How to Wrestle 2020 Corn Diseases

Familiar diseases like gray leaf spot join new ones like Physoderma brown spot and tar spot.

Corn farmers wrestled with old and new corn diseases in 2019 that likely will again be present in 2020. 

These included the usual suspects, such as gray leaf spot. One new one, though, is Physoderma brown spot. Symptoms of this fungal disease include numerous lesions that appear on midcanopy leaves. Nodal infection from this disease can cause stalk rot to occur. The good news is that after this spring’s prolific rainfall, dry weather curbed disease development. 

“Some diseases like southern rust appeared early, but dryness inhibited development,” says Randy Myers, Bayer Crop Science fungicide product development manager. “Last year, gray leaf spot around the Midwest was the worst I have ever seen. This year, it could be found everywhere around the Midwest, but it did not progress as rapidly due to dry conditions.”

However, wet weather has caused some diseases to pick up. Myers says a Bayer field trial check in late August indicated no or little signs of southern rust. Several days later, it scattered all over the field.

Tar Spot

Tar spot, a relatively new corn disease, started out slowly in 2019, but picked up steam in some areas in 2019. Symptoms can occur as early on corn as V-3, although symptoms can surface at any stage. 

The disease is also present in central America. There, it’s called tar spot complex because a secondary fungus, Monographella maydis, is often associated with the lesions, says Dean Malvick, a University of Minnesota Extension plant pathologist. 

In Latin America, cool temperatures and high humidity favor the disease spread. However, it is uncertain what conditions favor it in the United States. So far, continuous corn with minimum tillage and high rates of nitrogen (N) fertilizer appear to be factors, says Malvick. 

In Latin America, severe tar spot has been shown to reduce corn yields by more than 40 bushels per acre by increasing stalk rot and lodging. However, many other diseases may pile on top of it to cause severe losses, Malvick says. 

Growers need to be aware of this disease and monitor it closely,” says Myers. “There is not good resistance in Midwestern corn genetics. So the first line of defense has to be fungicides,” he says.

Timing is critical, as an effective fungicide should be applied soon after tassel emergence or at the first sign of disease.

“You have to spray proactively,” says Myers. “Playing catch-up with the disease is hard. You can’t fix the damage that is done.”

Target Diseased Fields For Harvest First

If 2020 mimics 2019, farmers will face tough choices in a rain-soaked, snow-packed harvest. If that happens, it’s more important than ever to target diseases in fields ready for harvest first. Diseased fields have thinner stalks, as they transfer carbohydrates from the stalk to the ear for grain fill. This makes stalks prone to breakage as the season progresses, says Myers.

It’s physically impossible to assess every acre of corn for stalk breakage potential. What can be done, though, is to assess field areas that have experienced stalk breakage in previous years, says Myers. If these fields have spindly stalks, target those areas for harvest first, he says. 

White Mold is Bold

On the soybean side, white mold again reared its ugly head, particularly in the Upper Midwestern states of Minnesota, Wisconsin, North Dakota, and South Dakota.

“Once a field has white mold, it lingers for a long time. You can assume you will have it for the next 10 years,” says Myers.

Some varieties tolerate white mold better than others, but there is no true resistance, says Myers. The fact that white mold can persist in soils for years also nixes crop rotation as a control measure. Fungicides can give good control, but timing is crucial. Because senescing flower petals are the medium for infection, treat when the first flowers develop and before canopy closure, he says.

READ MORE: 20 Strategies That Farmers Can Use In 2020

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