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Density and Spatial Pattern of Propagules of Macrophomina phaseolina in Corn Rhizospheres. O. M. Olanya, Graduate assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616, Current address: International Institute of Tropical Agriculture, P.M.B. 5320, Ibadan, Nigeria.; C. Lee Campbell, Associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 79:1119-1123. Accepted for publication 17 June 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-1119.

Propagule density of Macrophomina phaseolina in soil was monitored from May to September 1985 at a total of eight sites chosen to represent a range of propagule densities in two fields. Corn seedlings were removed from one of two adjacent plots of 25 contiguous quadrats at each site to give reduced root density. Propagule density did not vary significantly within a plot or between adjacent plots through the season. From a regression of Lloyd’s index of mean crowding (*) on mean propagule density, individual propagules, i.e., colony-forming units of microsclerotia, were the basic unit of contagion in eight of 10 cases and these colony-forming units had a random spatial pattern. Monthly values of Morisita’s index of aggregation ranged from 1.06 to 1.15 in Field A (Edgecombe County) and from 1.12 to 2.17 in Field B (Wayne County), confirming a random or slightly aggregated pattern of propagules. For frequency count data of propagule density, the Poisson distribution was appropriate in 18 of 40 and 20 of 40 cases for location A and B, respectively; however, the negative binomial distribution did not describe any data set. Because density and spatial pattern of propagules of M. phaseolina were generally similar throughout the growing season, this suggests either a lack of detectable reproductive activity and decay of propagules in soil during periods of active corn root growth or equal rates of propagule reproduction and death.