In a year when it seems like we’ve seen just about anything possible that could go wrong in our soybean fields… from too much rain and delayed planting, to too dry, to hail, to soybean gall midge, to delayed canopying and poor weed control, to herbicide-resistant weeds, and let’s not forget about all of those thistle caterpillars… now another problem is starting to show up in some soybean fields in Northeast Nebraska. Frogeye leaf spot is a fungal soybean disease that has been spreading across the state.
Frogeye leaf spot infection can occur at any stage of soybean development, but most often occurs after flowering and is typically in the upper portion of the canopy. Initial symptoms are small, dark spots on the leaves. These spots eventually enlarge to about ¼ inch and the centers of the lesions become gray to tan and have a reddish-purple margin. Individual leaf spots can coalesce to create irregular patterns of blighting on the leaf. Frogeye leaf spot can be difficult to identify because it is easily mistaken for other diseases and disorders such as herbicide injury.
Unfortunately we do not have good thresholds on when you need to treat for frogeye, or a lot of the other fungal foliar diseases. That is because the variety’s resistance to a disease and the stage of growth play a big role in disease development. Generally, the later it is in a plant’s development, the less likely you are to see an economic return from a fungicide application. Treatments at R3 to R4 are more effective while treatments after R5 to R6 rarely give an economic return over the cost of the application.
Also, how fast these diseases develop is largely dependent on the weather. Frogeye leaf spot is most severe when warm, humid weather with frequent rain persists for extended periods. Several days of overcast weather also favor disease development and spread. Overhead irrigation may increase the risk of severe frogeye leaf spot compared to flood or furrow irrigation or dryland production systems.
In the past, QoI (or strobilurin) fungicides have been used and were quite effective at controlling this disease. However, most populations of this disease in states where it has been present longer have developed resistance to this family of fungicides. There have been four counties in Iowa and another four counties in South Dakota where this resistance has been confirmed. This resistance is also suspected in two fields in Nebraska and more testing is currently being done to confirm if this is a resistant strain of the disease.
If you sprayed a soybean field with a straight QoI or strobilurin fungicide and it did not appear to slow the development and spread of this disease, please contact your local Nebraska Extension office. We would like to test a sample from that field to determine if resistance is present and how widespread this problem is. Please collect a gallon food storage bag of symptomatic leaves prior to an additional fungicide application, refrigerate the sample, and then take it to your local Extension office who will forward it to the UNL Plant & Pest Diagnostic Clinic.
Frogeye leaf spot’s effect on yield can vary greatly, depending on disease timing, varietal susceptibility to this disease, and weather conditions during soybean reproductive stages. However, severe cases of frogeye leaf spot have resulted in yield losses of 30 percent or more.
If you have a problem with frogeye leaf spot in your soybeans this year, there are several things you can do to reduce the problem in future years.
• Many soybean varieties have partial or complete resistance to this disease. Visit with your seed dealer and try to identify varieties with the Rcs3 gene. Also, planting high-quality, certified seed will reduce the risk of introducing infected seed into a field.
• Crop rotation helps reduce the disease that overwinters on plant debris left in the field, but a corn-soybean rotation may not be long enough to reduce the problem in fields with a severe problem. Where erosion is not a problem, tillage will reduce the amount of the fungus available to infect the next soybean crop.
• Well-timed foliar fungicide applications can effectively control frogeye leaf spot. Fungicides applied during pod development (R3-R4) are most effective for managing this disease and protecting against yield losses.
For more information on identification and control of frogeye leaf spot in your soybean fields, contact your local Nebraska Extension office.