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Yield Quest: Harvesttime Disease Checkup

Besides gleaning the fruits (well, crops) of your labor, harvest is also a good time to review any disease monitoring you did during the growing season. Although there are no remedies available now,  a checkup from the combine can help devise future disease management strategies.

“Disease management is all about being proactive,” says Mike Meyer, a Syngenta research scientist. 

Here are four diseases that you’ll want to monitor from the combine cab. 

Sudden Death Syndrome (SDS)

This fungal soybean pest (above) first surfaced in Arkansas in the 1970s, says Dean Malvick, University of Minnesota Extension plant pathologist. Now, it’s as far north as Minnesota, having been confirmed there in 2003. 

This fungal disease is aptly named, for it suddenly surfaces in late summer – right around the R4 and R5 stage (full pod to beginning seed) . Soybeans leaves will exhibit crinkly and chlorotic leaves before plants die. 

In reality, the symptoms you see in late summer actually started early in the season, when the soilborne fungus Fusarium virguliforme infects soybean roots as early as one week after crop emergence in cool and wet soils. 

Remember this year’s soggy spring? That’s exactly why SDS resulted in some fields this year.“Anything that spreads soil will spread SDS, such as wind, water, and machinery,” says Malvick. 

Unfortunately, by the time you view its aftereffects in late summer, there’s little that can be done. Foliar fungicides don’t work on SDS. However, Malvick says there are some varieties that tolerate SDS much better than others. 

Tools exist for farmers to manage it, such as BASF’s Ilevo seed treatment. Syngenta also has plans to soon launch Saltro, a new seed treatment fungicide that has an SDHI mode of action. At presstime, Saltro – which offers SDS protection – had not yet received approval from federal regulators. 

Goss’s Wilt 

Goss’s wilt in corn is a worry. Since it’s a bacterial disease, fungicides won’t control it. 

First discovered in Nebraska in 1969, its prevalence spiked in the mid-1970s and then faded away. However, it then surfaced in Nebraska again in 2006 and then spread into the central United States and Canada, says Malvick. 

“Serious leaf diseases in corn used to be rare in Minnesota,” he says. “But Goss’s wilt changed that.”

On the plus side, Goss’s wilt is much less of a worry than it was 10 years ago, says Malvick. 

“Many corn hybrids today have good levels of resistance,” he says. However, he adds Goss’s wilt still surfaces in susceptible hybrids.

Tar Spot 

Tar spot initially surfaced in the U.S. in 2014. Since then, it expanded, surfacing in northern states like Iowa, Michigan, and Wisconsin.  

tar spot
Caused by the fungus Phyllachora maydis, it produces small semicircle raised black structures call stromata on the upper and lower surfaces of leaves and on leaf sheaths and husks. 

“The stomata are often glossy and, unlike mature rust pustules, do not leave a sooty residue on fingers when rubbed,” says Malvick. 

Symptoms can occur as early on corn as V3, 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, Malvick says.

In Latin America, cool temperatures and high humidity favor the disease spread. However, it is uncertain what conditions favor it in the U.S. besides these favorable weather conditions, 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, he says. 

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

Application timing is critical. Myers advises applying an effective fungicide soon after tassel emergence or at the first sign of disease in the field. 

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

White Mold 

This fungal disease is most active in the Upper Midwest states of Minnesota, Wisconsin, North Dakota, and South Dakota. Some varieties tolerate white mold better than others, but there is no true resistance, says Myers. 

white mold
Unfortunately, crop rotation does little good, for white mold can persist in soils for seven to eight years. Fungicides can control white mold, but timing is crucial.

Myers advises applying an effective fungicide when the first flowers develop and before canopy closure. That’s because senescing flower petals are the medium for infection, he says.

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