IMPACT OF SULFONYLUREA HERBICIDES ON RHIZOCTONIA ROOT-ROT, GROWTH, AND YIELD OF WINTER-WHEAT

Chlorsulfuron and metsulfuron-methyl were examined for their potential to predispose winter wheat to Rhizoctonia root rot in eastern Oregon. Soil treated or not treated with chlorsulfuron in the field was collected as intact cores, infested with Rhizoctonia solani AG-8, RA. oryzae, or neither, and evaluated in the greenhouse for effects on disease severity, growth, and development of winter wheat. Seedlings in chlorsulfuron-treated soil had more severe root rot and reduced growth than did those in untreated soil. Eight field experiments were performed at two root rot-affected sites to examine the effect of preplant and postemergence applications of chlorsulfuron on tilled and untilled soil. These studies were performed over two crop seasons, and chlorsulfuron + metsulfuron-methyl was included during 1 yr. Seedlings always had more severe Rhizoctonia root rot and grew less vigorously in plots treated with sulfonylurea herbicides. With time, however, the negative impacts dissipated, and herbicide-treated plots had grain yields that differed from untreated plots in only one of eight experiments. Yield was increased by sulfonylurea herbicides in one experiment, and this was apparently not entirely related to differences in weed populations. Although sulfonylurea herbicides interacted with Rhizoctonia spp. to damage roots of winter wheat seedlings, this did not have a direct relationship with the yield of grain under the conditions studied.