Significant disease pressure is showing up in corn and soybeans earlier this summer than what Daren Mueller saw in 2024 crops. The Iowa State University plant pathologist says the early onset of disease pressure he has seen in Iowa – southern rust in corn, and sudden death syndrome (SDS) and white mold in soybeans – is concerning.
While he is closely tracking diseases in soybeans, he says there is “worse news” about the diseases showing up in corn.
“We actually are seeing a lot of southern rust in Iowa already. That is the one thing that I’m probably the most nervous about. I think we’ve found it in six or seven counties now,” he says.
Along with Iowa, the Crop Protection Network (CPN) has confirmed southern rust in at least 13 other states – including Illinois, Kansas, Missouri, Nebraska – and a “probable” finding in Indiana.
Southern rust can be difficult to distinguish from common rust.
While both are fungal diseases that affect corn and share similar symptoms, they have distinct characteristics. Common rust tends to favor cooler, wetter conditions, while southern rust prefers warmer, humid weather.
Southern rust can also be more aggressive and potentially more yield-damaging than common rust, especially in later planted fields.
“Southern rust likes heat, and we don’t have good levels of resistance. And it can move very, very quickly through a cornfield,” Mueller told AgriTalk Host Chip Flory on Thursday. CPN reports yield losses up to 45% have been reported with severe infections.
“Last year was the first year in a long time where we really had to deal with southern rust, and we’re finding it about 10 days earlier than last year.”
The good news in Iowa, so far: the incidence (number of plants affected) and severity (area of leaf diseased) are low, adds Alison Robertson, Iowa State professor of plant pathology and microbiology in an online article she posted on Thursday.
Mueller is encouraging farmers to get out and scout for southern rust now, so they can take action to address the disease.
“In a year like this, if we catch a couple more rains like we’re supposed to in the next week or so, and this inoculum is out there, a timely fungicide application is going to be very beneficial,” he says.
The Crop Protection Network, a multi-state Extension resource, offers a new mapping tool called Crop Lookout farmers can reference to identify various diseases and their locations.
“If you just click on that, there’s a couple of hot links on that map for tar spot and southern rust. When we find new spots of disease, it’s updated in real time,” Mueller says.
Speaking of tar spot, Mueller says the disease can still take a huge toll on corn, but that farmers are learning how to deal more effectively with the disease.
“I think we’re getting more used to tar spot, and I think people aren’t panicking as much,” he says.
Soybean Diseases Showed Up Early This Season, Too
Mueller says he found SDS and white mold (also known as Sclerotinia stem rot) in soybeans in mid-July.
“That was about a month earlier than what we want,” says Mueller, who serves as the coordinator of the Iowa State integrated pest management program. “The fact that (SDS) was showing up a good month in advance, it has us a little nervous.”
SDS is most severe when soybeans are planted in cool, wet soils and has delayed emergence – conditions that were prevalent this past spring in parts of Iowa and further into the East and Southeast.
Severe SDS can result in yield losses greater than 50%, according to University of Minnesota Extension.
To address SDS, there’s no curative action farmers can take, but Mueller offers one action farmers can take to address the problem.
“You get your pad and paper out and you take notes in the field. Record what varieties you selected, what seed treatments you put down. That’s all valuable information for the next time you plant soybeans,” he says.
This is an important action because key soybean planting states, like Iowa, have had three or more years of fairly dry weather in the latter half of the summer, so crop advisers and farmers have little data on which of the newer varieties perform best in the face of SDS.
“Use it to your advantage to just collect the data,” Mueller advises.
For white mold, Mueller says farmers can still apply a fungicide in many cases and get enough of a response to warrant the cost of treatment.
“We’re still, early enough, you can get a fungicide out there if you feel like the risk is staying high. You could spray all the way up to R3 and still get some money back on your fungicide,” he says.
“You’ve got to get some penetration through the canopy [for white mold],” he adds. “All the activity is done in that lower canopy. Anything you can do to get the fungicide as deep into that canopy as you can, that’s what we want.”
Unlike SDS, white mold is fairly easy to pinpoint definitively in the field. At this point in the season, Mueller says to look for individual dead plants “here and there” in the field.
“Then just peel back the canopy, and look for the white powder, it’s called mycelia, the fungal growth. You know that’s a telltale sign that you have white mold,” he says.
Mueller’s complete conversation on AgriTalk is available here:
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