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

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.

Interactions between the biocontrol agent Pseudomonas fluorescens CHA0 and Thielaviopsis basicola in tobacco roots observed by immunofluorescence microscopy

“…rp2 failed to penetrate through the endodermis and was not found within root cells (Campbell et al, 1987). The infection of tobacco roots by T. basicola inoculated singly into sterile soil proceeded similarly to that described for other plant species regarding fungal structures and temporal progress of root infection (Conant, 1927;Stover, 1950;Christou, 1962;Wick & Moore, 1983;Mauk & Hine, 1988;Hood & Shew, 1996). As for cotton (Mauke & Hine, 1988), root penetration involved the formation of appressoria (Fig.…”

HOST RANGE AND PERSISTENCE OF THIELAVIOPSIS BASICOLA IN TOBACCO SOIL

“…Although tobacco and two leguminous crops were hosts for the fungus, all attempts to recover T. b&sicola from roots of cereal plants were unsuccessful. Similarly, Johnson (1916) (Linderman and Toussoun 1967), or become converted into the thick-walled chlamydospores (Stover 1950 With the common 2-year tobacco rotation, the above evidence suggests a possible gradual build-up of T. basicola has been taking place over the years in some tobacco fields. High inoculum potential accompanied by the probable dominance of the more virulent wild type might explain why tobacco cultivars, e,g., Hicks Broadleaf and Delcrest, that exhibited a satisfactory tolerance to the disease in the past, are severely attacked by the fungus in some tobacco fields (Patrick 1964 …”

Infection and colonisation of white clover seedling roots by fungi

“…1978) and red clover (Sundheim 1970). T. basicola is the causal agent of root rot of tobacco (Nicotiana tabacum L.) and bean (Phaseolus vulgaris L.) (Stover 1950;Christou 1962;Pierre & Wilkinson 1970). The ability of C. fertilis, F. avenaceum, and T. basicola to invade roots of young clover seedlings under glasshouse conditions has been demonstrated recently by , although the extent of invasion and damage was very limited.…”