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Corn+Soybean Digest

Corn Killers

Wouldn't it be great if there were no such things as gray leaf spot or rootworms? If all you had to do was plant your crop, hope for good weather and then harvest?

Unfortunately, we live in a world of reality, where corn diseases and pests are abundant. To alleviate the pain and pressure of that, though, university and Extension personnel have a wealth of data and advice to help combat those pesky pests and dreadful diseases.

The 2007 growing season brought lighter-than-normal pest and disease occurrence for some states that experienced drier-than-average weather, including Kentucky and Mississippi. “Last year (2007) was a light year for Mississippi,” says Chris Daves, Extension entomologist for Mississippi State University. “We treated some non-Bt corn fields for southwestern corn borer, but it wasn't widespread.”

Texas, on the other hand, had a wet growing season and saw a problem with fumonisin. Their normal problems include aflatoxin due to the hot, dry conditions, says Thomas Isakeit, professor and Extension entomologist at Texas A&M.

Some areas had a hot, dry spell, followed by wet, humid weather. “Iowa had a worse problem with aspergilus in the northeast part of the state due to hot, dry conditions, says Alison Robertson, assistant professor, entomology, at Iowa State University (ISU). “However, August became the opposite with wet, humid conditions, raising the problems with gray leaf spot and anthracnose.” The state also had problems with corn rootworm (both northern and western), says Marlin Rice, professor and Extension entomologist at ISU.

Illinois saw the usual corn rootworm, which, according to Kevin Steffey, Extension entomologist at the University of Illinois, has been a problem for decades. “We also had problems with Japanese beetles clipping silks,” he says.

Pathologists and entomologists from across the South and the Midwest have given their predictions as to what to watch for in 2008. Granted no one can provide a solid outlook, what with so many variables, but these folks think this is what you should be looking for during the upcoming growing season.



  • SYMPTOMS: First generation: leaf feeding, tunneling into midribs and boring into stalks. Second generation: leaf feeding, boring into stalks, tassels and around ear zone. Larvae tunnel down through center of stalk.

  • TIME OF ATTACK: First generation: V6-VT; second generation: V12-R6


  • MANAGEMENT: First generation: insecticides, with critical timing. Once borers enter stalks, they cannot be controlled. Avoid late-planted corn; use fall tillage options.


  • SYMPTOMS: Fine, spider-like webbing on underside of leaf; will start on lower leaves and move up plant, piercing plant cell walls and removing contents causing leaf to dry up and die.

  • TIME OF ATTACK: Close to tassel when temperatures start to warm.

  • CONDITIONS FAVORING: Moisture-stressed fields and hot, dry, windy weather with low humidity and little rainfall. Fields with insecticides for European corn borer, western bean cutworm; fields next to grasses, ripening wheat, alfalfa.

  • MANAGEMENT: Reduce moisture stress if possible; treat only heavily infested areas. Corn reaching full-dent stage is unlikely to benefit from treatment.


  • FORMS: Western, western variant, northern (larvae and adults)

  • SYMPTOMS: Root systems with scars or elimination of roots due to larvae feeding on and tunneling into roots. Adults feed on and clip silks; western and southern species feed on leaf tissue.

  • TIME OF ATTACK: Larvae: V4-R2; adults: V8-R5

  • CONDITIONS FAVORING: Continuous corn in general, but rotation may not be a help when it comes to variants that lay eggs in soybeans.

  • MANAGEMENT: Transgenic hybrids, soil insecticides, seed treatments.


  • SYMPTOMS: Loss of stand before emergence or due to above-ground cutting from worm feeding on seedlings. Also, below-ground tunneling, irregular holes, dead plants.


  • CONDITIONS FAVORING: Reduced tillage and abundance of broadleaf weeds prior to planting.

  • MANAGEMENT: Early tillage and good weed control. Scouting and application of insecticide if/when threshold is reached.



  • SYMPTOMS: Small, linear, rectangular-shaped lesions, tan to gray in color. Lesions begin on lower leaves and spread upward.

  • TIME OF ATTACK: Lesions generally appear near first tassel; disease spread continues through maturity.

  • CONDITIONS FAVORING: Cloudy days with humid or wet conditions; heavy dew, fog or light rain. Continuous corn and reduced tillage also favor this disease.

  • MANAGEMENT: Resistant hybrids, tillage, crop rotation, scouting and fungicide application if necessary.


  • FORMS: Southern and common

  • SYMPTOMS: Small, circular raised blisters, reddish-brown in color. Found in common rust on both leaf surfaces; on southern rust on top side of leaf surface.

  • TIME OF ATTACK: Spores carried on spring winds from southern areas of U.S.

  • CONDITIONS FAVORING: Common rust: cool, humid weather; southern rust: warm, humid conditions.

  • MANAGEMENT: Resistant hybrids, scouting with fungicide application if necessary.


  • SYMPTOMS: White, moldy growth that can cover entire ear beneath the husk; develops from base to tip. Small, black fruiting structures can be seen on kernels; kernels appear stuck together.

  • TIME OF ATTACK: From silking through maturity.

  • CONDITIONS FAVORING: Warm, dry weather prior to silking; rainy, wet weather at pollination, during grain fill and after silking.

  • MANAGEMENT: Resistant hybrids, tillage, crop rotation.


  • SYMPTOMS: Elliptical brown to grayish to tan lesions from 1 to 6 in. long. If humid, lesions may have gray-green centers due to spores on dead tissue.

  • TIME OF ATTACK: As early as silking, but more prevalent during later development stages.

  • CONDITIONS FAVORING: Wet, humid weather; heavy dew and fog. Can also have higher rate in continuous corn and reduced tillage.

  • MANAGEMENT: Resistant hybrids, tillage, crop rotation.


  • FORMS: Leaf blight and stalk rot

  • SYMPTOMS: Leaf blight: lesions varying in size, generally brown and round-shaped with yellowish area surrounding. First appear on leaf tip, moving to midrib, then produce large, dead spots. Stalk rot: shiny, black streaks and blotches on the lower stalk. Internal stalk tissue can turn dark gray to brown and become shredded.

  • TIME OF ATTACK: Anytime from seedling emergence to maturity. Leaf symptoms can begin in May; stalk rot symptoms usually appear in late August to mid-September.

  • CONDITIONS FAVORING: Cool, wet weather.

  • MANAGEMENT: Tillage, crop rotation, resistant hybrids. Scout and apply fungicide if necessary for foliar stage of disease.

Editor's Note: Please keep in mind this is just a general list of pests and diseases to keep an eye out for. Others mentioned included chinch bugs, Japanese beetles, aflatoxin, ear rot and stalk rot. For specific information for your location, contact your local Extension office or university.

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