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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.
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.
Figure 2. Severe GLS leaf infection.
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.
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:
Figure 3. GLS symptoms on tolerant, moderately tolerant, and susceptible corn products (left to right).
Sources: 1 Robertson, A. and Mueller, D. 2011. Foliar fungicides on corn. Integrated crop management. Iowa State University. http://crops.extension.iastate.edu/cropnews/2011/07/foliar-fungicides-corn.
2 Rees, J.M. and Jackson, T.A. 2008. Gray leaf spot of corn. NebGuide G1902. University of Nebraska-Lincoln Extension. http://extensionpublications.unl.edu/. 3 Robertson, A.E. 2012. Gray leaf spot. USDA-NIFA Outreach Webcast. www.plantmanagementnetwork.org. 4 Stromberg, E.L. 2009. Gray leaf spot disease of corn. Virginia Cooperative Extension.
https://pubs.ext.vt.edu/450/450-612/450-612.html. 5 Wise, K. 2010. Diseases of corn: gray leaf spot. Purdue University Extension. www.extension.purdue.edu. 6 Wise, K. 2018. Fungicide efficacy for control of corn diseases. CPN-2011-W. Crop protection network. https://cropprotectionnetwork.org/2018/03/21/fungicide-efficacy-for-control-of-corn-diseases/. Web sources verified 07/05/18. 140318060805.