Gray leaf spot on corn
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.
click image to zoomFigure 1. Gray leaf spot lesions on a corn leaf. Early 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.
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