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Morphological Instability on a Chlorate Medium of Isolates of Macrophomina phaseolina from Soybean and Sorghum. G. L. Cloud, Former graduate student, Department of Plant Pathology, University of Arkansas, Fayetteville 72701, Present address: Division of Plant and Soil Sciences, Plant Pathology and Agricultural Microbiology, 401 Brooks Hall, P.O. Box 6057, West Virginia University, Morgantown 26506-6057; J. C. Rupe, associate professor, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. Phytopathology 81:892-895. Accepted for publication 1 March 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-892.

Soybean and sorghum were grown separately in soil infested with single, sclerotial isolates of Macrophomina phaseolina collected from either soybean or sorghum and together in soil infested with both isolates. Isolates were characterized by their growth on a chlorate-amended medium as having restricted or dense growth for the soybean or sorghum isolates, respectively. Roots from each treatment were assayed periodically for M. phaseolina, and the growth pattern on the chlorate-amended medium of the resultant isolates was determined. Significantly greater infection of soybean roots occurred when soybeans were grown in soil with the soybean isolate compared with either soil infested with both isolates or soil infested with the sorghum isolate. There were no differences in colonization of sorghum roots when sorghum was grown in soil infested with either or both isolates. The chlorate growth patterns of M. phaseolina obtained from infested soil in microplots or isolates in association with their respective hosts remained constant throughout the experiment. However, the chlorate growth patterns of isolates recovered from their alternative hosts shifted during reproductive development of the host plants.

Additional keywords: charcoal rot, Glycine max, Sorghum bicolor.