Tillage effects on Macrophomina phaseolina population density and soybean yield.

Reduced-tillage production systems have become popular with soybean farmers in the United States. During 1992 to 1994, soil and soybean root samples were collected from disk-tillage, no-tillage, and moldboard plow treatments in an experiment conducted since 1979 at the University of Tennessee in Jackson. The objective was to determine if reduced-tillage production systems affected charcoal rot incidence and severity. There were no differences at planting in soil population density of Macrophomina phaseolina in the 0- to 15-cm layer of soil among treatments. However, the soil population density of M. phaseolina in disk-tillage and no-tillage plots was significantly greater in the 0- to 7.5-cm layer of soil than in the 7.5- to 15-cm layer. Similarly, soil population densities were significantly greater in the 0- to 7.5-cm layer of soil in no-tillage than in either disk-tillage or moldboard plowed plots. Tillage did not affect the number of M. phaseolina infection sites in 6-week-old plant roots or the percentage of root segments of physiologically mature plants colonized by M. phaseolina. There was no significant correlation between soybean yield and M. phaseolina soil population density. These data suggest that longterm tillage did not affect charcoal rot incidence and severity.