Are you dealing with Septoria brown spot in your soybean fields? You already know this and other common foliar fungal diseases wreak havoc on soybean crops, but you might not know how to start developing an economically sound management plan.
While fungicides may feel like the cure-all, there are times when a foliar fungicide isn’t necessary or economically wise. So, how do you know whether you should treat a particular incidence of Septoria brown spot? We’ve compiled some guidelines to help you decide when and if you need to treat your fields. Learn how external conditions, disease severity and other factors impact the economic returns fungicide treatments may produce.
The disease is most severe where soybeans are grown continuously, especially in reduced tillage operations, since Septoria brown spot overwinters on infested plant debris.
When to spray fungicide for Septoria brown spot in soybeans
When Septoria brown spot is severe
Symptoms of Septoria brown spot in soybeans begin as small, irregular, dark brown spots on leaves. These often merge into larger brown blotches, particularly along leaf edges or leaf veins. In its early stages it can be misidentified as bacterial blight since both diseases have similar symptoms. However, infection of Septoria brown spot typically begins on oldest leaves and bacterial blight occurs on new leaves.
When a Septoria brown spot infection is severe, fungicide treatment is recommended. Signs of severe disease include yellowed leaves with large irregularly shaped brown blotches and subsequently premature defoliation.
With 25% to 50% premature canopy defoliation, yield loss estimates can reach 15%.¹ Additionally, severe Septoria brown spot infection usually leads to smaller seed size.
When the growing season is wet
Septoria brown spot in soybeans cannot survive in hot, dry conditions; hence, the risk of yield loss is higher if growing conditions are wet. Septoria brown spot epidemics are common during growing seasons with frequent rainfall since the disease spreads through splashing water droplets and develops best when periods of leaf wetness extend from 6 to 36 hours. The ideal temperature for Septoria brown spot development is 77 F, but disease symptoms can develop in temperatures ranging from 60 F to 85 F.
When the growth stage is optimal for foliar treatment
Since fungicide should be applied to lower leaves prior to canopy closure, for the most effective control experts recommend treatments be made before the R3 growth stage.
Nonfungicidal control for Septoria brown spot in soybeans should also be considered, including selecting seed tolerant to the disease, avoiding back-to-back soybean cropping, and managing weeds that act as hosts for Septoria brown spot such as velvetleaf and nightshade.
Experts are available to help you make your decisions. Reach out to your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
________________________________________________
Endnotes
- Giesler, Loren J. “Brown Spot (Septoria Leaf Spot).” CropWatch, 22 July 2019, cropwatch.unl.edu/plantdisease/soybean/brown-spot. Accessed 25 Oct. 2024.
