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Quantitative Enumeration of Macrophomina phaseolina in Soybean Tissues. G. E. Short, Former Postdoctoral Fellow, Department of Plant Pathology, University of Missouri-Columbia, Columbia, MO 65201; T. D. Wyllie(2), and V. D. Ammon(3). (2)Professor, Department of Plant Pathology, University of Missouri-Columbia, Columbia, MO 65201; (3)Assistant Professor, Department of Plant Pathology and Weed Science, Mississippi State University, State College, MS 39762. Phytopathology 68:736-741. Accepted for publication 14 October 1977. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-736.

Macrophomina phaseolina penetrated roots of soybean plants that ranged in age from 7 to 42 days. The fungus invaded cortical tissue intercellularly; later, intracellular invasion occurred and that was followed by the formation of sclerotia. The large size of the sclerotia, the apparent disruption of host cell walls by sclerotia, and the appearance of sclerotia in stem tissues only after loss of color suggested that sclerotia in tissues are indicative of host cell death. A technique was developed for determining mycelial and sclerotial propagative units in infected soybean tissue. The method was used to measure disease development in field-and greenhouse-grown soybeans. Mycelial propagative units always exceeded sclerotia until moribundicity of the host was apparent. Sclerotia were not abundant in root or stem tissues until after the onset of host moribundicity. Following plant death, sclerotia were most numerous in the roots, less abundant in the lower one-half of the stem, and least numerous in the upper one-half of the stem. We suggest that differential propagule enumeration in diseased tissues quantitatively measures the degree of compatibility between selected soybean cultivars and Macrophomina phaseolina.

Additional keywords: charcoal rot, Glycine max.