Northern Corn Leaf Blight in Parts of PA
Posted: July 22, 2015
Elongated tan lesions characteristic of Northern corn leaf blight on corn (Photo: Allison Robertson, Iowa State Univ.).
Northern corn leaf blight is a disease caused by the fungal pathogen Exserohilum turcicum. The symptoms are usually first observed on the lower leaves and the spread up the plant. The lesions are initially small, elliptical and gray-green in color. As the disease progresses the lesions will expand to 1 to 6 inches long, become tan in color and are not restricted by the leave veins. Eventually, the lesions will coalesce and cover the entire leaf. Under humid conditions, the lesions will produce dark gray spores on the lower leaf surface giving them a dusty appearance. A new lesion can produce spores in as little as one week under favorable conditions. The spores are then disseminated by rain splash to the leaves of nearby plants or they can be carried in the wind longer distances during storms.
Infection occurs after a period of 6 to 18 hours of leaf wetness under moderate (64 to 81°F) temperatures. Therefore, symptoms typically develop after periods of long heavy dews and overcast days as well as in areas of the field that are more prone longer dew periods (e.g. field edges adjacent to woods, low spots in the field, etc.). In cool, wet summers the disease will develop earlier in the season. In Pennsylvania, conditions are more favorable for disease later in the season as the temperatures start to drop and dew periods lengthen thus NCLB tends to be more problematic in later sweet corn plantings.
The greatest losses from NCLB occur when severe necrosis develops on the upper 2/3 of crop canopy by silking. The reduction in photosynthesis due to the necrosis results in reduced ear fill and when symptoms develop on the husks they appear older and are less marketable.
The increase in disease incidence over the past couple of years is likely due to a shift in the pathogen population making the host resistance in field corn varieties less effective. Race 0 of the pathogen is still thought to be dominant however, Race 1 has been detected and will overcome the primary resistance gene, Ht1. Therefore, the increase of NCLB in field corn increases pathogen pressure in nearby sweet corn fields.
Since the pathogen overwinters in crop residue, rotating out of corn for at least one year and managing the crop residue to encourage decomposition will reduce initial inoculum that can infect the lower leaves. However the soil health benefits of using reduced tillage outweigh benefits of plowing under crop residue.
Host resistance can still be an effective tool for managing NCLB especially for later sweet corn plantings. There are different types of resistance genes that have been introduced into sweet corn hybrids through traditional breeding (not GMOs). Hybrids can have polygenic (partial) resistance which confers resistance to both races of the pathogen however, the resistance is not complete for any of the races or monogenic resistance which confers resistance to only specific races of the pathogen. These various resistance genes will limit lesion size, lesion number and the amount of sporulation within each lesion. When resistance genes are present in a hybrid, lesion size, shape and color may vary. For example hybrids that contain one of the monogenic resistance genes Ht1, Ht2, and Ht3 will develop chlorotic lesions but sporulation will be limited so the disease does not spread quickly.
The crop should be scouted regularly, focusing on the lower leaves where symptoms develop first. Protectant fungicides like chlorothalonil can be applied when there are reports of NCLB in the area but symptoms have not been observed in the field. Good coverage is critical. NCLB specific fungicides include those in FRAC group 11 (strobilurins; e.g. Quadris and Headline) and FRAC group 3 (triazoles; e.g. Tilt). There are also a number of products that contain both FRAC groups (11 + 3; e.g. Quilt and Stratego). Rotate between these FRAC codes and tank mix with a broadspectrum protectant for resistance management when symptoms are first observed in the field will help manage NCLB. PHIs vary between the products so read the labels carefully when the crop is near harvest. Also depending on the label, NCLB might be referred to as Helminthosporium leaf blight which is collectively refers to both Northern corn leaf blight and Southern corn leaf blight.