An Overview of Northern Corn Leaf Blight

Introduction

Northern corn leaf blight (NCLB) is a yield-limiting disease in the United States and Canada. Despite its name, the disease can be severe across a broad geography, including the southeastern U.S. This publication describes the symptoms and signs of NCLB, environmental conditions that favor the disease, how to differentiate NCLB from other corn diseases and disorders, and effective management practices.

Symptoms and Signs

Symptoms of NCLB vary depending on hybrid resistance and the genotype of Exserohilum turcicum (commonly called race or races) that infects corn. On susceptible hybrids, early symptoms of NCLB appear as small, light-colored elliptical lesions that develop parallel to leaf margins or veins and have a diffuse margin and a water-soaked appearance (Figures 1,2). These lesions continue to expand as the disease progresses, sometimes reaching up to 6 inches long and expanding across leaf veins into the classic “cigar-shape” that is commonly associated with NCLB (Figure 2).  Over time, the lesions become grayish or tan, and can have dark-gray or green masses of fungal spores in the center that are visible with a hand lens or the zoom feature on a smartphone camera (Figure 3). Multiple lesions can develop on individual leaves and cover large portions of leaves. On susceptible hybrids, lesions can coalesce resulting in severe leaf blighting (Figure 4).

On hybrids resistant to NCLB, the symptoms of NCLB will express differently than on susceptible hybrids. In hybrids that have partial (non-race specific) resistance, lesions may be small and restricted in size, with fewer spores produced in the lesion. In this situation, symptoms of NCLB may be overlooked in the field because lesions will not be classically “cigar-shaped,” but could be small, narrow yellow or chlorotic streaks on the leaves (Figure 6).

Disease Cycle

The fungus E. turcicum causes NCLB and survives winter on corn residue.

In areas with large amounts of residue, lesions may be observed first on the lower leaves as spores are rainsplash- and wind-dispersed from residue within the field. Symptoms of NCLB will continue to develop in the middle and upper canopy if conditions favor secondary spore dispersal and disease development. Since spores of E. turcicum can spread by wind, disease symptoms may also be observed first in the upper canopy as spores produced in surrounding areas are deposited in new fields. Symptoms of NCLB can appear at any time in the season, but are typically observed from the late vegetative stages (V10-V14) through physiological maturity (R6).

Conditions that Favor Disease

The fungus that causes NCLB requires relatively long periods of leaf moisture (6 hours) for infection, which may be supplied by rain, heavy dew, or overhead irrigation. Subsequent disease development is favored by high environmental moisture and moderate temperatures (64°to 81°F; 18° to 27°C), although infection can occur at temperatures outside of this range. Under favorable conditions lesions can appear on leaves within a week after infection. Hot, dry weather reduces spread of the NCLB fungus and slows development of NCLB.

Yield Losses and Impact

If NCLB lesions are present in the canopy prior to and just after tasseling (VT), and conditions are favorable for disease development, significant yield losses can occur in susceptible hybrids. Up to 50% yield losses have been observed in susceptible hybrids if the disease is established prior to tasseling. Late season development of NCLB (after dough/R4) will likely have a reduced impact on yield.

Diagnosis

In mature NCLB lesions, fungal spores on the ends of fine fungal threads (hyphae) may be visible with the aid of a magnifying tool (x20), such as a hand lens or smartphone camera. These dark gray-green spores are typically in the center of a lesion and not confined within another fungal structure (Figure 7). Spores of E. turcicum are most noticeable after periods of dew or rain. Spores may be less easy to distinguish in dry conditions or in older lesions, where the lesion center may degrade. Submitting samples to a diagnostic laboratory can help ensure an accurate diagnosis.

Diseases, Disorders, and Injury with Similar Symptoms

Diseases

Diplodia leaf streak

Diplodia leaf streak symptoms begin with a small lesion that expands across the leaf and can range in size from a small streak to several inches long and wide. Lesions may have concentric rings and are commonly observed in the mid-to-lower canopy (Figure 8).

How to distinguish Diplodia leaf streak from northern corn leaf blight:

The fungus that causes Diplodia leaf streak produces round fungal structures in the center of lesions. These fungal structures are visible with the naked eye and resemble specks of pepper in a lesion, and thus are distinct from the fungal spores produced by E. turcicum.

Goss’s wilt

Early lesions of Goss’s wilt have a water-soaked appearance and as lesions progress, black “freckling” that is diagnostic of the disease can be observed in the water-soaked edges of the lesions. Bacterial exudate may also cause the leaves and lesions to have a shiny appearance (Figure 9). Goss’s wilt can be found across the Corn Belt and in Manitoba Canada, but is rarely observed in southern states.

How to distinguish Goss’s wilt from northern corn leaf blight:

NCLB is caused by a fungus and will not have the shiny bacterial exudate observed with Goss’s wilt. Lesions of NCLB will also not have freckling or water-soaking associated with Goss’s wilt.

Southern corn leaf blight

Southern corn leaf blight (SCLB) lesions vary depending on the genotype (race) that causes the disease, but are generally tan or brown rectangular lesions with a brown margin and are often surrounded by a yellow halo. 

How to distinguish southern corn leaf blight from northern corn leaf blight:

Southern corn leaf blight lesions typically remain in the lower canopy and are smaller than lesions of NCLB, but can resemble lesions resulting from a resistant (hybrid) reaction to NCLB. Laboratory diagnosis may be needed to distinguish SCLB from NCLB.

Management

Multiple management options are available to minimize the impact of NCLB. One of the most effective options is to plant hybrids that are less susceptible to NCLB (Figure 10). Partial resistance and hybrids with race-specific (Ht genes) resistance are available. However, it may be difficult to obtain information on which Ht genes are available in certain hybrids, and which race-specific resistance Ht gene(s) are needed in some states/provinces to manage NCLB. Research characterizing the race structure of E. turcicum in the United States is most likely outdated, so it is important to speak with state Extension specialists to determine what is known about the local E. turcicum populations (frequency) within a given state. Discussions with local seed dealers can also help to select hybrids that have reliable NCLB resistance ratings, including the source of any resistance (Ht genes), in a given hybrid for your area.

The NCLB fungus survives in corn residue, and consequently, continuous corn and conservation tillage practices increase the risk for disease. Crop rotation and residue management where possible can help reduce in-field sources of primary inoculum. However, conservation tillage practices should not be abandoned as the spores of the NCLB fungus can travel from one field to the next, rendering tillage and crop rotation ineffective as sole management options. 

Foliar and at-plant fungicides are available for NCLB management. Preventative applications are most effective, and foliar fungicide timings targeting tasseling/silking (VT/R1) have been most successful at reducing NCLB compared to a non-treated control in many University studies.

Fungicide efficacy ratings for NCLB are presented in the annually updated table, Fungicide Efficacy for Corn Diseases.

Acknowledgments

Authors

Kiersten Wise, University of Kentucky; Gary Bergstrom, Cornell University; Trey Price, Louisiana State University, LSU Ag Center; Alison Robertson, Iowa State University; and Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs.

Reviewers

Tom Allen, Mississippi State University; Alyssa Collins, Pennsylvania State University; Maira Duffeck, Oklahoma State University; Travis Faske, University of Arkansas; Andrew Friskop, North Dakota State University; Heather Kelly, University of Tennessee; Dean Malvick, University of Minnesota; Daren Mueller, Iowa State University; John Mueller, Clemson University; and Damon Smith, University of Wisconsin-Madison.

Photo Credits

All photos were provided by and are the property of the authors and contributors.

Sponsors

The authors thank the National Corn Growers Association and the Grain Farmers of Ontario for their support.