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Corn+Soybean Digest

Coddle Your Corn

Unpredictable. Change of pattern. We'll see what the weather brings. That's what entomologists and pathologists say when it comes to pests and diseases in corn.

The disease and insect pressure from 2008 can help to predict what farmers should be concerned about, and when, for the upcoming growing season, they say.

“Rootworms are No. 1 and are always going to be No. 1,” says Kevin Steffey, Extension entomologist at the University of Illinois. “Their numbers, generally speaking, were lower in 2008 than they were in the middle of the decade, but rootworms go through long cycles where during the middle few years of a decade they are bad, and then at the front and back ends, the numbers are lower. That's been the pattern for several decades.”

Though rootworms are No. 1, that leaves a laundry list to follow. Among those are chinch bugs and armyworms.

“Chinch bug infestation was widespread, and fall armyworm numbers were heavy in some fields in the whorl and ear filling stage especially in later-planted corn,” says Roy Parker, Texas A&M entomologist. “Also, spider mites were bad in fields that were treated for fall armyworm, more than I've seen in many years.”

The infestation didn't stop with insects. Many diseases plagued the cornfields of the U.S.

North Carolina saw gray leaf spot, southern and northern corn leaf blight, along with Columbia lance, root-knot and sting nematodes, according to Stephen Koenning at North Carolina State University. Gray leaf spot was also a problem in Nebraska.

“It continues to be common and resulted in harvest losses,” says Tamra Jackson, Extension plant pathologist at the University of Nebraska-Lincoln. “Also, Goss's bacterial wilt and blight occurred state-wide in 2008. Stalk and crown rot diseases at the end of the season were widespread, too.”

It seems the Corn Belt couldn't escape stalk rot, either.

“Stalk rot was the main corn disease problem in Minnesota,” says Extension pathologist Dean Malvick at the University of Minnesota. “Stalk rot caused economic damage, but we also had occurrences of common rust, eyespot, northern corn leaf blight and anthracnose. Some of these diseases will occur again in 2009. Weather, crop varieties and location will be major factors that drive how much disease pressure will occur from each disease.”

All variables aside, university Extension workers do their best to let you know what pests and diseases you should watch for in your cornfields. They spend hours finding patterns to help predict what could be attacking your crops. Following are their best estimates for the 2009 crop year.



  • 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 reach threshold.


  • FORMS: common and fall

  • SYMPTOMS: Ragged leaves to complete defoliation for armyworm; whorl and ear-feeding for fall armyworm.

  • TIME OF ATTACK: VE-V12, armyworm; V12-R6, fall armyworm.

  • CONDITIONS FAVORING: Wet spring seasons; no-till corn planted in grassy cover crops, sod or hay.

  • MANAGEMENT: High-risk areas not scoutable may warrant use of preventive treatment. Scouting and rescue treatment is preferred. Easy to control if detected early.


  • 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.

Continue on Page 2



  • TWO 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.


  • 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


  • 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: 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.

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

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