Propagule Densities of Macrophomina phaseolina in Soybean Tissue and Soil as Affected by Tillage, Cover Crop, and Herbicide

All current commercial soybean cultivars are susceptible to charcoal rot, a disease caused by Macrophomina phaseolina. Efforts to manage this disease through nongenetic means have not been effective. However, the combined effects of tillage, cover crop and herbicide use, and their roles in the population dynamics of this fungus have not been fully investigated. A field experiment was conducted in 2002 through 2004 at Stoneville, MS, to determine the population dynamics of Macrophomina phaseolina (Tassi) in soybean stem and root tissues at harvest and in soil at planting and harvest as affected by tillage, cover crop, and herbicide. The tillage treatments were conventional tillage (CT) and no-tillage (NT); the cover crops were hairy vetch, rye, and no cover crop; and the herbicide treatments were glyphosate- and non-glyphosate-based applications. Analysis of variance indicated that there was an effect due to tillage, cover crop, and tillage by year interaction on colony forming units (CFU) of M. phaseolina recovered from soybean tissue. Colony forming units in soybean tissue were greater under the CT than NT and were greater for hairy vetch and no cover crop than rye. Regardless of the cover crop system used, CFU in tissue was greater for CT in 2002 than in 2003 and 2004. Application of glyphosate did not affect the CFU in stem and root tissues or in the soil. The CFU from soil at harvest was significantly higher than at planting. The CFU in soil at planting and harvest was only affected by tillage and not by cover crop system. The CFU from stem and root tissues was greater than in soil suggesting that quantification of CFU in tissue may provide a better estimate of treatment effects at harvest. These results also suggest that charcoal rot may be better managed in the NT rather than in the CT system.