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. 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. On severely infected plants, lesions can become so numerous that the leaves are eventually destroyed. Late in the season, plants may look like they have been killed by an early frost. Lesions on products containing resistance genes may appear as long, chlorotic, streaks, which can be mistaken for Stewart’s wilt or Goss's wilt. 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. Sporulation requires a 14-hour dew period between 20 and 25 °C (77 °F). When there is not a sufficiently long period of continuous humidity, the fungus will stop making spores and resume conidia production only when humidity level rises again. For this reason, sporulation often occurs during nighttime and is halted when humidity falls during the day. In the United States, NCLB is a problem during the spring in southern and central Florida and during the summer months in the Midwestern states. On a global scale, NCLB is a problem in corn-growing areas in the mid-altitude tropics, which have the wet, cool environment that is favorable for disease development. These susceptible areas include parts of Africa, Latin America, China, and India. Management of NCLB can be achieved primarily by using hybrids with resistance, but because resistance may not be complete or may fail, it is advantageous to utilize an integrated approach with different cropping practices and fungicides. Scouting fields and monitoring local conditions is vital to control this disease. Major (vertical) resistance of corn hybrids comes from the race specific Ht1, Ht2, Ht3, and HtN genes, with the Ht1 gene being most prevalent. Plants with Ht1 Ht2, or Ht3 genes have smaller, chlorotic lesions and reduced sporulation. The HtN gene delays symptoms until after the pollen shed. Individually, each Ht gene has limited effectiveness because there are races of E. turcicum that are virulent in the presence of one or the other. For example, widespread use of the Ht1 gene has reduced the prevalence of the Race 0 to which it has resistance against, but has increased Race 1. Breeders are now focusing on incorporating several resistance genes into corn hybrids. Incorporating both the Ht1 and Ht2 provide resistance against both Races 0 and 1. Thus far, this multigenic approach has proven to be effective. However resistant plants still show some symptoms, and the threat of new races showing up lends to the need for other management practices, especially in areas where the disease is present.