Colonization of Soybean Roots by Macrophomina phaseolina. C. A. S. Pearson, Department of Plant Pathology, Kansas State University, Manhattan 66506. F. W. Schwenk, and F. J. Crowe, Department of Plant Pathology, Kansas State University, Manhattan 66506; and K. Kelley, Southeast Kansas Branch Station, Kansas State University, Parsons 67357. Plant Dis. 68:1086-1088. Accepted for publication 11 June 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-1086.

Nine soybean cultivars, representing maturity groups III, IV, and V, were monitored for root colonization by Macrophomina phaseolina in naturally infested, fumigated, and fumigated-infested soils. Fumigation reduced initial soil populations by 80% but did not significantly reduce subsequent disease incidence. Cultivars differed in the rates at which they were colonized. On the basis of colonization rates calculated using time after planting and host ontogeny, cultivars can be better evaluated at specific growth stages than at specific times after planting. Bay, Essex, and Forrest (group V cultivars) had the slowest rates of colonization. Sprite (a group III cultivar) had a rate similar to the group V cultivars and yielded like Essex. Fungal population in root systems of soybean at growth stage R8 (harvest maturity) was negatively correlated with yield.