Frogeye leaf spot on soybean. (Purdue Botany and Plant Pathology/Greg Shaner)
WEST LAFAYETTE, Ind. - A variety of foliar diseases have shown up in soybean fields across Indiana this season and need to be correctly identified before producers consider control options, a Purdue Extension plant pathologist says.
Moderate temperatures, moisture, and humidity have led to many foliar diseases appearing in certain areas of Indiana.
"We're seeing increased levels of diseases such as bacterial blight, downy mildew, and, in some cases, there's brown spot in the lower canopy," Kiersten Wise said. "Growers may be concerned about these diseases, but, in most cases, these particular diseases are of minor economic importance."
Symptoms for both bacterial blight and brown spot include brown to black lesions on leaf surfaces. Bacterial blight lesions are angular and surrounded by a yellow ring or halo, and they may have a water-soaked appearance. Symptoms usually are present only in the upper canopy. The affected areas of leaf tissue will often drop out, giving plant leaves a tattered appearance.
Symptoms of brown spot on soybean (left) compared with symptoms of bacterial blight. (Purdue Botany and Plant Pathology/Kiersten Wise)
Brown spot lesions usually remain in the lower canopy of the plant but can sometimes spread to upper leaves. They might have yellow halos, and affected leaves will usually turn yellow as the disease progresses.
Symptoms and signs of downy mildew are pale green to yellow spots on upper leaf surfaces and tufts of gray spores on lower leaf surfaces during moist conditions. Sometimes producers also might notice white fungal growth on the interiors of soybean pods.
Listen to Wise talk about foliar diseases in soybean.
With all three diseases, management options include crop rotation, plowing under crop residue and, in some cases, fungicide application.
"If you're going to apply a fungicide, you want to make sure you get a good disease diagnosis first," Wise said.
She said fungicides will not control bacterial blight, and that commonly used fungicides are not labeled for use on downy mildew.
It's also important to properly identify soybean diseases so producers know which fields are most susceptible to which diseases. Tillage and planting less susceptible varieties can help lower the risk of diseases developing in future soybean crops.
Wise said another foliar disease, frogeye leaf spot, also is present in some Indiana fields. This disease is significant because it can reduce yields when present at high levels. Disease symptoms include brown to gray circular spots on leaves that are surrounded by purple halos.
Listen to Wise talk about frogeye leaf spot.
Fungicides can be used to control frogeye leaf spot, but producers should consider the timing of disease onset, level of disease within a field, plant growth stage and variety susceptibility before deciding on an application.
"If you are going to apply a fungicide for frogeye management, it's important to avoid applying any solo strobilurin fungicides," Wise said.
Some populations of frogeye leaf spot might be resistant to strobilurin fungicides, meaning they cannot be managed by those fungicides.
"We want to avoid and delay resistance from building up in fields, so we would recommend applying triazole fungicides or triazole-strobilurin mixes to fields that have frogeye leaf spot," Wise said.