Gray leaf spot is one of the most important foliar diseases of corn and it is caused by Cercospora zeae-maydis. This fungus survives the winter on corn residue and as temperatures warm up in the spring, spores produced by the overwintering fungus will infect developing corn plants. Because the majority of the spores will be coming from corn residue on the ground surface, the symptoms are first observed on lower leaves and move up the plant to newer leaves. When spores are blown in from neighboring fields, the symptoms will sometimes show up in upper leaves without much lower canopy symptoms. Ideal conditions for disease infection include prolonged, warm (75°F to 85°F) humid periods (>90% HR). Spore development can be slowed down or stopped during periods of low humidity and resume when humidity rises. The appearance of symptoms might not occur for several weeks after infection depending on weather conditions and the disease tolerance of the hybrid selected. 

Symptoms first appear on lower leaves and include small tan to brown spots with yellow margins or “halos”. As the disease progresses, lesions are observed in the middle and upper canopy. Mature lesions are much more distinctive and appear as narrow rectangular brown to gray spots on the leaf surface, parallel to the veins (Fig. 1). As severity increases, these lesions may coalesce into large areas of blighting on the leaf surface, interfering with photosynthesis. In severe cases of gray leaf spot, the stalk integrity may be affected and result in lodging. As disease spreads widely throughout the field and lesions continue to develop and coalesce, it will appear as if the corn plants are senescing or drying down (Fig. 2).

If lesions develop on the upper canopy reaching the ear leaf during the period of time right before and after tasseling, yield loss can be significant. However, if the infection happens later in the crop cycle and symptoms do not develop or reach the ear leaf until after the grain fill period, then the impact on yield is much less.  Hybrids with partial resistance will restrict lesion growth and expansion. The extent of the yield loss will depend on corn growth stage and the severity of the disease.

A combination of preventative practices that include hybrid selection, residue management and crop rotation, along with timely fungicide applications can help manage gray leaf spot. Since the pathogen survives on corn residue, fields at greater risk of infection include those growing continuous corn and managed with reduced or no-till practices. Planting susceptible hybrids in disease favorable environments can also increase GLS severity and potential for yield losses.

Selecting resistant hybrids is particularly important in high risk fields. Resistance to GLS is not well understood and although there are a number of hybrids with resistance to this disease, none are immune. Resistant hybrids will get infected with GLS but will produce much smaller lesions and disease development may be slow. Seed companies will provide information on the GLS resistance ratings for their hybrids. Different companies use different scales to rate resistance so keep that in mind when looking for resistant hybrids.

The amount of GLS inoculum available at the start of the season can be reduced by rotating crops and reducing crop residue with tillage. However, soil conservation practices that minimize erosion and maintain soil quality are very important to farm productivity. Practices that accelerate corn residue decomposition or incorporation (fall tillage, vertical tillage), will help reduce the amount of local inoculum available during the next growing season.

A number of fungicides are labeled for management of gray leaf spot. Their efficacy will depend on a number of factors that include, timing of the application, location of the symptoms on the canopy at the time of the application, weather conditions before and after the application, residuality of the active ingredients used, spray coverage, etc. Several fungicides are labeled against gray leaf spot and research indicates that these products are most effective at protecting yields when applied from tasseling to early silking (VT-R1). Corn Fungicide efficacy tables can be found here: https://crop-protection-network.s3.amazonaws.com/publications/fungicide-efficacy-for-control-of-corn-diseases-filename-2020-03-18-150007.pdf. Scout fields and determine if GLS lesions are present on the third leaf below the ear or higher prior to tasseling. Additionally, economic factors (the value of the crop, fungicide cost, hybrid resistance) must be considered when making fungicide application decisions, as economic returns are not always guaranteed.