REDUCING THE SEVERITY OF PHYTOPHTHORA ROOT-ROT IN SOYBEANS

Phytophthora root rot (PRR) caused by Phytophthora megasperma f. sp. g lycinea (Pmg) can cause severe yield reductions in soybeans [Glycine m ax (L.) Merr.]. This study was conducted to determine if management pr actices (variety selection, metalaxyl [Ridomil] -(2,6-dimethylphenyl)- N-(methoxyacetyl)-DL-alanine methyl ester] use, potash fertilization) could reduce PRR impact on soybean production where PRR was known to b e a problem. The study was conducted at three experimental field sites in both 1988 and 1989. All sites had a history of damage from PRR. Th e management treatments were combined in a complete factorial arrangem ent with four replications. Potassium chloride (0-0-60) was broadcast and incorporated before planting to supply 40, 80, and 160 lb K/acre. Three varieties selected for varying degrees of tolerance or resistanc e to PRR were planted in mid to late May. When Ridomil was used, it wa s applied in-furrow at planting at a rate of 6 lb/acre. Throughout the study, use of K had no effect on yield, stand, or plant height. These parameters, however, were significantly affected by both variety sele ction and Ridomil use. Yield reduction caused by PRR was most severe w hen the susceptible variety, 54-254, was grown. The PRR caused smaller yield depressions when the varieties, BSR-101 and Corsoy 79, were gro wn. Use of Ridomil also had a positive effect on yield, stand, and pla nt height. The magnitude of response to Ridomil was related to variety . This product produced the greatest response when the 54-254 variety was grown. The results of this study show that the soybean grower can use Ridomil and variety selection or a combination of the two manageme nt practices to reduce the severity of PRR.