Soybeans: What Are the Biggest Yield Robbers to Watch For?

© Debra L Ferguson

Let’s face it: Farming would be a lot easier and less risky if each year came with its own instructions. But in the real world, a solid game plan is needed to weather whatever surprises the growing season may bring. Here’s an overview of some of 2015’s top pest possibilities:

Watch for Weeds

  • Waterhemp – Whether you call it waterhemp or pigweed, Mark Bernards, Ph.D., assistant professor of agronomy, crop science, and weed control at Western Illinois University, says waterhemp will continue to be a challenge in 2015. The key, he says, is telling the difference between waterhemp and Palmer amaranth, especially when plants are young.
  • Palmer amaranth – Palmer amaranth continues to spread by multiple means and can be a formidable foe for soybean farmers.

    “Palmer amaranth makes waterhemp look wimpy,” says Tranel. “It can look okay one day, and a week later you can find it’s grown more than six inches.”

    In fact, Palmer infestations can lead to rapid crop loss. “Once you have a heavy infestation of Palmer amaranth, you can find that it’s not economically viable to even drive through the field to attempt to harvest the crop,” he says.

  • Marestail – Marestail, also commonly called horseweed, with widespread glyphosate resistance is becoming a problem in many areas.
  • Giant ragweed – While herbicide-resistant giant ragweed is not as widespread as some other species, it’s on the move and could become a problem.

Best the Bugs

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    Soybean aphids – Glen Hartman, Ph.D., research plant pathologist and professor, USDA-Ag Research Service based at the University of Illinois, says aphids tend to occur in northern areas, but there have been recent reports of aphid outbreaks farther south. It is impossible to predict where outbreaks will occur, however, so choosing resistant varieties is an important consideration.

    “If we had a crystal ball, we could say, ‘this is the place to plant resistant varieties,’ but there’s no way to be sure,” he says. Farmers may use insecticides when aphids hit economic thresholds during the season.

  • Japanese beetles – Beetles are similarly unpredictable. Many farmers have sprayed to manage localized outbreaks that hit economic thresholds.
  • Marmorated stinkbug – This insect has been more of a problem in the East, but could be headed west.
  • Kudzu bug – Similarly, these insects have been an issue in the South but are marching north.
  • Bean leaf beetles – In addition to feeding on leaves, these beetles also can transmit the bean pod mottle virus. “In some years, this is a very significant problem,” Hartman says. Researchers are studying the virus and believe overwintering may affect severity and risk.

Disarm Diseases

  • Soybean Cyst Nematode (SCN) – SCN is an annual concern and widely claimed as the leading yield-robber in soybeans. “It is easy to forget about SCN when we have good yields and few SCN symptoms,” says Carl Bradley, Ph.D., associate professor of plant pathology at the University of Illinois. “Farmers need to practice proper management, or SCN will cause yield losses.”
  • Sudden Death Syndrome (SDS) – Bradley says catching SDS early in the season can create major soybean yield losses.
  • Phytophthora root rot – Variety selection is the most effective option for managing Phytophthora root rot, says Hartman. It is most likely to occur on heavy, poorly drained soils or soils that have been saturated for an extended time. The disease can attack soybeans as seed rot, seedling blight or by attacking the roots.

    For farmers who have had problems with SCN, SDS or Phytophthora root rot, Hartman says the best bet is to choose resistant varieties. “Variety selection is the number one management choice for those diseases,” he says. “There really are no options for managing these conditions once they occur, so it is important to pick the right variety.”

  • Sclerotinia, or white mold – This disease is only a concern in fields with a history of it. Because no sclerotinia-resistant varieties exist, farmers will need to adopt other management practices, such as wider rows and foliar fungicides.

    “Some years, it’s rare, and other years, it is devastating. It really depends on the environment,” says Hartman. “There’s really only a short window when it affects yields – typically when conditions are moist and warm at flowering.”

  • Frogeye Leaf Spot – This disease is very weather dependent and thrives under good growing conditions. Hartman says management options include selecting resistant varieties, as well as scouting and spraying if an outbreak happens early in the season.
  • Brown spot – This disease occurs during good growing conditions throughout the state. “Brown spot tends to come up from the bottom of the plant, defoliating leaves,” says Hartman. “However, it does not tend to affect the top of the canopy and has minor effect on yields.” Brown spot can be controlled with a late fungicide, but yield benefits may be minor. Hartman advises using fungicides to control severe outbreaks.

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