Slice these soybean stressors

If you’re tired of wrestling corn diseases like northern corn leaf blight and gray leaf spot, there’s an easy way to manage them. 

“Our first recommendation is to rotate into a non-host crop,” says Todd McRoberts, NK agronomy manager. A year of a non-host crop like soybeans can help break cycles of some corn diseases. 

The trouble is, though, that’s not always feasible. Livestock producers, for example, often need the corn that growing corn continuously can bring. 

Rotating to a non-host crop also may not work because some leaf diseases can spread from nearby fields, says Dean Malvick, a University of Minnesota Extension plant pathologist. 

Meanwhile, alternative crops such as soybeans also have their own disease issues that include:

  • Pythium and Phytophthora root rot. These early-season diseases can plague soybeans early in the growing season, particularly in years with wet spring soils. 
  • Frogeye leaf spot. This fungal disease has moved from the South and mid-South into the Midwest. In many cases, the yield-robbing disease resists Qol (strobilurin) fungicides, making a fungicide mix of multiple modes of action paramount. 
  • White mold. This fungal pathogen is present in soils for years. The disease flares up in years of rain, cool temperatures, high relative humidity, and moist soils.   
  • Sudden death syndrome (SDS). Symptoms surface in fields that seemingly are on their way to stratospheric soybean yields. Although farmers don’t see the chlorotic leaf and dying soybeans until August, SDS is rooted in early-season infections. 

Predicting diseases such as these is not likely to get easier either. 

“Changing weather patterns are leading to more changes in disease risk to corn and soybean production,” says Malvick. Increasing weather volatility makes it more difficult to accurately predict which diseases will be a problem. 

“Scouting is becoming more important,” he says. 

Fortunately, there are ways to deal with these maladies that may pop up in your soybean fields. Varietal tolerance gets a jump on the fight even before a seed is planted in the spring. Seed treatments can help nix
early-season stressors and ones that plague soybeans early and later on like SDS. Foliar fungicides also can help curb the impacts of some of these diseases. 

Early-Season Stressors 

Cool and wet soils are prime movers for early-season yield robbers like Pythium and Phytophthora root rot. Several seed treatments do a good job of managing them. Still, farmers should concentrate on seed selection first and seed treatments second. 

“While seed treatments have a tremendous value around them, they should not influence the seed decision,” says Lance Tarochione, an Asgrow/DeKalb technical agronomist. “They are two separate things.”

Farmers do have more flexibility in choosing soybean seed treatments than they do with corn, adds Tarochione. “Soybeans are treated downstream by dealers who have more flexibility to treat the seed for what the farmer needs,” he says. “With soybeans, you can spend $40 per unit (mainly 140,000 seeds) on seed treatments or nothing. Some $40 with $8 (per bushel) beans is a lot of money, so growers are trying to identify the treatments for which they can get the most benefit.”

In the case of SDS, though, they can be beneficial. “There are two phases to this disease,” says Daren Mueller, Iowa State University Extension plant pathologist.

Daren Mueller of Iowa State Extension in a soybean field
Photo credit: Gil Gullickson
The first is the root rot phase that occurs soon after planting. In spring, the soilborne fungus Fusarium virguliforme can infect roots as early as one week after crop emergence in cool and wet soils. These infections that key Fusarium root rot can produce the toxins you see in August of chlorotic and crinkled leaves. SDS keys yield loss via flower abortion, poor pod set, and fewer seeds per pod. 

Resistant varieties remain the best way to manage SDS, says Mueller. Resistant varieties showed a 15.1% yield advantage over susceptible ones on fields with an SDS history in university trials at around 50 locations from 2013 to 2019 in Illinois, Indiana, Iowa, Michigan, South Dakota, Wisconsin, and Ontario. 

“As you pick a resistant variety, you are protecting your field from both phases of the disease,” says Mueller. 

The university researchers in this study and others have also examined seed treatments that companies say has some impact on SDS. Two seed treatments – BASF’s Ilevo and Syngenta’s Saltro – rose to the top, providing similar control of SDS in university tests. 

White Mold 

White mold is another yield robber that often surfaces in August. Management starts with selecting varieties that tolerate white mold. Tolerance can differ between varieties, though, says Mueller. 

Recent university research also shows that some Roundup Ready 2 Xtend varieties – featuring tolerance to glyphosate and dicamba – may be lacking in tolerance to white mold.

Studies showed a 2- to 4.1-bushel-per-acre statistically significant edge that Roundup Ready 2 Yield (glyphosate-tolerant) had above Roundup Ready Xtend varieties where white mold was the major yield-limiting factor. 

Officials for Bayer say it has not reviewed the research, but company research has
not shown differences in susceptibility to white mold between traits. 

Foliar fungicides can curb white mold, although control can hinge on timing the ability of the fungicide to penetrate into the canopy. Mueller recommends applying a fungicide to manage white mold at or close to the R1 (flowering) stage. 

Frogeye Leaf Spot

This fungal disease has migrated from the mid-South to the Upper Midwest in just a few years. 

“Yield losses have occurred up to 30% in Southern states when the disease is severe,” says Malvick. 

Resistant varieties are one way to combat frogeye leaf spot. Fungicides are another option, although management is complicated by widespread resistance to Qol (strobilurin) fungicides. 

“If you see frogeye leaf spot in your (Iowa) field, chances are it is 100% resistant to Qol fungicides,” says Mueller. 

Thus, make sure your fungicide includes another mode of action, such as a DMI (triazole) or SDHI mode of action for effective control of frogeye leaf spot. 

“Undercover sprayers can improve fungicide coverage,” Mueller says. “But if you use the wrong product, it will not improve performance (of the fungicide).”

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