The recent warm, humid weather has been favorable for development of the corn disease gray leaf spot. This disease has been observed mostly on lower leaves of corn in fields, but lesions may be found on leaves right below the ear leaf in some susceptible hybrids. We recommend scouting fields now to determine the level of disease present. The fungus that causes gray leaf spot, Cercospora zeae-maydis, infects the corn plant during prolonged warm (75°F to 85°F), humid (more than 90 percent relative humidity) periods. Symptoms are commonly observed following long periods of heavy dew and overcast days.

Gray leaf spot on cornEarly gray leaf spot symptoms are observed on leaves as small, pinpoint lesions surrounded by yellow halos. At this stage, it can be hard to correctly identify the disease, but as lesions mature, they elongate into narrow, rectangular, brown to gray spots (Figure 1). Lesions on susceptible hybrids expand parallel to leaf veins and may become 1.5 to 2 inches long.

Cercospora zeae-maydis spores can cease development during low humidity periods, and then resume the infection process once humidity rises. Each lesion can produce many spores, which are splashed or blown to the upper leaves or to other plants where they can survive until conditions are favorable for infection. This cycle makes it appear that the disease is moving up the plant.

Due to the length of the infection process, symptoms may not be noticeable for up to two weeks after infection, depending on weather conditions and hybrid susceptibility. Hot, dry weather will restrict disease development and spread.

Yield loss may depend on the number of lesions and how far up in the canopy they occur as the plant enters tasseling and pollination. If lesions have reached the ear leaf or higher during the two weeks before and after tasseling, yield loss could occur. If lesions develop on upper leaves later in the season, the economic impact will be less.
Preventative management strategies can reduce economic losses due to gray leaf spot. In-season disease management options, such as fungicides, are also available. Susceptible hybrids planted in no-till or reduced-till fields are at high risk for gray leaf spot development, but weather is the primary influence on disease development.

It is important to remember that a fungicide application is an additional cost to corn production, and growers must consider economic factors (corn market price, and fungicide application cost) and other disease factors before deciding whether to apply a fungicide for gray leaf spot management.

Research in Indiana indicates that strobilurin and strobilurin/triazole premix fungicides are most effective at preventing yield loss when applied in response to disease presence, and at the tasseling to early silking (VT-R1) growth stage. Scouting fields around V14, or just prior to tassel emergence, can help determine the level of disease pressure in a field.

Iowa State University developed guidelines to determine when a fungicide may be necessary to prevent yield loss. These thresholds and incorporate hybrid susceptibility ratings and disease levels prior to tasseling:

  1. Consider a fungicide application if:
    The hybrid is rated as susceptible or moderately susceptible AND 50 percent of the plants in a field have disease lesions present on the third leaf below the ear leaf or higher prior to tasseling. Please see the following video for identifying the area on the plant to scout.
  2. Consider a fungicide application if:
    The hybrid is rated as moderately resistant AND 50 percent of the plants in a field have disease lesions present on the third leaf below the ear leaf or higher prior to tasseling AND additional factors or conditions that favor disease development are present (residue present, favorable weather conditions).

Scout even resistant hybrids for disease problems, but, in general, fungicide applications to these hybrids are not recommended and will not consistently result in increased yield.

The thresholds available for fungicide application decisions are not hard and fast rules. It is important to remember that gray leaf spot severity can be unpredictable in Indiana, even when factors favoring disease are present. For instance, in 2010, we had very favorable weather conditions for gray leaf spot development at tasseling/silking stage of corn development. However, the environment became very hot and dry after silking, and disease did not progress as expected. Consequently, fungicide applications were not profitable in many of our research locations. Therefore, consider threshold guidelines, cropping practices, planting date, predicted weather conditions, and economic factors when deciding whether to use a fungicide to manage gray leaf spot, and manage expectations on what type of yield response a fungicide application will provide.