In most soybean-producing areas, foliar fungicide use has increased to help manage common foliar diseases and potentially improve yields. Before using a foliar fungicide, it is important to scout and determine the type of disease(s) present. Foliar fungicides only control fungal diseases, particularly anthracnose, Septoria brown spot, Cercospora leaf blight, frogeye leaf spot, pod and stem blight, and soybean rust (Figure 1). Bacterial diseases such as bacterial blight or bacterial pustule, and viral diseases such as the soybean vein necrosis virus are not controlled by fungicides.
Not all fungal diseases are well controlled by foliar fungicides, mainly due to when the infection occurs. For example, stem canker can cause severe yield losses in soybeans, but results of fungicide applications have often been inconsistent. This is likely because infection occurs during early vegetative growth, and fungicides are generally not applied in time to prevent it. Charcoal rot and sudden death syndrome also can cause severe yield losses, but because infection occurs in the roots, they are not controlled by foliar fungicides.
To better understand the value of foliar fungicides in soybeans, DuPont Pioneer conducted extensive on-farm and small-plot research trials. On-farm trials were conducted over 4 years at various locations in 11 states and 2 Canadian provinces. Researchers conducted 148 trials comparing untreated soybeans to soybeans treated with foliar fungicides. Across all of these trials, 82% resulted in a positive yield response, and the average yield response to a foliar fungicide application was 2.5 bu/acre (Figure 2).
Figure 2. Average soybean yield response to foliar fungicide across DuPont Pioneer on-farm trials conducted from 2007 to 2011.
In addition to on-farm trials, DuPont Pioneer conducted small-plot, replicated trials in Illinois, Indiana, Iowa, Minnesota and Nebraska over 4 years. These trials compared yield response to foliar fungicides applied at different soybean growth stages (Table 1). Results showed that applying a fungicide around R3 (beginning pod development) provided the greatest average yield response (for a single application) over the untreated check.
Table 1. Average yield response for fungicide treatments applied at different growth stages.
N* = Number of comparisons