THE BLACK ROOTROT DISEASE OF TOBACCO: I. STUDIES ON THE CAUSAL ORGANISM THIELAVIOPSIS BASICOLA

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Abstract
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.