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Issue No. 17, Article 2/July 16, 2004

Gray Leaf Spot of Corn Causing Concern in Illinois

Gray leaf spot (GLS) can be one of the most significant diseases on corn in the Midwest. Weather and other conditions have been favorable recently for development of gray leaf spot in Illinois. Numerous fields with significant levels of this disease have been reported in the past week. Information about this disease and management options are discussed.

Symptoms and effects of gray leaf spot. Lesions of GLS begin as small, necrotic spots with halos. The spots can be oval to elongated. As the lesions grow and mature, they become rectangular in shape. Mature lesions typically have distinct parallel edges and a gray appearance, and they appear opaque when held up to the light.

Developing lesions of gray leaf spot showing lesions of different sizes and shapes.

Typical, mature lesions of gray leaf spot on corn.

GLS can have several effects on corn. Yield reduction has been shown to occur due to smaller ears, unfilled kernels, and reduced kernel size. Losses from GLS have also been reported to occur due to increases in stalk and root rots.

Disease cycle and infection period. GLS is caused by the fungal pathogen Cercospora zeae-maydis. GLS infections are initiated from fungal spores that are produced on infested corn residue on the soil surface. Large numbers of spores of the GLS pathogen can be produced on residue beginning in May or June. After the spores are disseminated by wind and rain to the new crop, there may be a 2- to 3-week lag (latent) period before lesions fully develop. Infection may occur within about 1 week after spores arrive on a leaf surface under favorable environmental conditions, but mature lesions may not develop for another week for susceptible hybrids or 2 weeks for moderately resistant hybrids.

Conditions favorable for GLS. To cause significant damage, GLS generally requires high humidity (>95%) for at least 24 hours, warm temperatures (75°F to 85°F), and susceptible inbreds or hybrids. The disease typically occurs from silking to maturity. The GLS pathogen overwinters in corn debris on the soil surface; hence the disease appears to have historically increased along with increases in no-till or minimal-till field management practices. Corn appears to be the only known host of this pathogen. Corn-on-corn rotations may favor GLS.

Management options. Management for future reductions in GLS should focus on resistant corn hybrids, although crop rotation should also be considered. Immediate concerns with GLS when it occurs in the field are often focused on fungicides. Many trials in the midwestern and eastern United States, including in Illinois, have shown that fungicides can reduce GLS disease severity and increase yields when GLS is a problem. Fungicide decisions should take into account susceptibility of the hybrid (or inbred), stage of crop development, disease severity, and to some extent weather conditions. Studies (for example, a study from Iowa, Munkvold et al., 2001) suggested that three conditions are needed to maximize the probability that some fungicides will provide an economic return:

  • An application at or before stage VT (tasseling)
  • Hybrids susceptible to GLS
  • High disease pressure and prolonged high humidity

Timing appears important to get maximum benefit from fungicides for GLS control. Several studies suggest that initial fungicide applications should occur no later than tasseling (VT) when GSL is developing. Later applications may also be beneficial, but studies suggest one application at or before VT may be important. For example, studies from Iowa (Munk-vold et al., 2001) suggested that "optimal timing for one application is at VT (tasseling) and for two applications at V7-V8 and VT." Another study from Indiana (Shaner et al., 1998) suggested that an initial fungicide application at silking (R1) may have been too late to prevent much of the infection.

It has been very difficult to develop a threshold level of GLS infection for hybrids that should trigger fungicide applications, because disease levels at any one time have not been reliable predictors of subsequent disease development. For example, a threshold of 10% leaf area infected on the lower six leaves of susceptible hybrids at the V18/VT stage may work under some situations and may not work under others. There are no hard and fast rules, but GLS can be destructive and should be managed with resistant hybrids and perhaps with properly timed fungicides where a susceptible hybrid is planted, the yield potential is high, disease pressure is high, and the environment is favorable for GLS.--Dean Malvick

Dean Malvick

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