Northern Corn Leaf Blight (NCLB) is caused by Exserohilum turcicum, a foliar fungal pathogen, which can be found in corn-producing regions around the world (Wise, 2011). NCLB manifests as lesions and tissue necrosis across large areas of leaves, resulting in reduced photosynthesis. Consequently, there is great potential for yield loss (up to 30%), particularly when lesions develop early in the growing season or when lesions reach the ear leaf or higher during two weeks before and after tasseling. Once infected, plants continually shed NCLB fungal spores throughout the growing season, and during periods of high humidity and moderate temperatures, large NCLB outbreaks can occur. The disease is difficult to control because the fungus overwinters on corn residue, but tilling, annual crop rotation, or the use of NCLB resistant corn varieties may help reduce the chances of an outbreak (Wise, 2011). Fungicides are also effective against NCLB, but no disease thresholds for fungicide application have been established because the relationship between disease severity and loss of yield is unclear for most corn varieties.
One problem for managing fungal pathogens like NCLB is that accurate diagnosis can be difficult in the field. The symptoms of NCLB can be mistaken for several other pathogens. For example, lesions manifest as tan or gray streaks that form parallel to the leaf margins, which are similar features to other foliar fungal diseases such as Diplodia Leaf Streak and Gray Leaf Spot. Additionally, symptoms of foliar bacterial diseases such as Stewart’s wilt, Goss’s wilt, and Bacterial Corn Streak can produce similar symptoms. Since fungicides are not effective against bacterial pathogens, misdiagnosis could lead to further financial loss through inappropriate fungicide application. Further adding to the confusion of visual diagnosis is that the physical appearance of lesions from the same disease can look different across corn varieties that differ in susceptibility to NCLB. With current laboratory protocols, producers may have to wait weeks for proper diagnosis because testing facilities use time-consuming or labor-intensive methods, such as waiting for cultures to grow to view fungal structures under a microscope. Thus, there is a need to develop a NCLB test that provides faster results, so producers have time to decide and budget for purchasing resistant varieties before planting (e.g., NCLB positive soil test) or purchasing fungicides on more susceptible varieties during the growing season.
Here at the National Agricultural Genotyping Center (NAGC), we have already validated pathogen detection assays for Bacterial Corn Streak and Goss’s Wilt, two diseases that cannot be treated with fungicides, but symptoms closely resemble NCLB in the field. Surprisingly, there appears to be no peer-reviewed studies that have developed an assay to detect NCLB. NAGC has developed a quantitative PCR assay to detect NCLB in both infected plant and soil samples.
This project was funded by the National Corn Growers Association and the North Dakota Corn Utilization Council.
Wise, K. 2011. Diseases of corn: Northern Corn Leaf Blight [Online]. Purdue University Extension. Available: www.extension.purdue.edu/extmedia/bp/bp-84-w.pdf.