Date of Award

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Plant Pathology and Crop Physiology

First Advisor

Johnnie P. Snow

Abstract

The national, local, and in-field distribution of Calonectria crotalariae (Loos) Bell and Sobers, as reflected in the distribution of Cylindrocladium black rot (CBR) of peanut, red crown rot (RCR) of soybean and microsclerotia densities in soil, respectively, are described. The current distribution of RCR in Louisiana is probably due to the introduction of the pathogen on indigo (Indigofera tinctoria (L.)) and to subsequent movement by man. RCR has been reported in 17 Louisiana parishes. In field studies, delayed soybean planting was efficacious in reducing RCR incidence. Effects of delayed planting on yield were maturity group dependent and confounded with RCR effects. A statistical model was constructed that predicted 50% yield loss at 100% RCR symptomology on a susceptible cultivar. Associations between weed infestation and RCR incidence were observed in the field, and indicated that the amino acid biosynthesis-inhibiting herbicides glyphosate, imazaquin, and chlorimuron ethyl may have fungitoxic properties. Glyphosate significantly reduced in vitro C. crotalariae colony area, at field-use rates. Preplant applications of glyphosate also reduced RCR incidence in two field studies. To study the effects of these materials on microsclerotia production, a technique was developed which employed fungal culture on solid medium in combination with colorimetry and a standard curve to indirectly count microsclerotia. Imazaquin significantly reduced production microsclerotia in vitro. Chlorimuron ethyl was less effective than imazaquin. Simulated repeat applications of these materials were effective in restricting colony growth. In field studies, RCR incidence was not significantly reduced by preplant imazaquin applications. Long-term cumulative effects of the herbicide on RCR are postulated. A procedure was developed and used over a six year period to evaluate cultivars for reaction to RCR. These trials showed clear differences in RCR reaction among cultivars. Resistant cultivars can be selected within any maturity group. Location instability of RCR reaction on several cultivars indicates the pathogen population may be heterogenous. Two mechanisms of resistance are proposed.

Pages

142

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