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Macrophomina phaseolina: Spatio-Temporal Dynamics of Inoculum and of Disease in a Highly Susceptible Crop. J. D. Mihail, Department of Plant Pathology, University of Arizona, Tucson 85721, Present address: Department of Plant Pathology, University of Missouri, Columbia 65211; Phytopathology 79:848-855. Accepted for publication 29 March 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-848.

The spatio-temporal dynamics of microsclerotia of Macrophomina phaseolina were investigated over 2 yr in a field plot of 100 contiguous quadrats (1.52 ◊ 3.05 m), arranged in a 10 ◊ 10 grid. The population of microsclerotia increased from a preplant population of 8.9 microsclerotia per gram to 98.6 per gram after two successive crops of the latex-producing plant Euphorbia lathyris. Inoculum remained spatially aggregated over the 2 yr as measured by Morisitaís index of dispersion. However, the large patch of quadrats with a similar inoculum level, demonstrated by the high-order positive autocorrelation measured by Moranís I statistic, was not detectable at the end of the second growing season, largely as a result of continuing cultivation operations during the first fallow period. Charcoal rot symptom development and mortality were strongly influenced by the timing of host reproduction and unaffacted by plant density. Unexpectedly, during the second epidemic, symptom expression declined following a period of cool weather. During both epidemics, a large patch of high charcoal rot mortality was visually evident and statistically confirmed by the high-order positive autocorrelation as measured by Moranís I statistic. Spatio-temporal autocorrelation analysis was used to find a model that would account for the observed temporal changes in mortality patterns for the two epidemics and in inoculum density patterns during the two fallow periods.