The effect of fungicides on soybean in Iowa applied alone or in combination with insecticides at two application growth stages on disease severity and yield

Bestor, Nathan
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The use of foliar fungicides on soybean, Glycine max [L.] Merr., has not been recommended in most years in Iowa. But with economic factors such as price of grain, rebate offers on early purchases of fungicides, and rising production costs, the use of fungicides and other pesticides has increased dramatically over the last decade. There is little information regarding the impact of spraying foliar fungicides in Iowa. We tested the effect of fungicide applications at growth stage R1 (beginning of flowering) and R3 (beginning of pod set) on disease control and yield responses. Diseases present in this study included Septoria brown spot, caused by Septoria glycines Hemmi, Cercospora leaf blight, caused by Cercospora kikuchii ((T. Matsu. & Tomoyasu) Gardener)), and frogeye leaf spot, caused by Cercospora sojina Hara. Insecticides also have been used increasingly in soybean production, primarily to manage soybean aphid, Aphis glycines Matsumura, which is a problem somewhere in Iowa most years. We also studied the yield benefit of adding a fungicide to an insecticide at R1 and R3 applications. Soybean grain prices are near record levels and the yield responses of pesticides needed to break even is as low as ever. We used Bayesian inference methods to determine the probabilities of fungicides making an economic return under various estimated economic conditions designed to simulate application costs from airplane applied and ground applied fungicides.

The second portion of this paper examines anthracnose stem blight, caused by Colletotrichum truncatum Schwein. Anthracnose stem blight can be a yield robbing disease, especially in the southern United States. However, it is not understood how anthracnose stem blight affects yield in Iowa and if fungicides are warranted for its control. We examined how various fungicides control anthracnose stem blight at applications R1 and R3. We also examined how anthracnose stem blight severity (%) was related to yield and individual yield components, to determine if anthracnose stem blight was detrimental to yield in Iowa.

The third portion of this paper discusses the importance of fungicide trials in applied research today, especially in light of fungicide use on soybean being relatively new in Iowa and other Midwestern states. Our main objective was to answer questions surrounding the validity of small plot research since on-farm trial data are preferred by some growers and agribusiness professionals. Using analysis of variance (ANOVA) we compared yield responses of pyraclostrobin and premix products used in small plot and on-farm research trials in Iowa from 2008 to 2010.

Anthracnose, Application timing, Brown spot, Fungicide, Insecticide, On-farm research