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Populations of Macrophomina phaseoli in Soil as Affected by Fumigation and Cropping. Tsuneo Watanabe, Former Research Assistant, Department of Plant Pathology, University of California, Berkeley 94720, The senior author is now Plant Pathologist, National Institute of Agricultural Sciences, Tokyo, Japan; Richard S. Smith, Jr.(2), and William C. Snyder(3). (2)Research Plant Pathologist, Pacific Southwest Forest and Range Experiment Station, Forest Service, USDA, Berkeley, California 94704; (3)Professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 60:1717-1719. Accepted for publication 12 June 1970. DOI: 10.1094/Phyto-60-1717.

A technique for the direct isolation of Macrophomina phaseoli from soil by the differential flotation of microsclerotia is described. Using this technique, the effects of soil fumigation and subsequent cropping on the populations of M. phaseoli in soil were determined. Populations of microsclerotia, greatly reduced by soil fumigation, increased very little after 2 years in fallow soils or soils cropped to ponderosa pine. In soils sown to white fir, a more susceptible species, there was a greater increase in the M. phaseoli populations. These studies indicate that with proper soil fumigation, the seed beds need not be fumigated every year in order to control M. phaseoli losses.