If and when to make a foliar fungicide application is a decision soybean producers must make each growing season. Every year is slightly different, and it is difficult to predict whether or not a fungicide application will be profitable. However, the risk factors listed below can be used to make an educated decision of whether or not to spray. The greater the number of risk factors in your field, the more likely it is that a fungicide application will be profitable.
- Growth stage of the crop. Disease is most likely to impact yield during pod and seed development since this is when the yield of the crop is being formed. Fungicides are more likely to result in a yield response if disease occurs between the beginning pod (R3) and full seed (R6) growth stages, a period that lasts approximately 30 days. Results from research conducted in 2014 suggest that a fungicide application is most likely to result in a yield response if weather conditions (see #2 below) conducive for disease development occur within two weeks of the R3/R4 growth stage. Overall, the optimum timing for a fungicide is at the R3/R4 growth stage, though later applications can be beneficial if diseases develop later in the season. Fungicide applications after the R5 growth stage are not recommended.
- Weather conditions. Currently, research is being conducted to develop a weather-based fungicide decision aid for soybean. Infection, growth, and sporulation by foliar pathogens occurs when temperatures are moderately warm and during periods of rainfall or high humidity. To determine the relative disease risk based on weather conditions, find a source of hourly temperature, rainfall, and relative humidity data (e.g. Weather Underground or a personal weather station). If over the past week the following conditions occurred on two or more days, the risk of disease development in the crop is high:
• Average daily temperatures from 65-80 °F AND
• 10 or more hours of relative humidity at or above 95% OR a rainfall event
These conditions have occurred in many areas throughout Virginia over the past week as storms have moved through. However, before making a fungicide application consider the other risk factors below.
- Susceptibility of soybean variety to disease. Soybean varieties vary in their susceptibility to diseases, and more susceptible varieties are more likely to need a fungicide to protect yield. Keep in mind that a variety may be relatively resistant/tolerant to one or a few diseases but susceptible to others, so be aware of which diseases are most likely to occur in your field (see #4 and #5 below).
- Field history. Soybean debris is a source of pathogen inoculum, so diseases will show up earlier and be more severe in fields planted to soybean year after year compared to those in rotation with corn or another crop. History of a particular disease in a field should also be considered since it is likely to occur year after year.
- Confirmation of disease in a field. If you see a foliar disease starting to develop in your soybean crop, it may be a sign that it is time to apply a fungicide. Scouting may be useful for some diseases such as frogeye leaf spot, but other diseases (e.g. Cercospora blight) do not show symptoms until long after infections have occurred. Fungicides are more effective when applied preventatively, but applications shortly after the onset of symptoms will slow the progress of the disease. Thresholds have not been established for frogeye leaf spot or other foliar diseases of soybean.
- Yield potential. This year some soybean fields got off to a poor start and never really recovered. If growth is poor and yield potential is low, you do not have much yield to protect and fungicide applications are less likely to be profitable. In contrast, if a crop has high yield potential, a fungicide application can help to maximize that yield, especially if the variety is susceptible to disease.