Studies on two soilborne fungi of agronomic importance in Arkansas, Macrophomina phaseolina of soybean and Thielaviopsis basicola of cotton

Two soilborne fungi of agronomic importance in Arkansas, Macrophomina phaseolina (Tassi) Goid. of soybean (Glycine max (L.) Merr.) and Thielaviopsis basicola (Berk. & Broome) Ferraris (Syn. Chalara elegans Nag Raj & Kendrick) of cotton (Gossypium hirsutum L.) were studied to determine how two common management practices, irrigation and winter cover crops, might be used to reduce plant damage from these pathogens.Field studies in 1988, 1989, and 1990 indicated that M. phaseolina root densities dramatically increased or decreased in soybean roots as irrigation was terminated or initiated, respectively, at soybean flowering (R2). Irrigated soybeans had fewer microsclerotia in their roots than those receiving less irrigation. Soil water stress was not needed for M. phaseolina root infection or microsclerotia development, however, increased water stress increased colonization. The effects of accumulative soil water stress (calculated from soil water matric potentials) on colonization of soybean roots by M. phaseolina were greatest early in the season during vegetative growth stages and were not significant during late reproductive growth. Microsclerotia densities in soil were not influenced by soil water conditions.Soil populations of T. basicola were significantly reduced in controlled environmental studies with only 0.0016% hairy vetch. Suppression of the populations did not differ between 20 or 24 C, but was greater at 25 C and 28 C than at 20 C. T. basicola suppression in soils amended with hairy vetch was significant in soils maintained at matric potentials of These studies demonstrate that (1) irrigation can help to minimize M. phaseolina colonization of soybean root tissue, especially during vegetative growth stages and early flowering can reduce the infection and root colonization by this fungus. (2) Early season incorporation of a winter cover crop of hairy vetch into the soil can suppress populations of T. basicola in the soil.