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The range in cultural variability of Fusarium oxysporum f. cubense on potato dextrose agar was studied. Several hundred isolates obtained from diseased varieties of Musa were categorized into four morphological groupings or cultivars. These were termed 'Sporodochial', 'Ropy', 'Cottony Alba', and 'Slimy Pionnotal'. The morphological cultivar 'Sporodochial' was further divided physiologically into 'Odoratum' and 'Inodoratum' cultivars. Clones producing sporodochia were the parental forms from which all other cultural types could arise by mutation. Mutants did not revert to their parental types. A system is proposed for cataloguing clones of plant pathogenic Fusarium species. Based on morphological characters, the catalog is intended to serve as a collection of data which provides also for the categorizing of cultivars.

Intensive cultural studies with Thielaviopsis basicola (Berk. & Br.) Ferraris, the cause of the black rootrot disease of tobacco, showed that it exists in nature in two distinct forms, which have been called brown and gray wild types, and which are differentiated by their cultural characteristics on potato dextrose agar. The brown wild type usually is the predominant form isolated, although in some instances the gray predominates in mixed infestations. It is suggested that the balance between the two types is determined, in part, by a differential selective action of the soil flora. The gray wild type is less pathogenic and is more poorly adapted than the brown to withstand long dormant periods. The latter occasionally mutates to the gray in soil, host, and in artificial culture. In a few instances, but only in association with host passage, the gray wild type has mutated to the brown. The wild types do not remain stable on media which support abundant saprophytic growth but give rise to and are crowded out or replaced by cultural mutants. The wild type can be maintained with a minimum of variation in soil, soil agar, roots, or similar substrates (low in certain nutrients). Variations in the morphology and behavior of endoconidia consisted of the production of endoconidial yeastlike colonies and the formation of thick-walled chlamydosporelike structures from thin-walled endoconidia. Some endoconidia are as resistant as chlamydospores to long periods of dormancy, heat, and drying. Fungus morphology in the host was characteristic for each of the three phases of cell penetration, cell colonization, and the production of chlamydospores and endoconidia. Root invasion by Thielavia basicola was common only in association with Thielaviopsis basicola which suggests a commensalistic relationship between these two fungi.

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Six species and forms of the genus Fusarium show optimum growth and survival in soil at 15% saturation. Optimum soil moisture content for actinomycete growth and survival is similar to that for the Fusarium species whereas that for bacteria is at 75% soil saturation. The present studies indicate that Fusarium spp. are strongly aerobic and that Fusarium populations can be greatly reduced by maintaining the soil in a saturated condition in the absence of hosts.

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Flood fallow and silting \Irere the only practices that consistently reduced incidence of banana wilt incluced by F. o.vyspoi.~r~rz f. cubem-e ( E . F. S.) Snpd. ancl I-Ians. The efhciency of flood fallow was greatly increased by interflood plo\ving, and in most locations by postflood fungicide drenches. Even these treatments, however, did not insure economic control in some areas. I t was necessary, therefore, to select areas where soil factors terldetl to retard rapid disease development. Also, the absence of the banana host for a number of years previous to flood fallow appeared to influence treatment success.'Manuscript