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Agronomic Crops Network

Ohio State University Extension


Continue to Scout for Corn Foliar Diseases

Figure 1. Gray leaf spot

A dry early June delayed disease progression in corn, but we have received reports of gray leaf spot (GLS), tar spot, and to a lesser extent, northern corn leaf blight (NCLB) after recent wet, humid conditions. Corn growth and development is variable across Ohio, ranging from VT to R3 (“milk” stage). Continued scouting is needed, especially in fields with susceptible hybrids and a history of foliar diseases. Scouting efforts should also focus on continuous-corn, no-till fields, since fungal pathogens causing disease can overwinter on crop residue. When walking fields, keep an eye out for these foliar diseases:

  1. Gray leaf spot: symptoms of GLS include tan, rectangular lesions (Figure 1) up to 2-4 inches long that first appear on lower leaves. Lesions are bordered by leaf veins and can turn gray later in season. Infection requires dew or foggy conditions and high relative humidity in the canopy.
  2. Figure 2. Progression of tar spot from a few lesions (stromata) to leaf blighting.Tar spot:  this relatively new foliar disease prefers wet weather and moderate temperatures and can progress rapidly under the right conditions. The main signs of tar spot are raised, black spots called stroma that cannot be rubbed off and appear on both upper and lower leaf surfaces (Figure 2). Learn more about how to differentiate tar spot from insect frass (poop) and other diseases by clicking here. In general, fungicides with multiple active ingredients (AI) are more effective against tar spot and should be applied between VT and R3 in a high disease environment (>5% severity).
  3. Figure 3. Northern corn leaf blight lesion.Northern corn leaf blight:  keep an eye out for tan, “cigar-shaped” lesions (Figure 3) that will expand up to six inches in length across leaf veins. NCLB is favored by high relative humidity and wet, but slightly cooler conditions than those that favor the development of GLS.

Having trouble with early disease diagnosis? See the article in this issue by Paul and Rotondo for information on how to handle and send samples to the plant disease clinic for laboratory diagnosis, confirmation, and monitoring of disease spread, particularly tar spot.

Based on our studies, when fungal disease is present in corn silage fields at harvest, a fungicide application at VT-R1 reduced fiber concentration and improved nutritional value compared to the untreated control. Corn treated with fungicide had improved fermentation due to more consistent dry matter values. When disease was severe, dry matter yield was also improved. When fungal disease infects corn, one of its natural responses is to increase lignin around the infected area to stop the disease spread, which reduces digestibility. Be cautious utilizing fungicide after R1 due to the preharvest interval, which for most products is 14-30 days, and may cause a delay in silage harvest which can be even worse than damage from disease.

Overall, fungicides are most effective in managing disease and protecting yield when applied at silking (R1) or tasseling (VT). However, yield response is variable when under low disease pressure or absence of disease. View efficacy of various corn fungicides based on multi-state trials by clicking here.

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.