The two most common foliar diseases in Illinois that will respond to fungicides are Frogeye leaf spot and Septoria brown spot. Cercospora leaf blight is a close runner-up. Both Frogeye leaf spot and Septoria brown spot can be considered diseases that favor wet, warm, humid weather—conditions we experience here in Illinois. Sometimes to help farmers scout, I tell them that if they are seeing Gray leaf spot in their corn, then it is time to scout their soybeans. These diseases can infect early in the soybean growth stage if conditions are favorable. The other less-common foliar diseases in soybeans such as bacterial blight and downy mildew will not respond to fungicides because one is a bacterial disease and the other is an oomycete (fungal-like).
Septoria Brown Spot
Another factor that can enhance the risk of disease is the amount of residue in fields, because disease overwinters on that residue; therefore, no-till and soybeans as a previous crop may increase disease severity. Frogeye leaf spot could also be lurking on the seed, which is why it’s important to have pathogen-free seed and use fungicide seed treatments. Finally, soybean varieties can differ in their susceptibility to foliar diseases. There are no disease ratings for Septoria brown spot, but you can check soybean varieties for their resistance to Frogeye leaf spot.
Cercospora Leaf Blight
Most research indicates that the yield response or return on investment increases with a fungicide when foliar disease is present. Timing of a fungicide application is critical and most research has proven that an R3 (beginning pod) fungicide application is best to protect the plants during grain fill. Earlier fungicide applications are usually a waste of money. The exception to this rule is white mold, when R1 (one open flower) is the best time to apply fungicides. White mold is a cool, wet season disease and will be more severe on certain varieties planted in fields with a history of this disease, highly populated and with early canopy closure.
When scouting for Septoria leaf blight, look at lower leaves first and if it appears on greater than a third to one-half of the canopy, then you may want to think about a fungicide application. Only a few universities have done research trying to establish economic thresholds for fungicide application for Frogeye leaf spot. Some say to pull the trigger on fungicides if you find 1 or 2 lesions every 25 feet of row. Others say a fungicide may be warranted for Frogeye leafspot if you have a susceptible variety, the weather is favorable (humid and warm), and you have found disease before the R3 growth stage.
Frogeye Leaf Spot
I have talked to many farmers over the past few years and many claim they are seeing a yield response from fungicides. However, as grain prices remain low, many may start to scout fields or start asking their agronomist, crop consultant or university specialist to help them identify the brown spots on their soybeans. Does the type of fungicide applied matter? It could. Purdue University has shown that some fungicide active ingredients may give you more efficacy for control of certain diseases.
You can find a chart with the Fungicide Efficacy for Control of Soybean Foliar Diseases here.
The other issue to remember is that several years ago, it was documented that there were strains of Cercospora sojina (Frogeye leafspot) resistant to strobilurin fungicides; therefore, this class of fungicides may not kill these resistant strains.
In the future, I suspect that more research will be done on the effects of fungicide on retention of green leaf area and longer seed fill, but for now, the more foliar fungal disease that you see in your field, the more return on investment you should see from the use of fungicides.
Stephanie Porter is a Sales Agronomist with Burrus® Hybrids with responsibilities that include educating growers and Burrus staff on all types of pests, weeds, diseases and other agronomic issues that affect corn, soybean and alfalfa production. Her territory encompasses Southern Wisconsin as well as Northern, Eastern and Southern Illinois.