Host-Specific Pathogens Do Not Account for the Corn-Soybean Rotation Effect

Soybean [Glycine max (L.) Merr.] generally yields more when rotated with another crop rather than grown continuously. The specific reasons for this yield response are unknown. The objective of this study was to investigate the extent to which a soybean-corn (Zea mays L.) rotation limited the buildup of host-specific pathogens of soybean, and to evaluate whether plant diseases were involved in the rotation effect. Incidence and severity of seven diseases and seed yield were recorded at two locations in the field in 1987 and 1988 for soybean maintained in four cropping sequences ranging from annual alternation with corn to 1,2, or 5 yr of continuous cropping. Brown stem rot (BSR) caused by the fungus Phialophora gregata (Allington & D.W. Chamberlain) W. Gains was found to be the most prevalent disease during both years and at both locations. Rhizoctonia root rot (Rhizoctonia solani Kühn) and stem canker (Diaporthe phaseolorum Cooke. & Ellis Sacc. var. Cavlivora Athow & Caldwell) were noted in 1987 at both locations among <5% of all plants monitored. The BSR-prevalence ratings were relatively high (mean 60%) for Hodgson 78, a susceptible cultivar, and relatively low (mean 22%) for BSR101, a resistant cultivar. Seed yield data from the various cropping sequences for the two cultivars indicated no clear relationship between BSR severity and seed yield. In 1987, rotation with corn resulted in a yield benefit to both cultivars. In 1988, both yield and development of BSR were adversely affected by drought. A comparison of the seed yield of severely infected and uninfected plants within cropping sequence indicated that BSR had a minimal effect on soybean yield in 1988. Therefore, within the conditions of this study, the yield benefit to soybean from rotation with corn did not appear to be due to the reduced incidence of plant diseases. Contribution from the Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108. Paper no. 19,570 of the Scientific Journal Series, Minn. Agric. Exp. Stn.