Yield losses of soybeans due to Phytophthora megasperma f.sp. Glycinea

Abstract

Field trials were conducted in two seasons to determine the influence of Phytophthora megasperma f.sp. glycinea on seed yields of soybean genotypes with different levels of resistance. The fungicide metalaxyl, applied as a seed dressing and soil drenches, was used to control the disease. The cultivar Davis, which has high field resistance, did not suffer significant losses in either year. whereas the yield depression of less resistant genotypes ranged up to 72%, depending on the genotype and year. In 198 1 - 82 high death rate of plants occurred early in the season, but yield losses were less than in 1982-83 when mortality occurred late in the season. The declining ability of surviving plants to compensate for disease losses during the latter growth stages may account for the differences in yield losses between seasons.Genotypes with immunity to P. megasperma f.sp. glycinea have been widely grown in the United States because they offer complete protection against some races. However, new pathogenic races have developed quickly, and previously immune genotypes have then suffered severe yield losses. The results of these trials demonstrate that yield losses can be prevented without resorting to fungicide treatments. It is likely that a stable long-term solution to phytophthora root and stem rot will be conferred by field resistance.