Impact Of Foliar Diseases On Soybean In Ohio

Abstract: One of the most common foliar diseases of soybeans in Ohio is Septoria brown spot, caused by Septoria glycines Hemmi. The economic impact caused by Septoria brown spot has not been documented in this State, but the disease has been developing earlier in the season. In contrast, during 2006 frogeye leaf spot caused on average 35% yield loss across 500,000 acres in Ohio. While frogeye leaf spot, caused by Cercospora sojina Hara, is common in the southern U.S., no documented losses have occurred previously in Ohio. In the last three years there has been a strong pressure by industry to increase fungicide applications for a perceived "plant health" benefit. Therefore, the objectives of this study were i) to determine the yield losses associated with S. glycines on soybean, and ii) to determine the cause(s) of the increase in prevalence of C. sojina in Ohio. The yield loss associated with S. glycines was determined with two approaches: weekly applications of chlorothalonil and timed applications of pyraclostrobin, tebuconazole, and flutriafol. Both of these approaches allowed for different levels of Septoria brown spot to develop. The ability of C. sojina to overwinter and the prevalence of seed transmission were both evaluated to determine what was the source of primary inoculum. In addition, a collection of C. sojina isolates from Ohio was compared to isolates from the southern U.S. to determine if there was a shift in temperature growth curves which would have favor infections of C. sojina during the growing season in the northern U.S. This same isolate collection was also used to determine if the Rcs3 gene was effective against Ohio's C. sojina population. From this three-year study it was concluded that brown spot in Ohio does contribute to reductions in yield, albeit at 196 to 293 kg/ha. Strobilurin fungicides alone or in combination with a triazole, reduced the severity of brown spot, but did not maintain the yield potential of plants grown under conditions of water stress. However, this family of fungicides may increase soybean yields under certain conditions. In addition, it is important to consider the net return in order to determine if producers have a real benefit or only an extra expense. The use of a highly susceptible cultivar widely planted during 2006, and warmer winter temperatures were both the cause of the increase in prevalence of C. sojina during the previous two years in Ohio. If winter indices continue to increase, then these conditions will enhance survival of this and other plant pathogens that did not previously overwinter at these latitudes.