EDGING CLOSER: The black dots, indicative of tar spot, are fungal reproductive structures called ascomata. In 2019, tar spot quickly moved across Iowa and was confirmed within one county of the Missouri River.
Be on the lookout for problems that are growing in prevalence in Nebraska corn and soybean fields.
To put it mildly, the 2019 growing season was a wild one. However, after a rough, wet growing season in 2019, the 2020 growing season has potential for some new diseases in corn and soybeans.
Tamra Jackson-Ziems, Nebraska Extension plant pathologist, notes that over the course of the 2019 growing season, a couple of emerging diseases have come closer to Nebraska, or have expanded their reach in the state.
While it hasn’t yet been officially confirmed in Nebraska, tar spot has been working its way farther west over the past couple of years.
“It turns out that temperatures in the 60- to 72-degree [F] range really favor that disease,” Jackson-Ziems says. “In a matter of a couple weeks, tar spot appeared to move halfway across Iowa and was confirmed within one county from the Missouri River. Originally, we all thought Nebraska wouldn’t need to worry about this until late 2020 or maybe next year, but the weather last year seemed to help push it toward us faster than anticipated.”
However, she notes, there’s a chance tar spot is already in Nebraska, but hasn’t been confirmed. Fortunately, the disease isn’t hard to identify.
“This disease has really characteristic black dots, just like the name implies,” Jackson-Ziems says. “It looks like someone took a big paintbrush and dipped it in black paint or tar and slung it across the leaves. These dots are fungal reproductive structures called ascomata. They are somewhat raised above the leaf surface.
“They can be tiny and barely visible, or larger — about the size of a pencil eraser. They’re primarily on leaves, but can be on the ear husk, too. The second symptom that’s much less common is there can be secondary lesion or ring around those black dots that we call a fisheye lesion.”
Because the disease favors cooler, wetter conditions, Jackson-Ziems recommends starting to monitor for tar spot early in spring and early summer, especially in the eastern counties — although, since there hasn’t been much inoculum built up in Nebraska, there aren’t many concerns about yield loss because of tar spot this year.
“This disease was first identified in 2015 in northern Indiana and northern Illinois,” Jackson-Ziems says. “It took two or three years to build up and become severe enough that it required some fungicide applications. At this point, foliar fungicides are the only management recommendations we can make, but I don’t think we need to worry about that yet.”
Growers can check for tar spot by rubbing black spots — if they rub off the leaves, it’s more likely insect frass, which often is mistaken for tar spot. If it can’t be rubbed off, growers should submit samples to the University of Nebraska-Lincoln Plant and Pest Diagnostic Clinic.
GROWING PRESENCE: While fungicide-resistant frogeye leaf spot has traditionally been more of a problem in states south of Nebraska, where continuous soybean rotations are more common, it was confirmed in 10 Nebraska counties in 2019.
Soybean diseases on the radar
Also showing up in neighboring states in recent years is strobilurin-resistant frogeye leaf spot in soybeans. In 2019, Jackson-Ziems collected leaf samples and sent them to the University of Kentucky for advanced testing, and samples came back positive from 10 Nebraska counties.
“I think we’ve probably had it for a while but didn’t have proof until now,” she says. “Most of these samples weren’t targeted because fungicides didn’t work. For most, we just collected samples where we knew we had frogeye and didn’t look into history of fungicide applications, and every one of them came back with fungicide resistance.”
“The sad reality is while herbicides are available to farmers from a dozen or so classes, right now in row crops in the Midwest, we’re only using fungicides from about three classes,” Jackson-Ziems adds. “It’s going to require people to spray a product or tank mixture of two or more fungicide classes, and taking advantage of a third one we haven’t been using as much. The three-way mixes that are commercially available are going to be more expensive, but those products and tankmixes are options.”
In addition, if planting in a field where frogeye has been identified, it’s a good idea to plant a resistant variety. Growers having issues with fungicide efficacy should contact Nebraska Extension.
“We know it’s in surrounding states,” says Brady Kappler, BASF technical service representative in Nebraska. “It’s been slower to develop here, because many of the fields in states south of here where it’s been traditionally identified have been in continuous soybeans. Now that it has been identified in Nebraska, it’s something we want to be aware of. If we get the right weather conditions, we can see more of it.”
Another soybean disease to watch for next year is white mold, which Kappler says has been popping up in Nebraska fields more often.
“White mold is really tricky because a lot of times you don’t know you have it bad until it’s too late, because it’s located underneath the canopy,” he says. “Also, the infection occurs while the plant is flowering. So for the best control, you need fungicide on at R1, flowering for a first shot, and will probably need a second application at R3-R4. If the grower hasn’t had white mold before, he may not realize it until it’s too late, and treating white mold reactively isn’t going to be successful.”