Crop Insights written by Leroy Svec¹, Bill Dolezal², Madeline Henrickson3, and Mark Jeschke4
Northern corn leaf blight (NCLB) is caused by the fungus Exserohilum turcicum, also known as Setosphaeria turcica and previously known as Helminthosporium turcicum. The disease organism overwinters as mycelia and conidia in diseased corn leaves, husks, and other plant parts. Spores are produced on this crop residue when environmental conditions become favorable in spring and early summer. These spores are spread by rain splash and air currents to the leaves of new crop plants, where primary infections are produced. Infection occurs when free water is present on the leaf surface for 6 to 18 hours and temperatures are 65 to 80° F.
Secondary spread occurs from plant to plant and field to field as spores are carried long distances by the wind. Infections generally begin on lower leaves and then progress up the plant. However, in severe NCLB outbreak years (that have high spore levels), infections may begin in the upper plant canopy. This can occur when weather systems deposit spores from southern growing areas, such as Mexico and the Caribbean. In recent years, weather patterns with large storms moving from south to north over the North American continent have spread the NCLB organism into additional northern regions.
Figure 1. NCLB disease cycle.
Heavy dews, frequent light showers, high humidity, and moderate temperatures favor the spread of NCLB. Development of disease lesions on the ear leaf or above and significant loss of green leaf area can result in yield loss.
There are multiple races of Setosphaeria turcica documented in North America. These races can be region specific and are able to undergo race shifts. This requires corn breeders to be mindful of the different races and tailor their breeding programs accordingly. The resistance genes available to corn breeders are named “Ht” based on the previous NCLB fungal name (H)elminthosporium (t)urcicum. The common sources of resistant Ht genes are dominant genes and provide resistance to key races of Setosphaeria turcica (St) as shown in Table 1.
Table 1. Common sources of resistance Ht genes.
To provide disease resistance to NCLB when multiple races might be present, two or more Ht genes may be needed. Because of these multiple races of NCLB, Pioneer breeders are incorporating additional Ht genes in their hybrid development programs (i.e., a “multigenic” approach). Resistant phenotype and inheritance of NCLB resistance genes are shown below (Table 2).
Table 2. “Ht” resistance genes.
¹sht1 is a dominant inhibitor of Ht2, Ht3, and Htn1 (but not of Ht1) in some parent lines.
The resistant phenotype, which appears with Ht1, Ht2, and Ht3 genes, is tissue chlorosis, where normal green color begins to change to a yellow hue in leaf lesions (Figure 3, left). These NCLB lesions are slower to develop, and there are fewer spores produced per lesion.
With the Ht4 gene, a chlorotic “halo” appears around the lesions, which are somewhat smaller in size and fewer in frequency.
The Htn1 gene prolongs the latent period before lesions occur; fewer and smaller lesions develop with fewer spores produced per lesion. The plant can maintain its health longer even with the disease organism present (Figure 3, right).
The Htm1 and NN genes provide complete resistance, and minimal lesions are noted in plants with these genes present.
Figure 3. Left - Ht1 “chlorotic” reaction – slower to develop and fewer spores produced per lesion. NCLB disease cycle. Right - HtN type reaction – fewer, smaller lesions develop and fewer spores produced per lesion.
Susceptible and resistant reactions are shown in Figures 4-6.
Figure 4. Susceptible response, early lesions. Plant has no resistance, but lesions have not had time to fully develop.
Figure 5. Susceptible response, later lesions. Lesions have expanded to form large areas of necrotic tissue. Entire leaves may eventually become necrotic.
Figure 6. Resistant response. Note chlorotic halo surrounding lesions and restricted development of lesions, indicative of resistant response.
Pioneer evaluates corn hybrids in multiple environments to observe their reaction to NCLB infection. Inoculated plots as well as “natural infection” sites are used to establish disease pressure. Both basic research trials (small plots) and advanced testing trials (larger IMPACT™ plots) are used for this hybrid characterization process. Use of numerous widespread locations, including those with a history of extreme NCLB incidence, helps ensure that some environments will provide severe NCLB pressure to challenge even the best hybrids. It also helps provide exposure of hybrids to as many race variants of NCLB as possible. The critical time for evaluating disease damage begins in the early reproductive stages of development.
"Development of disease lesions on the ear leaf or above and significant loss of green leaf area can result in yield loss."
The Pioneer 1 to 9 NCLB scoring system is based on “leaf loss” from the disease. A score of “9” indicates no leaf loss, and a score of “1” denotes 95% leaf loss in the presence of the disease. In determining overall hybrid ratings, experimental hybrids are compared to hybrids of “known” response to NCLB. This provides a “relative” rating system in which new hybrids are characterized as accurately as possible relative to established hybrids that are more familiar in the marketplace.
Figure 7. Illustration of Pioneer scoring system for NCLB.
When photosynthesis is limited by loss of green leaf area due to disease lesions, corn plants remobilize stalk carbohydrates to developing ears. When this occurs, stalk quality is reduced, often resulting in harvest losses. Hybrids with higher leaf disease scores tend to maintain leaf health and overall plant health longer into the grain filling period. This maintenance of plant health results in higher yields, better stalk standability, and increased grain harvestability.
Effective management practices that reduce the impact of NCLB include selecting resistant hybrids, reducing corn residue, timely planting, and applying foliar fungicides.
An important first step in managing NCLB is selecting resistant hybrids based on disease reaction characterization scores. The Pioneer NCLB rating reflects the hybrids’ expected performance against the major NCLB races predominant in your area. As race shifts inevitably occur, continued testing by Pioneer researchers may result in a rating adjustment for some hybrids. Use of multigenic resistance by breeders increases hybrid stability as NCLB races shift over time.
Hybrids should be selected based on all important traits needed for a field. In addition to NCLB resistance, select hybrids with high yield potential, appropriate insect resistance traits, suitable (usually full-season) maturity for the area, and consistent performance demonstrated in data from multiple locations and years. Strong emergence, stalk strength, and drought tolerance are other agronomic characteristics to consider in helping to optimize stands and harvestable grain yields.
Reducing corn residue decreases the amount of NCLB inoculum available to infect the subsequent crop. Crop rotation is one effective method of reducing residue. In addition, any form of tillage that places soil in contact with corn residue promotes decomposition and decreases the amount of residue that survives to the subsequent cropping season. Stover harvest for cellulosic ethanol production or animal feed is another means to reduce corn residue and disease inoculum. However, reducing corn residue does not protect against spore showers carried into a field on wind currents.
Timely planting can often help hybrids escape the most severe damage from NCLB if crop development outpaces normal disease progression. The latest-planted corn in an area may be infected when plants are smaller, resulting in the disease progressing more rapidly relative to the crop. However, in cases of high disease incidence, both early- and late-planted corn may be severely damaged.
Various foliar fungicides are available to help control or suppress NCLB development (Table 3).
Table 3. Common corn foliar fungicides and efficacy against NCLB5,6 (Wise, 2019).
Though fungicides are routinely used by growers to protect against several common leaf diseases, NCLB may not always be controlled as completely as some other diseases. This is due to the more rapid life cycle of NCLB, which may be as short as one week under favorable conditions. Because NCLB sporulates so rapidly, it is more difficult to time a single fungicide application. Consequently, selecting resistant hybrids is a crucial first step in managing NCLB where incidence is historically high.
Decisions to use a fungicide must be based on the disease risk factors of the field, including hybrid susceptibility, cropping sequence, tillage system, location, disease history, yield potential, the price of corn, and expected weather during reproductive development. Weather conditions anticipated during ear fill are a primary factor for disease development and often have the most impact (along with hybrid disease rating) on the profitability of fungicide applications.
Pioneer on-farm trials were conducted at 40 locations in Iowa in 2015 to evaluate corn yield response to foliar fungicides applied at different timings. Northern corn leaf blight pressure was high in much of Iowa in 2015 and it was the predominant foliar disease at the trial locations (Figure 8). Trials compared yield of corn treated with DuPont™ Aproach® Prima fungicide at the VT, R1, or R2 stage to non-treated corn.
Figure 8. Field trial comparing fungicide treated (left) and non-treated corn (right) at a location with high northern corn leaf blight pressure in 2015.
Figure 9. Average fungicide yield response of Pioneer® brand hybrids with different levels of genetic resistance to northern corn leaf blight in 40 Pioneer Agronomy trials in Iowa in 2015.
Results showed that yield response to fungicide application varied by hybrid genetic resistance to northern corn leaf blight. A yield response of 13 bu/acre was observed with hybrids rated a 3 on a 1-9 scale for northern corn leaf blight, while hybrids rated a 6 for northern corn leaf blight had an average yield response of 9 bu/acre (Figure 9). Fungicide yield response was greatest at the VT application timing (Figure 10).
Figure 10. Average yield response to fungicide applications at the VT, R1, or R2 growth stages in 40 Pioneer Agronomy trials in Iowa in 2015.
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1Leroy Svec, Pioneer Research Scientist (retired)
2Bill Dolezal, Pioneer Research Fellow (retired)
3Madeline Henrickson, Pioneer Agronomy Sciences Intern
4Mark Jeschke, Pioneer Agronomy Manager
5Fungicide performance is variable and subject to a variety of environmental and disease pressures. Individual results may vary.
6Always read and follow all label directions and precautions for use when applying fungicides. Labels contain important precautions, directions for use and product warranty and liability limitations that must be read before using the product.
The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary.
Aproach® and Aproach® Prima are not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.
