Gray leaf spot (GLS) is considered one of the most significant yield-limiting diseases of corn worldwide. Distinct symptoms of GLS are the rectangular, brown to gray necrotic lesions that run parallel to the leaf, spanning the spaces between the secondary leaf veins. Fields at highest risk for loss of yield potential include those that have a history of GLS and are planted to continuous corn under no-till or reducedtillage cultivation practices.
Symptoms of GLS appear on corn at the eight-leaf growth stage (V8) and later. Due to an extended latent period, initial symptoms of the disease typically develop two to four weeks after infection. Lower leaves of the canopy are the first and usually most severely affected. Immature lesions appear as small (1/8 inch), brown or tan spots on the leaf surface that are surrounded by a yellow halo when held up to light. As the lesions mature, they elongate and develop into distinct symptoms of GLS, which are characterized by rectangular, brown to gray necrotic lesions that run parallel to the leaf, spanning the spaces between the secondary leaf veins (Figure 1). In severe infestations, the lesions will coalesce and form large areas of dead leaf tissue.
The fungal pathogen that causes GLS, Cercospora zeae-maydis, survives between growing seasons in infested corn residue. In late spring, when temperatures are warm and humidity is high, spores are produced on the residue and blown onto the lower leaves of corn. A few weeks after infection, mature lesions develop and produce additional spores on the underside of leaves that continues the spread of the disease upwards into the plant canopy and throughout the field (secondary spread). Toward the end of the season when corn is ready to harvest, the fungus produces survival structures called stromata within the infected leaf tissues that give the lesions a blackened appearance (Figure 2). Infected corn residue left in the field after harvest will provide a source of inoculum for the following season.
Favorable Conditions. The fungal pathogen that causes GLS is favored by prolonged periods of high relative humidity (90% or greater) and warm temperatures (70-90° F). Extended periods of fog or heavy dew are particularly favorable for GLS infection. If environmental conditions, including adequate leaf wetness, humidity, and warm temperatures, are not favorable for infection when the spores land on leaf surfaces, the spores may sit dormant until conditions improve. The infection cycle may take two to four weeks depending on the environment and susceptibility of the corn product used.
Fields at highest risk for potential yield loss include those that have a history of GLS and are planted to continuous corn under no-till or reduced-tillage systems. Generally, GLS results in a yield impact ranging from 5-40%, but losses of more than 90% have been reported in unique situations. Yield loss due to GLS is a direct result of leaf tissue death, which reduces the area available for photosynthesis. Reduction in photosynthesis results in lost sugar production and reduced grain fill (smaller ears, lighter and smaller kernels). The earlier the infection occurs in the crop development cycle, the more time available for secondary spread and leaf damage, resulting in greater yield losses. The impact on yield depends on the severity of the disease in mid-canopy through the dough stage (R4). Yield losses should be minimal if the disease does not appear on the ear leaf or above by six weeks after tasseling. Stalk rot brought about by severe infestation can also cause secondary yield losses due to premature plant death and reduced standability.
The primary preventative management strategies to help control GLS include using tolerant products, crop rotation, and residue management. Other preventative measures that can help lessen the impact of GLS include early planting and weed control. In-season, fungicides are available to help minimize yield losses.
Product tolerance. All corn products currently on the market are potential hosts of the GLS fungus. None are immune to infection. GLS reactions vary in corn products and are scored from tolerant to susceptible. Figure 2 shows the difference in corn products classified as tolerant, moderately tolerant, and susceptible.
Crop rotation and residue management. The pathogen responsible for GLS survives in infested corn residue. Since corn is the only known host of the GLS fungus, rotation to a non-host crop for one to two years allows time for the infested residue to break down, which can lessen the severity of GLS. Burying infested residue also reduces the survival of the fungus and subsequent levels of the disease. If a large amount of infested residue occurs when in-season environmental conditions are favorable, severe levels of GLS may develop. Using a combination of both rotation and tillage greatly reduces survival of the fungus.
Fungicides. The determination of whether a fungicide application is warranted ultimately comes down to if it will be profitable. Profitability of fungicide use depends on factors associated with disease severity and yield potential: 1) product susceptibility to GLS, 2) severity of GLS infection, and 3) if weather conditions are favorable for GLS development. Secondary benefits of fungicides, in addition to control of GLS, may also be a factor.
Timely fungicide applications can be very effective at minimizing yield losses when susceptible and moderately susceptible products are planted in situations where GLS is likely to occur. Because of the lengthy latent period, there is no standard economic threshold for GLS. From tasseling to early silking (VT-R1) growth stages is the critical time to implement controls to protect yield potential if environmental conditions have been or are likely to be favorable for infection. Many fungicides are labeled for GLS in corn, typically a strobilurin or triazole, or a combination of the two, is recommended for control. Be sure to check specific product labels for application rates and further details.
Planting date. The impact of GLS is more severe if plants are affected early in their development. Early planting can help reduce yield losses by ensuring the crop is at a later stage of grain fill when conditions are typically favorable for GLS development.
Weed management. Fields with heavy weed cover have reduced airflow which can hold humidity in the crop canopy and prevent the drying of leaf surfaces. Keeping a handle on weed control can help reduce the favorable conditions for GLS development.
Choosing the right corn product for each field situation is very important. Channel® Brand corn products are rated for GLS tolerance on a 1-9 scale: 1 = Excellent and 9 = Poor. Specific product ratings can be found online at www.channel.com and you are encouraged to consult with your local Channel Seedsman for further details.
Rees, J.M. and Jackson, T.A. 2008. Gray leaf spot of corn. NebGuide G1902. University of Nebraska-Lincoln Extension. www.ianrpubs.unl.edu.
Robertson, A.E. 2012. Gray leaf spot. USDA-NIFA Outreach Webcast. www.plantmanagementnetwork.org. Stromberg, E.L. 2009. Gray leaf spot disease of corn. Virginia Cooperative Extension. www.pubs.ext.vt.edu.
Wise, K. 2010. Diseases of corn: gray leaf spot. Purdue University Extension. www.extension.purdue.edu.
Web sources verified 07/05/18. 140318060805