Alabama producers have long been planting soybeans in regular crop rotations. These producers go to great lengths to familiarize themselves with diseases common in soybeans. In 2015, producers dealt with 14 different diseases during the growing season.
Dr. Edward Sikora, professor and Extension plant pathologist with the Alabama Cooperative Extension System, said three different plant diseases have already been spotted in crops growing thus far this season. This included Septoria brown spot, soybean rust and the thrips transmitted-soybean vein necrosis virus.
Sikora said symptoms of many foliar diseases are often hard to differentiate from one another, even for a plant pathologist. This can make diagnosing the disease in the field quite difficult. There are four foliar diseases Sikora expects to see in Alabama soybeans within the next month: brown spot, target spot, downy mildew and frog-eye leaf spot.
Septoria Brown Spot
Septoria Brown Spot gets its name from the chocolate brown spots on the foliage. The spots are often surrounded by a yellow halo. Symptoms typically begin on the oldest leaves, than slowly move up the plant. This disease can be easily confused with soybean rust. Brown spot is not a yield reducer like rust, so being able to differentiate between the two diseases is critical.
Sikora said brown spot is common in fields where soybeans are planted year after year because the fungus survives the winter on plant debris left after harvest. He said Brown Spot is not a disease he is concerned with and has never recommended a fungicide application for its control.
Downy mildew is another fungal diseases characterized by bright yellow, irregularly shaped spots on the upper leaf surface, visible in the upper plant canopy. On the underside of leaves, fuzzy, gray tufts of fungal growth can be seen particularly with high humidity or when leaves are wet with dew.
This disease thrives under conditions of high humidity and extended periods of wet weather. Sikora expects to see it popping up as a result of the spotty thunderstorms we’ve experienced in recent weeks. Like brown spot, downy mildew rarely causes extensive damage to soybeans but it is important for the grower to properly identify downy mildew to avoid unnecessary fungicide applications.
Target spot, also a common issue for cotton producers, has been a disease Sikora said he has seen in his soybean trials for the past 10 years. Typically, target spot isn’t visible until the canopy begins to close.
The disease shows up in the lower canopy as reddish brown lesion surrounded by a yellow halo. Concentric rings may develop in older lesions, hence the name target spot.
Target spot can cause defoliation, but is not considered a threat to soybean yield at this time. Sikora said he and his counterparts will continue to monitor the effects of the disease on yield and determine when fungicide applications for target spot control may be economical for farmers.
Frogeye Leaf Spot
Frogeye leaf spot is one disease Sikora is concerned with every year. This disease has been reported to cause yield losses of 20 to 40 percent. Initial symptoms are small tan spots with purple or brown margins on leaves in the upper canopy, so they are relatively easy to detect. Heavily infected leaves may have a tattered appearance and premature defoliation is possible.
Warm, humid weather promotes the development of frogeye leaf spot. Rain and heavy dews during bloom and early pod set will favor disease development.
His best suggestion is to treat before symptoms show up.
Frogeye can be managed through the use of frogeye resistant soybean varieties or with a timely fungicide application.
“There are frogeye leaf spot resistant varieties, so if you had this problem last year I would hope you chose a frogeye resistant variety to grow in 2016,” Sikora said. “If you are growing a susceptible variety, you should consider a fungicide application at the R3 to early R4 soybean growth stage when weather conditions favor disease development.”
Sikora has detected several populations of frogeye in Alabama that are resistant to strobilurin-type fungicides. Using a product that has active ingredients from at least two different fungicide classes is important for resistance management.