Date of Award

1988

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Plant Pathology and Crop Physiology

First Advisor

Johnnie P. Snow

Second Advisor

Gerard T. Berggren

Abstract

The role of a toxic metabolite in the stem canker disease of soybean was investigated. Isolates of Dpc produced a metabolite in the culture filtrate which produced symptoms similar to stem canker on soybean plants. The severity of symptoms was dose dependent. The isolate from Opelousas, Louisiana produced significantly higher amounts of the toxin than the other isolates. A positive correlation was seen between the amount of toxin produced and the severity of disease produced by the Dpc isolates. Among twelve plant species tested, only soybean and lima bean, were sensitive to the toxin. Reaction of five soybean cultivars to the culture filtrates of the Dpc isolates did not reflect the resistance of the cultivars to the pathogen as seen in the field. Spectral analyses indicated that the toxic metabolite is a low molecular weight aromatic compound, has one or more hydroxyl groups, methyl groups and C=C, and has a reducing nature. The trifoliates were not sensitive to cytochalasins H and B, but were sensitive to a toxic metabolite produced by a Phomopsis sp. isolated from wilted pine trees. Some of the chemical properties of Dpc toxin are similar to the Phomopsis/pine wilt toxin. Preliminary studies indicated a similar molecular ion peak of the two toxins. The initial symptom of toxin action was a red discoloration of midrib and veins before chlorosis was observed. The toxin induced yellow autofluorescence of vascular bundles. The fluorescence was also associated with lesions on the plant parts. The discoloration of the midrib and veins, and lesions on the plant were due to accumulation of phenols which are alkali soluble, can be extracted and separated by thin layer chromatography, and react with Folin-Ciocalteu reagent and ferric chloride. Two phenolic compounds, and a compound showing yellow fluorescence, not seen in untreated samples were observed in the toxin treated and fungus inoculated samples suggesting a common host response to toxin action and fungal inoculation. The role of toxin in the stem canker disease appears to be as a virulence factor.

Pages

70

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