Weather patterns could set the stage for certain diseases. Here’s what to watch for…
Flooding in parts of the Midwest, wet conditions in other areas and dry conditions elsewhere make for an interesting summer. Here are soybean disease issues you may encounter, depending on the hand you’ve been dealt by Mother Nature.
If you experienced ample to excess moisture, depending upon where you live, white mold, sudden death syndrome and septoria brown spot could show up. Whether they become serious will depend on weather conditions during July and August.
White mold. “While increased precipitation should increase risk, warmer temperatures may slow it down,” explains Daren Mueller, Extension plant pathologist at Iowa State University. “Development will rely on continued favorable weather conditions for the disease.”
Mueller looked at weather data for three random weather stations in northwest, north-central and northeast Iowa for the four worst years for white mold in Iowa since 1999. They were 2004, 2009, 2015 and 2019. Precipitation for April, May and June combined was 16% greater than the 30-year average, and temperatures were 0.7% cooler than the 30-year average.
For the 2024 season, through June 20, precipitation ran 37% ahead of average, and temps were 2.9% warmer.
What’s important now are rainfall and temperature trends in July and August. For the four worst white mold years, precipitation in July and August was 12% higher than average, but temperatures were 3.9% cooler.
“Moisture levels would need to continue high through July and August, and temps would need to cool to get an outbreak,” Mueller says.
What about fungicides? “In recent years, we’ve encouraged folks to hold off on their first fungicide application until R2 or even R3,” Mueller explains. “However, with all this rainfall, keep a close eye on fields at high risk for white mold.
“You may need to apply fungicide earlier, at R1 to R2, especially if rain continues and it cools down. Field Prophet, available at no charge, is a useful monitoring tool. Remember that not all fungicides are equally effective against white mold.”
Sudden death syndrome. Mueller did comparisons for the four worst SDS years too. Note that they’re different than the worst white mold years. SDS years were 1999, 2010, 2014 and 2016. For SDS, precipitation averaged 33% higher in April, May and June, and temperatures were 1.8% warmer. In 2024, by June 20, precipitation was 37% above average and temperature was 2.9% higher.
Still, July and August will tell the story, Mueller says. In SDS years, average rainfall was 33% above average in July and August, while temperatures were 0.6% cooler than average.
“There are no foliar fungicides for SDS,” Mueller says. “Our data shows that all foliar fungicides tested have no effect on SDS.”
Septoria brown spot. In southern Iowa, Greg Shepherd, an agronomist with Beck’s, observed some septoria brown spot in late June. While typically not as devastating to yield, this disease is also linked to wetter weather.
The Purdue University Corn and Soybean Field Guide indicates brown spot is favored by prolonged periods of leaf wetness. The kicker may be temperature. This fungus prefers moderate temperatures in the range of 60 to 85 degrees F.
Lesions are small, angular and brown, later becoming brick red to chocolate brown. These lesions are often found only in the lower canopy until plants approach maturity.
Source: https://www.farmprogress.com/soybean/keep-eye-out-for-these-soybean-diseases