Corn Gray Leaf Spot


Gray leaf spot, caused by the fungus Cercospora zeae-maydis, occurs virtually every growing season. If conditions favor disease development, economic losses can occur. Symptoms first appear on lower leaves about two to three weeks before tasseling. The leaf lesions are long (up to 2 inches), narrow, rectangular, and light tan colored. Later, the lesions can turn gray. They are usually delimited by leaf veins but can join together and kill entire leaves.

All corn hybrids and inbreds are susceptible to Gray Leaf Spot (GLS). In minimum tillage fields with a history of GLS, symptoms can occur on the lower leaves prior to tasseling. Immature GLS lesions can be easily confused with lesions caused by other pathogens; e.g., eyespot (Kabatiella), anthracnose (Colletrichum). Mature lesions on leaves are rectangular in shape and restricted by leaf veins; reverse lighting reveals a yellow halo on most hybrids. Mature lesions are diagnostic and easily distinguishable from other diseases. As lesions expand they coalesce resulting in a blighting of large portions of the leaf. When the source of primary innoculum is within the field, symptoms originate on the lower leaves and progress up the plant as infection cycles occur. When severe, the entire plant may be blighted. Stalk infections have been reported and can result in lodging if severe. Lesions on sheath or husk tissues may be be oval to ellipsoidal in shape and are not usually diagnostic. Severely affected fields can appear to be experiencing early dry-down. The time from infection to the production of more innoculum can be 14-21 days on a susceptible hybrid and 21-28 days on a tolerant hybrid.

Management

Genetic Resistance

Hybrid tolerance/resistance is the best option for management of GLS. Tolerant hybrids yield well under moderate to high disease pressure.

Cultural Practices

Tillage can reduce the risk of GLS when the residue is incorporated into the soil and decomposition results. However, when other GLS management measures are employed; e.g., crop rotation, the use of tolerant hybrids, and the application of fungicides, the benefits from reduced tillage (e.g., increased water retention, reduced erosion) outweigh the negative impacts of GLS.

Rotation to crops other than corn for at least one year in low risk fields (minimum two years in high risk fields) may minimize the severity of GLS.

Chemical/Biological Control

Under high disease pressure, timely fungicide applications can greatly minimize impact on yield. These applications are costly and usually only practical when disease pressure is high.




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