Northern corn leaf blight

Northern corn leaf blight (NCLB) is a foliar disease of corn (maize) caused by Exserohilum turcicum, the anamorph of the ascomycete Setosphaeria turcica. With its characteristic cigar-shaped lesions, this disease can cause significant yield loss in susceptible corn hybrids.

There are several host-specific forms of E. turcicum. The most economically important host is corn, but other forms may infect sorghum, Johnson grass, or sudangrass. The most common diagnostic symptom of the disease on corn is cigar-shaped or elliptical necrotic gray-green lesions on the leaves that range from one to seven inches long. These lesions may first appear as narrow, tan streaks that run parallel to the leaf veins. Fully developed lesions typically have a sooty appearance during humid weather, as a result of spore (conidia) formation. As the disease progresses, the lesions grow together and create large areas of dead leaf tissue. The lesions found in Northern corn leaf blight are more acute if the leaves above the ear are infected during or soon after flowering of the plant. In susceptible corn hybrids, lesions are also found on the husk of ears or leaf sheaths. In partially resistant hybrids, these lesions tend to be smaller due to reduced spore formation. In highly resistant hybrids, the only visible disease symptoms may be minute yellow spots.

In nature, E. turcicum lives and reproduces in an asexual phase with a relatively simple life cycle. In temperate regions, the fungus overwinters mycelia, conidia, and chlamydospores in the infected corn debris. When conditions become favorable the following season, conidia are produced from the debris and dispersed by rain or wind to infect new, healthy corn plants. Once on a leaf, conidia will germinate and directly infect the plant. The damage to the plant is relatively localized, although diseased corn plants are more susceptible to stalk rot than are healthy plants. In conditions with high humidity, the fungus will produce new spores at the leaf surface, which are spread by rain or wind through the crop and create cycles of secondary infection. One complete cycle on susceptible plants takes approximately 10 to 14 days, whereas it takes about 20 days on plants with resistance. At the end of the season, E. turcicum goes into a state of dormancy in crop residue.

The ideal environment for NCLB occurs during relatively cool, wet seasons. Periods of wetness that last more than six hours at temperatures between 18 and 27 °C (64 and 81 °F) are most conducive to disease development. Infection is inhibited by high light intensity and warm temperatures. Leaving large amounts of infected residue exposed in the field and continuing to plant corn in those fields will promote disease progress by providing large amounts of inoculum early in the season. Also, the number of conidia produced in an infected field increases significantly after rain due to the increase in moisture.

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