PLANTING DATE AND ROW WIDTH EFFECT ON SEVERITY OF BROWN STEM ROT AND SOYBEAN PRODUCTIVITY

Early planting date and narrow rows increase the yield potential of soybean [Glycine max (L.) Merr.]. The purpose of this study was to determine the effect of these management practices on severity of, and yield loss due to, brown stem rot (BSR) (caused by Phialophora gregata W. Gams). Field studies were conducted for 2 yr (1988 and 1989) at the Hancock, WI, Agricultural Research Station, on a Plainfield loamy sand (mixed, mesic Typic Udipsamments). Treatments were three planting dates (I May, 15 May, and 30 May), two row widths (7 and 30 in.), and two cultivars (BSR-susceptible cv. Corsoy 79 and BSR-resistant cv. BSR 101). Stem and foliar symptoms were less severe on plants in the delayed planting. In 1988, severity of stem symptoms in Corsoy 79 was 2.0 to 2.4 times that in BSR 101 at all planting dates, with the least difference between cultivars at the 30 May planting date. In 1989, foliar symptoms of BSR were 4.5 times greater in Corsoy 79 than in BSR 101 planted 1 May, 5 times if planted 15 May, and 2 times if planted 30 May. Although the severity of BSR in both years was not affected by row width, cultivars responded differently in yield to planting date and row width. Corsoy 79 did not respond in yield to planting date or row width, but BSR 101 did respond to these management practices in the presence of BSR. Averaged over both years, BSR 101, planted on 1 May, yielded 49% more than-Corsoy 79, 39% more if planted 15 May, but was not different from Corsoy 79 at the 30 May planting date. Yield of BSR 101 was 31% less if planted on 30 May compared with a 1 May planting date, and 25% less if planted on 15 May. Yield of BSR 101 was 19% greater if planted in 7-in. rows than in 30-in. rows. Yields of Corsoy 79 at the two row widths were 28% less than BSR 101 in 7-in. rows and 21% less than BSR 101 in 30-in. rows. We conclude that delaying planting reduces severity of BSR; however, any yield advantage from disease control would be offset by lower yield potential of late-planted soybean. Similarly, yield losses to BSR are greater in high yield potential environments, and BSR negates the benefits of management practices designed to improve soybean productivity and profitability.