Northern corn leaf blight (NCLB), caused by the fungus Exserohilum turcicum, is currently the most common and economically important fungal leaf disease of corn in Ontario, Canada. During the past 10 years in Ontario, severity and incidence of NCLB have increased, possibly because of the appearance of new races.1

Disease Cycle

The NCLB fungal pathogen overwinters as conidia (external spores) and mycelia (vegetative part of a fungus) in and on previously infected corn residue. During the warm, moist weather of early summer, new conidia are produced on the old corn crop residue. Wind and rain then spread the 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. Disease development is favored by heavy dew, frequent rainfall, high humidity, and moderate temperatures. The infection process occurs when water is present (free water) on the leaf surface for 6 to 18 hours and temperatures are between 18 and 27°C (Figure 1).2

Northern corn leaf blight disease cycle
Figure 1. Northern corn leaf blight disease cycle.3 Graphic reproduced with permission of Crop Protection Network.

Symptoms and Severity

Northern corn leaf blight lesions are typically gray-green to tan colored, elliptical or cigar-shaped, and two to 15 centimetres in length (Figure 2).2 As lesions mature, they turn tan and develop distinct dark areas of fungal sporulation giving the lesions a dirty appearance. Lesions first appear on lower leaves and the disease spreads into the upper canopy as the season progresses. On severely infected plants, almost all leaves could be infected, and leaves can become entirely blighted (NCLB disease severity rating 9). Late in the season, plants may look like they have been killed by an early frost. For corn products with race-specific resistance, lesions are often small and yellow and produce no spores.4

Elliptical or cigar-shaped lesions typical of northern corn leaf blight.
Figure 2. Elliptical or cigar-shaped lesions typical of northern corn leaf blight.

Yield losses of more than 30% have been reported when NCLB lesions are present on upper leaves prior to or at tasseling (NCLB disease severity rating 7) (Table 1).4 Under conditions favorable for NCLB, yield losses from infections beginning before and at tasseling can be as high as 50%. Yield losses are minimal when leaf damage is moderate or delayed until six weeks post-silking.5

Table 1. Example of 1 to 9 rating scale showing the severity of foliar fungal diseases, like northern corn leaf blight.
Severity Rating Description
1 Slight Infection
Few lesions on lower leaves
3 Light Infection
lesions on lower leaves
5 Moderate Infection
Lesions on lower and middle leaves
7 Heavy Infection
Lesions on lower leaves, extending to upper leaves
9 Very Heavy Infection
Lesions on all leaves. Plants may be prematurely killed


The primary management strategy to help 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 in plants 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 a single dominant gene and can be expressed as chlorotic lesions with decreased sporulation.5 Monogenic and polygenic resistance can work together to reduce the severity of NCLB.

Protecting leaves from the ear leaf and above as corn plants enter reproductive growth stages is especially important. Fields should be scouted prior to tassel emergence, around the V14 growth stage, to determine disease pressure. Economic returns are more likely to be realized when fungicides are applied from tasseling to early silking. A general recommendation for corn foliar diseases caused by fungal pathogens is to consider a fungicide application if a fungal disease is present on the third leaf below the ear leaf or higher on 50% of the plants at tasseling and the corn product is susceptible to the disease.6

Consider costs and predicted weather conditions before deciding to apply fungicides. Delaro® Complete fungicide is a recommended broad-spectrum systemic fungicide in Eastern Canada for the control of NCLB. Apply Delaro Complete fungicide when disease first appears. A second application can be made 7-14 days later if favourable conditions for disease development persist. This pest control product is to be used only in accordance with direction on the label. It is an offence under the Pest Control Product Act to use the product in a way that is inconstant with the directions on the label. For additional information about this product contact your retailer.

A combination of rotating away from corn for one year followed by tillage is recommended to help reduce the potential of NCLB development in the following corn crop. 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.