Gray Leaf Spot of Corn

KEY POINTS

  • Gray leaf spot (GLS) is one of the most significant yield-limiting disease of corn worldwide.
  • Management strategies to reduce the impact of GLS include planting tolerant seed products, foliar fungicide application, residue management, and crop rotation.

Symptoms

Due to an extended latent period, initial symptoms of gray leaf spot (GLS) typically develop two to four weeks after infection. Immature lesions appear as small, brown or tan spots on the leaf surface surrounded by a yellow halo and may resemble lesions caused by other pathogens such as anthracnose, eyespot, or initial common rust pustules. As the lesions mature, they elongate and develop into distinct symptoms of GLS, which are characterized by rectangular, brown to gray necrotic regions that run parallel to the leaf spanning the spaces between the major leaf veins (Figure 1). Mature lesions are distinguishable from other diseases due to their rectangular shape.


GLS

Figure 1. GLS rectangular lesions.


In severe infestations the lesions will coalesce and large portions of the leaf may be blighted (Figure 2). Severe blighting results in premature death of leaves which contributes to a reduction of available photosynthate for kernel fill. Also, severe blighting may contribute to stalk deterioration resulting in increased plant lodging.


GLS

Figure 2. Severe GLS leaf infection.


Disease Cycle

The fungal pathogen overwinters as stromata on infected corn residue left on the soil surface. The pathogen rarely survives if infected residue is buried below the soil. In the spring, with adequate moisture and humidity, fungal infection structures, called conidia, are produced from the stromata and disseminate to newly planted corn by wind and splashing rain. Often the lower leaves of the corn plant are the first to become infected. Mature lesions will develop after several weeks, from which secondary inocula (conidia) will be produced and continue the cycle of infection.

The pathogen that causes GLS is favored by prolonged periods of high relative humidity (90% or greater) and warm temperatures (70-90 °F). Long lasting fogs or heavy dews are particularly favorable for GLS infection. When environmental conditions are not optimal for germination and infection (adequate leaf wetness, humidity, and temperature), conidia may remain dormant on the leaf surface until conditions improve. The length of the infection cycle may vary from two to four weeks, depending on the environment and on the susceptibility or tolerance of the corn product.

Management

Fields that are at highest risk for potential yield loss due to GLS are those that have had a history of the disease and are planted to continuous corn under no-till or reduced-till practices. Preventative and in-season management strategies to reduce the impact of GLS include the following:

  • The use of tolerant corn products is the most effective strategy to preserve yield potential, especially in fields with a high probability of GLS occurrence. All corn products that are currently on the market have some degree of susceptibility to GLS but some products have levels of tolerance that adequately prevent yield losses (Figure 3).

  • GLS

    Figure 3. GLS symptoms on tolerant, moderately tolerant, and susceptible corn products (left to right).


  • Timely fungicide applications may be effective at minimizing yield losses when susceptible and moderately susceptible products are planted in situations where GLS is likely to occur (Table 1). Because of the lengthy latent period, there is no standard economic threshold for GLS. However, a general recommendation to reduce the impact of foliar fungal diseases is to consider a fungicide application if a fungal disease is present on the third leaf below the ear leaf or higher on 50% of the plants at tasseling and the product is susceptible to the disease.1 Tasseling to early silking (VT-R1) growth stage is the critical time to implement controls to protect yield potential if environmental conditions have been or are likely to be favorable for infection.
  • Tillage can have a substantial impact on the occurrence of GLS because as stated earlier the pathogen does not survive well when host debris is incorporated into the soil.
  • Rotating to a crop other than corn for one year, followed by tillage, or for two years or more for reduced-tillage fields, can reduce levels of disease inocula in fields where there has been a recent history of GLS.
  • Growing corn for silage significantly reduces the amount of inocula available to infect corn the following season because silage corn is usually harvested before significant blighting occurs and a very limited amount of corn residue remains in the field after harvest.

GLS
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