Field Monitor News

By Gabe Hardman, Regional Agronomist

July 8, 2007

This summer, conditions have been right for the development of gray leaf spot. Monitor fields closely as they head into tassel and silking. Many available fungicides need to be applied at this time.

In order for a disease to exist, three components need to be present:
•    a susceptible host,
•    a virulent pathogen,
•    and a favorable environment for disease development.
If one of these components is absent then the plant disease cannot exist.

One such important corn disease across the Corn Belt is gray leaf spot. Gray leaf spot overwinters on plant debris and affects corn, grain sorghum, sudangrass, Johnson grass and barnyard grass.

Fungal structures, called conidia, form in the spring. These spores are blown by the wind to susceptible hosts where germination occurs through leaf structures called stomata.

Symptoms first occur as chlorotic spots, or lesions, that are elongated and rectangular in appearance. Lesions are tan to brown and have a grayish hue to them when sporulation begins. Gray leaf spot will spread in humid conditions and move more readily through splashing affects under rain and overhead irrigation.

If gray leaf spot is allowed to move above the ear leaf into the upper canopy of corn the potential for significant yield loss is likely.

Certain hybrids show varying susceptibility, tolerance, or resistance to gray leaf spot. Also, researchers believe that the fungus has different races; therefore a certain hybrid may show resistance to one race of gray leaf spot but susceptibility to another race.

There are several management strategies available to combat gray leaf spot. Mechanical tillage buries affected residue lowering the threat of infection. Rotation to non-host crops offers another effective method for combating infection. However, in minimum and no-till situations, significant host-crop debris may still be present, even under crop rotations, for infection to occur.

Chemical options are also available with fungicides offering a preventative, curative or a combination of both preventative and curative products. However, these fungicides need to be applied early, often before it is known for sure if the disease will cause economical yield loss.

Finally, as mentioned before, planting tolerant or resistant varieties offers another method to manage gray leaf spot.