Northern corn leaf blight, caused by the fungus Exserohilum turcicum, was one of the most important sweet corn diseases in South Florida causing significant losses some years. It is still a potential threat, occurring every spring and occasionally late fall. Resistant
varieties have helped reduce impact of the disease in recent times.
Initial symptoms of the northern corn leaf blight (NCLB) include yellow spots that develop on the foliage. These enlarge to form tan or straw-colored dead areas about four to six inches long and one-half inch wide. NCLB produces a long, elliptical lesion, while those of southern corn leaf spot tend to be oblong and much smaller than those produced by NCLB. Southern blight lesions also are lighter in color (light tan to brown), and have parallel sides rather than the tapering sides of lesions caused by E. turcicum.
NCLB moves from the lower canopy to the upper canopy. Fungal sporulation may be observed with a hand lens on foliar lesions following periods of high humidity. When severe, lesions may become so numerous that they coalesce and turn the entire leaf necrotic.
Survival And Spread
Sources of spores include volunteer corn, debris on the soil from previous crops, seed, and nearby corn plantings. Spores, which may be carried long distances, are spread by rain and wind. Lesions can produce spores in as little as one week, allowing NCLB to spread much faster than many other corn leaf diseases.
Disease development is favored by heavy dews, frequent showers, high humidity, and moderate temperatures. Infection occurs when free water is present on the leaf surface for six to 18 hours and temperatures are 65°F to 80°F.
Infections generally begin on lower leaves and progress up the plant, but infections may begin in the upper plant canopy when spore loads are high.
Although some control can be reducing inoculum through the use of crop rotation and deep plowing of old crop debris, control is best achieved with resistant varieties. Resistant varieties are available and should be considered, particularly for spring plantings.
Fungicide application can effectively control E. turcicum when applied at the right time. Fungicide should be applied when lesions first become visible on the lower leaves or when disease is reported to be in the area. Threat is highest from mid-February into April, but it may be seen during the fall as well.
Triazoles and strobilurins both provide control, with some pre-mixes giving superior control. These products should be used with a broad-spectrum protectant to minimize development of fungal resistance.
Use ethylenebisdithiocarbamate fungicides such as mancozeb as a protectant before disease is present. Apply four to six sprays on a five-to-seven-day basis. Use a surfactant/sticker as corn leaves are waxy and spray tends to run off. Rotate with a stobulurin like Headline (pyraclostrobin, BASF). As corn matures or disease becomes present, rotate between triazoles such as Folicur (tebuconazole, Bayer Crop Science), Monsoon (tebuconazole, Loveland Products), Propimax (propiconazole, Dow AgroSciences), and strobilurins or premixes of the two.