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Influence of Crop Rotation on Population Density of Macrophomina phaseolina in Soil Infested with Heterodera glycines. L. J. Francl, Research Plant Pathologist, USDA, ARS, Beltsville, MD 20705. T. D. Wyllie, and S. M. Rosenbrock. Professor, and Former Graduate Research Assistant, Department of Plant Pathology, University of Missouri, Columbia 65211. Plant Dis. 72:760-764. Accepted for publication 18 April 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0760.

Soil populations of Macrophomina phaseolina, cause of charcoal rot of soybean and various rots and blights of a large number of cultivated plants, might be managed by crop rotation if the density of infective microsclerotia in soil could be reduced below damaging levels. The soybean cultivar Forrest was grown in monoculture and in a 2-, 3-, or 4-yr rotation with corn, cotton, and grain sorghum from 1980 to 1984 in soil infested with the soybean cyst nematode, Heterodera glycines. This was to determine the population dynamics of M. phaseolina in response to cropping history and the quantitative effect of fungus and nematode on soybean yields. Highest population densities of M. phaseolina were found after soybeans were grown, but often not until early spring or at planting the following year, after crop residue had decomposed. Lower densities of M. phaseolina occurred as soybeans appeared less frequently in rotations. Cotton, in rotation with soybean, consistently reduced the population density of M. phaseolina more than corn-soybean rotations. The effect of sorghum on microsclerotial density was intermediate between corn and cotton. Soybean yield could be related to densities of M. phaseolina and H. glycines in 1984, but not in 1983. There was no evidence of an interaction between H. glycines and M. phaseolina in 1983 or 1984 with respect to yield. Crop rotation may be an effective method of reducing charcoal rot in soybeans, even though the other crops in the rotation are hosts of M. phaseolina.