters. The first chapter is an introduction to hosta petiole rot, caused by Sclerotium rolfsii and S. rolfsii var. delphinii, and sets the stage for...

A rapid assay was developed to assess hosta cultivars for resistance to petiole rot caused by Sclerotium rolfsii var. delphinii. Leaves of greenhouse-grown hosta (Hosta kikutii and Hosta spp. cultivars Munchkin, Lemon Lime, Tardiflora, Pearl Lake, Zounds, Gold Drop, Halcyon, and Honeybells) were treated with 20 ml of oxalic acid (50 mM) on a cotton swab, then incubated at 100% relative humidity and 27°C. After 4 days, incidence of leaves with lesions was evaluated. Cultivars Munchkin, Lemon Lime, and Tardiflora had a relatively high incidence of leaves with lesions, whereas Gold Drop and Halcyon had much lower incidence. These results were generally consistent with those of field and greenhouse cultivar screening tests in which whole plants were inoculated with the pathogen and rated for disease incidence. Additional screening methods, including spray application of either oxalic acid or mycelial fragments of S. rolfsii var. delphinii, were not as repeatable or simple to conduct as the cotton swab assay. The cotton swab method showed potential to accelerate identification of highly resistant hosta cultivars, and thereby aid efforts to breed resistance to petiole rot. Accepted for publication 1 December 2008. Published 28 January 2009.

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