E. turcicum overwinters as conidia and mycelia in and on corn residue. During the warm, moist weather of early summer, new conidia are produced on the old residue. Wind and rain then spread conidia to the lower leaves of young corn plants. Conidia are produced abundantly in lesions on susceptible plants and are responsible for secondary spread within and between fields. The infection process can begin when water is present on the leaf surface for 6 to 18 hours and moderate temperatures exist.2
Symptomatic lesions of NCLB are typically gray-green and elliptical or cigar-shaped (Figure 1). As lesions mature, they turn tan and develop distinct dark areas of fungal sporulation. Lesions first appear on lower leaves and move upwards as the disease progresses. The lesions can be as large as 3/4 inch wide and up to 6 inches long. Symptoms can progress rapidly after anthesis. On severely infected plants, almost all of the leaves may be infected and leaves can become entirely blighted. Late in the season, plants may look like they have been killed by early frost. Lesions on products containing resistance genes may appear as long, chlorotic, streaks, which can be mistaken for Stewart’s wilt.1
The primary management strategy to reduce the incidence and severity of NCLB is planting resistant products. Two types of resistance to NCLB exist in corn. Polygenic (multiple gene) resistance is expressed as a reduction in lesion size, lesion number, and sporulation and a longer latent period before conidia are produced. Monogenic (single gene) resistance is controlled by four single dominant genes: Ht1, Ht2, Ht3, and HtN. Resistance conferred by Ht1, Ht2, and Ht3 is expressed as chlorotic lesions with decreased sporulation. Resistance conferred by HtN is expressed as both an extended period between initial infection and when spores are produced, and fewer lesions. Monogenic and polygenic resistance can act together to reduce the severity of NCLB.
There are no thresholds for fungicide use for NCLB. However, it is especially important to protect the ear leaf and those above it as corn plants enter reproductive stages of growth. Fields should be scouted prior to tassel emergence, around V14 growth stage, to determine disease pressure. Economic returns are more likely to be realized when fungicides are applied from tasseling to early silking. Consider costs and predicted weather conditions before deciding to apply fungicides.
A combination of rotating away from corn for one year followed by tillage is recommended to prevent development of NCLB. Rotating to a non-host crop can reduce disease levels by allowing the corn debris on which the fungus survives to decompose before corn is planted again. Burying residue may help reduce infection levels by decreasing the amount of primary inoculum available in the spring. In no-till and reduced tillage fields with a history of NCLB, a two-year rotation away from corn may be necessary.
1 Compendium of Corn Diseases. 1999. APS Press; 2 Lipps, P.E. and D. Mills. Northern corn leaf blight. Ohio State University Extension. AC-20-02. http://ohioline.osu.edu (verified 6/20/2014); Robertson, A. 2009. Goss’s wilt and NCLB showing up in Iowa. Iowa State University Extension http://www.extension.iastate.edu (verified 6/20/2014); Wise, K. 2011. Northern corn leaf blight. Diseases of Corn, BP-84-W. Purdue Extension. www.extension.purdue.edu (verified 6/20/2014).