Northern Corn Leaf Blight

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Symptoms

Corn leaf with early lesion of northern leaf blight

Symptoms - Early

Early northern corn leaf blight lesions are gray-green and elliptical, beginning 1 to 2 weeks after infection.
In a susceptible reaction, fungal sporulation will begin within a few days.

Developing lesions of northern corn leaf blight

Symptoms - Fully Developed

Northern corn leaf blight lesions become pale gray to tan as they enlarge to 1 to 6 inches or longer.
Distinct cigar-shaped lesions unrestricted by leaf veins make northern corn leaf blight (NCLB) one of the easiest diseases to identify.
Under moist conditions, lesions produce dark gray spores, usually on the lower leaf surface, giving the lesions a "dirty" appearance.
As many lesions enlarge and coalesce, entire leaves or leaf areas may be covered.
Heavy blighting and lesion coalescence give leaves a gray/burned appearance.

Facts

Caused by Exserohilum turcicum (previously classified as Helminthosporium turcicum), a fungus found in humid climates wherever corn is grown.
Survives in corn debris and builds up over time in high-residue and continuous corn cropping systems.
Favored by heavy dews, frequent showers, high humidity and moderate temperatures.
Spores are spread by rain splash and air currents to the leaves of new crop plants in spring and early summer. Spores may be carried long distances by the wind.
Infection occurs when free water is present on the leaf surface for 6 to 18 hours and temperatures are 65 to 80 F.
Infections generally begin on lower leaves and progress up the plant, but infections may begin in the upper plant canopy when spore loads are high.
New NCLB lesions can produce spores in as little as 1 week, allowing NCLB to spread much faster than many other corn leaf diseases.

Disease Cycle

Impact on Crop

Field showing northern corn leaf blight damage

Infections by NCLB can occur at any growth stage, but plants are more susceptible after pollination.
Yield losses may be caused by:
- decreased photosynthesis resulting in limited ear fill
- harvest losses if secondary stalk rot infection and stalk lodging accompany loss of leaf area.
Yield losses are most severe when NCLB infects corn plants early and progresses to the upper plant leaves by pollination or early ear fill.
If ear development outpaces disease progression, yield losses will be lower.

Hybrid Selection

Pioneer researchers select for resistant parent lines and hybrids in multiple environments where NCLB pressure is consistently high year after year.
Pioneer hybrids are rated for NCLB resistance and ratings made available to customers.
Most hybrids are rated from "3" to "6" on Pioneer's 1 to 9 scale, where 9 indicates highly resistant.
Growers should choose hybrids rated a "5" or "6" for fields at risk of NCLB infection.
Two types of resistance are available in hybrids:

Two types of resistance are available in hybrids

HT lesion type: A chlorotic region may surround the lesion

Management

Crop rotation to reduce previous corn residues and disease inoculum
Tillage to help break down crop debris and reduce inoculum load
Fungicide application to reduce yield loss and improve harvestability
Consider hybrid susceptibility, previous crop, tillage, field history, application cost, corn price

Corn foliar fungicides and efficacy against NCLB. Adapted from Wise, 2014.

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