Motile zoospores of P. dissotocum and P. catenulatum were added to 20 and 60 mesh silica sand and a sandy loam soil to investigate the duration of motility. Both Pythium species remained motile for up to 24 hours in all soil textures tested. However, the duration and percentage of the motile population varied depending upon the soil type and species tested. The duration of survival of motile and encysted zoospores of P. dissotocum and P. catenulatum in air dried and saturated sterile silica sand was also tested. P. dissotocum, under air dried conditions, did not survive while P. catenulatum was capable of surviving 4 and 16 days as motile and encysted zoospores, respectively. Both fungi could be recovered at high percentages after 5 weeks under saturated conditions. The survival structure is believed to be in the form of a zoospore cyst.

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Specific questions regarding the community structure of VA-mycorrhizal fungi and the population biology of an ectomycorrhizal Basidiomycete, Suillus granulatus, were addressed. The distributional ecology of VAM fungal communities with grasses was studied across a climatic gradient in the hyper-arid central Namib dune field. VAM fungal communities were primarily structured by substrate stability and moisture availability. Five VAM species were found throughout the study area and were not host specific. Percent mycorrhizal colonization was correlated with moisture availability, whereas spore abundance was correlated with substrate stability. Moisture availability was the key factor influencing VAM fungal phenology: growth, assessed as increased colonization of roots, continued as long as moisture was available, and spore production occurred in response to declining moisture availability. While abiotic factors determine community structure of VAM fungi in the Namib dune field, preliminary studies suggest that the phytobiont mediates fungal response to these abiotic factors. Genetic analyses of S. granulatus single spore isolates using RAPD markers showed that a post-meiotic mitosis in the basidium produces heterokaryotic spores. Secondary homothallism provides an effective means for long distance dispersal, and may account for the broad geographic range of this ectomycorrhizal fungus. Secondary homothallism contributed to the failure of somatic incompatibility tests to delineate the spatial distribution of individuals in a natural population of s. granulatus. Analysis of genetic relatedness using RAPD markers demonstrated conclusively that somatically compatible individuals were not necessarily genetically identical. I concluded that RAPD marker analysis provides a more effective means for determining clonal distribution in ectomycorrhizal populations, than does somatic incompatibility testing. / Ph. D.