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Corn and Soybean Diseases to Watch

07:14AM Jun 25, 2016
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In July and August it’s not uncommon to see corn and soybean diseases rear their ugly heads. Learn how to identify key diseases and what, if any, action you can take this season to preserve yield.

While scouting, remember, for diseases to exist they need a complete disease triangle: the pathogen must be present, there must be a susceptible host and environmental conditions need to favor the disease. Since conditions change from year to year, it’s important to be ready for whatever Mother Nature might throw your way. 

Take good notes where you see phytophthora, Sudden Death Syndrome, Brown Stem Rot, frogeye leaf spot and white mold in soybeans and where you see northern corn leaf blight, gray leaf spot, Goss’s wilt and anthracnose leaf blight or stalk rot in corn. Apply fungicide if the return on investment is obvious with frogeye leaf spot, white mold, northern corn leaf blight and gray leaf spot. Also consider tillage or rotation to help break the disease cycle next year. Ultimately, the battle against fungus and disease starts at seed selection. If you recognize a soil-dwelling pathogen in your fields, consider resistant hybrids or varieties in the future.

As you get closer to the finish line it’s important to do everything you can to maximize return on investment, and use scouting to your advantage. 


Sudden Death Syndrome

Cool and wet spring conditions can encourage soybean sudden death syndrome development. The disease infects the plant after emergence but symptoms don’t show up until flowering.

Sudden death syndrome is second only to soybean cyst nematode as biggest yield robber in the Corn Belt. The disease overwinters in soybean residue and once you spot symptoms it’s too late to treat.

Warning signs and management tips:

  • Yellowing and interveinal chlorosis starting with lower leaves
  • Discoloration when the stem is split with a pale-green colored cortex
  • Leaves fall off, leading to early plant death and yield loss
  • Seed treatments or planting sudden death syndrome resistant varieties could offer relief for your next soybean rotation since the disease will overwinter in residue

White Mold

High-yielding soybeans are at the biggest risk of developing white mold. Spores survive in the first 2" of soil profiles and infect soybeans at canopy. The fungus favors cool temperatures (no higher than 85°F) and moist conditions.

It’s important to act quickly with fungicide to slow the fungus if found in your fields.

Warning signs and management tips:

  • 20" rows and smaller increases the likelihood of white mold
  • Before disease symptoms appear, small fungi-like apothecia can be found in soil
  • Initial symptoms show water-soaked stem lesions that eventually encircle the stem
  • Lesions can found on stems, pods, petioles and leaves 
  • White cotton-like mycelia growth can be found inside or outside of pods and stems, and in some cases, black raised spores can be found along the stem

Northern Corn Leaf Blight

Northern corn leaf blight favors mild temperatures from 65°F to 89°F. Fungal spores overwinter in corn residue. Next year, select resistant hybrids to reduce the likelihood of developing this disease.

Northern corn leaf blight has spread from Minnesota to Tennessee and is one of the most common corn diseases.

Warning signs and management tips:

  • Corn-on-corn fields and no-till fields with a history of the disease are more likely to develop the disease again this year
  • Cigar-shaped tan to gray lesions start on lower leaves and work their way to the top leaves
  • If resistant plants show infection the lesions will appear yellow and transparent, with irregular borders and fewer spores
  • Scout at or after silking during warm, wet conditions
  • Fungicides can slow the infection

Brown Stem Rot

Brown stem rot infects the plant in early stages and needs cooler temperatures (60°F to 80°F) while the plant is setting pods to show foliar symptoms. The disease can be more severe in fields with poor fertility.

Sudden death syndrome and brown stem rot can be difficult to tell apart. Scouting and getting your boots dirty will help distinguish the difference between the two soil-borne diseases so you can plan for next year. There are no in-season treatment options for brown stem rot but you can select resistant varieties next year.

Warning signs and management tips:

  • Pith will appear dark brown when the stem is split
  • Leaf necrosis and chlorosis (similar to signs of sudden death syndrome)
  • Plant death in some cases
  • Risk of brown stem rot is higher in soils with a pH less than 6.5

Frogeye Leaf Spot

In contrast to many soybean diseases, frogeye leaf spot flourishes in hot, humid areas. Soybeans can be infected at any point in the growing season but most commonly after flowering. Frogeye overwinters in soybean residue.

You can manage the disease in season, if you catch it early enough. Be mindful if you are having weather conditions that favor the disease so you can contact your aerial applicator faster.

Warning signs and management tips:

  • Initial symptoms are small yellow spots about ¼" in diameter; centers turn gray to brown and have reddish to purple margins
  • Lesions on stem will appear red when young and darken as they age
  • Spots on pods are reddish brown and can appear to be sunken and can infect the seeds inside the pod
  • Apply fungicide between R3 and R5 to maximize yield

Gray Leaf Spot

From corn silking to maturity, watch for warm, humid weather to encourage gray leaf spot. The disease overwinters in corn residue, but fungicide can slow the disease. 

Gray leaf spot not only limits the plant’s ability to photosynthesize, it also opens it up for more fungus and disease to enter the plant. Scout to catch this disease early.

Warning signs and management tips:

  • Early symptoms show small yellow or tan spots with halos
  • Symptoms later appear as gray rectangular-shaped lesions up to 4" long and 1⁄8" wide with parallel edges
  • Lesions might appear in early July on lower leaves, but the disease is much stronger and spreads faster in late July and August
  • Use resistant hybrids, tillage and rotation to minimize risk since it’s a soil-borne disease

Goss’s Bacterial Wilt and Leaf Blight

Goss’s wilt thrives in warm, humid conditions. If you’ve had Goss’s wilt in a field before, you might see it again this year from overwintered spores. Consider resistant hybrids next year.

Fields will be more susceptible if corn has undergone stress from wind, hail, machinery damage or an infection from other diseases that open it up for Goss’s wilt.

Warning signs and management tips:

  • Lesions are gray, yellow or red striped with wavy margins following leaf veins that might contain dark, watery spots
  • After splitting the stalk, look for orange to brown bundles visible in vascular tissue that can degrade the stalk
  • There is no in-season fungicide or other treatment available for Goss’s wilt
  • Look for damage near corn silking, but it can appear anytime from May to September

Anthracnose Leaf Blight and Stalk Rot

Watch for this disease during warm, wet summers. Since it can infect both the leaves and the stalk make sure you’re scouting early for stressors such as insect, disease, high populations or nitrogen loss. Leaf symptoms appear in late April through July, and stalk rot appears from August to late October.

Warning signs and management tips:

  • The disease overwinters in corn residue, select resistant hybrids to mitigate risk
  • Leaf blight shows ¼" to ½" round or oval lesions that might have small black specks in the center of the lesions; when humid, disease starts on lower leaves and works up to the top leaves
  • Dark brown to black stalk lesions go through the rind
  • After tasseling, infection might cause the top leaves of corn plants to die prematurely