Northern Corn Leaf Blight and Gray Leaf Spot in Corn

Northern corn leaf blight  

Midwestern summers often bring humid conditions, which in turn lead to a greater chance of your crop being infected by northern corn leaf blight (NCLB). A result of cool temperatures, high humidity and moist conditions, NCLB is a disease caused by the fungus Exserohilum turcicum.

The fungus overwinters as mycelium and spores in corn residue, which are dispersed by wind and splashing water. NCLB development typically occurs at or after silking and is favored by extended periods of leaf wetness, such as rain or heavy dew and moderate temperatures of 65-80 degrees.

The earlier infection occurs, the greater the damage. If the disease becomes established before tasseling, The Ohio State University reports yield losses may be as high as 30%-50%. Symptoms are most often observed following heavy dew and on overcast days.

What to look for:

• Symptoms usually first appear on the lower leaves.
• Cigar or elliptical-shaped lesions develop longitudinally with leaf veins.
• Lesions are light green or gray in color, then soon turn tan.
• Dark spores appear, making the leaf surface seem dirty within the lesion.
• Severely affected plants will kill entire leaves, making it difficult to find individual lesions.

NCLB can result in less photosynthetic area, leading to potential yield loss. Additional plant damage includes stalk rot development and lodging.

To help prevent NCLB, manage residue with a one-year rotation away from corn as well as a fungicide management plan. Purdue University recommends applying fungicides during tasseling to early silking stages (VT-R1) for the best opportunity of success.

Gray leaf spot

Gray leaf spot (GLS) is also one of the biggest yield-robbing corn diseases farmers face in the U.S. GLS can rapidly increase with warm temperatures (70°-90°), high humidity (greater than 90%) or periods of prolonged leaf wetness (12+ hours). The likelihood of GLS increases greatly in fields with prior year corn residue that can host the pathogen, such as continuous corn crop rotations and/or minimal/no-till management.

What to look for:

• Infection starting in the lower canopy, progressing upward.
• Lesions beginning as small oval, light-tan spots with circular halos.
• Lesions typically expand into long, narrow, rectangular lesions parallel to the leaf veins and later turn gray.
• Symptoms also can be confused with bacterial leaf streak (BLS), however, GLS has blocky/rectangular lesions, whereas BLS has wavier lesion margins.

Potential yield loss

Both diseases hurt yield by reducing the corn plant’s photosynthetic area, which lowers the carbohydrate production needed for kernel development. This reduction in photosynthetic leaf area results in carbohydrates being scavenged from the stalks, predisposing the plant to stalk rots and lodging. Early infection and conducive conditions for development can lead to significant yield losses. Table 1 illustrates the yield loss associated with varying levels of GLS infection at R5 or dent, roughly 25-30 days before reaching maturity (R6 to black layer).

During high moisture conditions conducive to pathogen development and early establishment of NCLB, yield losses can be as severe as 30–50%. However, with lower levels of infection or delayed infection (6 weeks after silking), yield losses are usually minimal.²

Treatment and management practices for NCLB and GLS

Corn hybrids do have some levels of resistance to both diseases. However, there are truly no resistant or immune hybrids. Each Golden Harvest® corn hybrid has a specific rating for its genetic ability to handle GLS and NCLB. More information on hybrid disease tolerance is available in the Golden Harvest corn product finder, which should be referenced for future product placement.

Since both pathogens can overwinter, residue management also decreases future GLS and NCLB pressure. Practices that bury residue, such as crop rotation and tillage, can reduce spore carryover to the following year.

Fungicide applications are also effective for controlling both diseases. When lesions are present at the ear leaves 2 weeks prior to and after tasseling, using a proven product with good activity on target disease is recommended. If considering an early application to proactively prevent GLS or NCLB, spray a fungicide with preventive and curative disease control, such as Trivapro®, to help maximize yield and profit potential.

Contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights. 

Sources:

1. Patrick Lipps, 1998, The Ohio State University. Gray leaf spot and yield losses in susceptible corn hybrids. Crop Observation and Recommendation Network. Issue 98-23.
2. Salgado, J.D., Schoenhais, J, and Paul, P.A. 2016. Northern corn leaf blight. Ohio State Extension PLPATH-CER-10. https://ohioline.osu.edu/