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Be aware of what is going on in your fields

By Staff | Sep 26, 2019

Frogeye leaf spot is another disease that has made it’s way into producer’s soybean fields this year. The problem with controlling this and other diseases is that they have become resistant to a particular class of fungicides.



As the growing season has come to an end for several soybean fields, did you notice any irregular areas where the soybeans seemed to die off earlier than other areas of the field?

Daren Mueller, associate professor and Iowa State University Extension and Outreach plant pathologist, suggests keeping those spots in mind while combining in order to help with management decisions next spring and beyond.

“We’ve had a lot of SDS (sudden death syndrome) showing up, and I would encourage producers to always be on the lookout for where it is at,” he said. “Because, in future years when you plant soybeans again, variety selection is the best place to start with the management of that disease.”

Sudden Death Syndrome (SDS) has been prevalent in several Iowa soybean fields this fall. Daren Mueller, associate professor and Iowa State University Extension and Outreach plant pathologist said now is the time to be making notes to help manage for the future.

According to information from the University of Minnesota, the first symptoms are typically on leaves in late July or early August in the Midwestern U.S.

Leaf symptoms often begin as scattered yellow, diffuse spots between veins. The spots typically expand between veins to become brown lesions surrounded by chlorotic areas, and the leaves may be cupped or curled. Leaves detach from the petioles as the disease progresses. Brown to grey discolored areas develop in the vascular tissue of the lower stem, and can typically be seen by removing the epidermis of the stem of fresh plants. The pith remains white, which is a diagnostic feature that distinguishes SDS from brown stem rot (BSR). SDS also causes root rot, and roots may have surface blue fungal growth in moist conditions.

Infection of roots may occur early in the season, but symptoms of SDS usually don’t develop until late July or in August. Infection and disease development are favored by early planting, cool and wet soil, SCN infection, and susceptible varieties.

For any field that has SDS, Mueller advises to start with finding a good variety of soybeans.

“There are some that have good levels of resistance and I would say if you are really concerned about it, there are seed treatments available, and come next year, there will be even more options,” he said. “Do your homework on what works best, but I would say the cheapest option is to start with a resistant variety.”

Frogeye leaf spot

Mueller said another disease that was prevalent in soybean fields this year was frogeye leaf spot.

“I would say, go out and look. Do your homework. It is beginning to adapt to our climate,” he said.

According to ISU Extension, disease symptoms for frogeye leaf spot typically start out as small, water-soaked spots (or lesions) in the upper plant canopy. As the disease progresses, lesions enlarge and become round to angular.

Eventually, the lesion center changes color to gray or brown and is surrounded by a narrow reddish-purple margin. In some soybean varieties, a light green halo around the lesion border can be observed. If environmental conditions are favorable, fungal sporulation can occur, which gives the underside of lesions a fuzzy gray appearance. The lesions can then begin to coalesce to create blighted areas on leaves. When the disease is severe, plants could experience premature defoliation.

In addition to foliar symptoms, the pathogen can infect stems and pods late in the growing season, though these symptoms can be challenging to identify. When lesions appear on the stem, they are elongated. When lesions appear on pods, they tend to appear oblong, and resemble the foliar symptoms. If pods are severely diseased, seeds can become infected and can experience discoloration, turning them a light purple-to-gray color. Infected seeds may also be symptomless.

Conditions that favor frogeye leaf spot include warm, humid weather, with frequent rains that persist over an extended period of time. Several days of overcast weather can also increase the spreading of the fungus.

Field conditions can also increase the susceptibility of plants to the disease. These conditions include continuous soybean production (the fungus that causes frogeye leaf spot can survive in infested soybean residue for at least two years), short rotations between soybean crops, practicing conservation tillage as well as planting a susceptible soybean variety in a field with a history of frogeye leaf spot.


When it comes to treating soybean diseases, Mueller warns to be aware of fungicide resistance.

“We have some pretty serious resistance problems in Iowa on soybeans and it’s pretty misunderstood as to what the solutions are,” he said. “In general, there are three classes of fungicides that farmers can pick from, and most of the products are a combination of two or three of those classes, which contain a strobulurin and the two most common foliar pathogens that are infecting soybeans are the ones that cause frogeye leaf spot and brown spot, and they are both resistant to the class of fungicide that is a strobulurin.”

Mueller added that, unfortunately, most fungicides are a combination of the strobulurin and it’s difficult to find a treatment that doesn’t include it.

“Strobulurin is 100 percent resistant and 100 percent worthless towards those pathogens,” he said. “We need to get off of the strobulurin bandwagon. It works in corn, but doesn’t work in soybeans. I can’t be harsh enough about how worthless they are.”

Mueller said some of those problem areas you may have already noticed, or will when you get in the seat of the combine, may not have been caused by diseases.

“Some of those might be potassium deficiency and, if it is, you don’t manage it like a disease. Get your soil tested and do all of those things you need to do to mitigate that,” he said. “If you’re in the combines, seeing dead spots, hopefully you took notes before and observed them, but if not, SDS, frogeye, brown stem rot, potassium deficiency these could have been the culprits.”

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