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
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
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
Symptoms usually are present only in the upper canopy. The
affected areas of leaf tissue will often drop out, giving plant leaves a
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.
With all three diseases, management options include crop
rotation, plowing under crop residue and, in some cases, fungicide
“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.
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
“If you are going to apply a fungicide for frogeye
management, it’s important to avoid applying any solo strobilurin fungicides,”
Some populations of frogeye leaf spot might be resistant to
strobilurin fungicides, meaning they cannot be managed by those
“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.