Description

Here at the National Agricultural Genotyping Center (NAGC), a real-time PCR multiplex assay has been developed for the detection and quantitation of Cercospora zeae-maydis (Czm) and Cercospora Zeina (Cz), two pathogens that cause gray leaf spot (GLS), as well as Exserohilum turcicum, the pathogen that causes northern corn leaf blight (NCLB).

Gray Leaf Spot (GLS) is a destructive foliar disease, which can be found in corn-producing regions around the world (Ward et al., 1999). This disease is caused by two closely related fungal species Cercospora zeae-maydis (Czm) and Cercospora Zeina (Cz), which have different distributions in North America. C. zeae-maydis is thought to be found throughout North America compared to C. zeina, which appears to be restricted to the eastern Corn Belt. When plants are infected by either species, GLS first manifests as small pinpoints, which eventually turn into brown to gray, rectangular lesions. These lesions can result in necrosis across large areas of leaves, resulting in reduced photosynthesis. Consequently, there is great potential of reduced grain development leading to yield loss, up to 50% (Ward et al., 1999), 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 (Wise, 2010). Once infected, plants continually shed GLS fungal spores throughout the growing season and can be transferred to adjacent plants or fields by wind-driven rain. During periods of high humidity and moderate temperatures, large GLS outbreaks can occur. The disease is difficult to control because the fungus overwinters on corn residue on the soil surface, but tilling, annual crop rotation, or the use of GLS tolerant corn varieties may help reduce the chances of an outbreak. Fungicides are also effective against GLS, 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 (Wise, 2010).

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.

This project was funded by the National Corn Growers Association and the North Dakota Corn Utilization Council.

References

Ward, J. M. J., Stromberg, E. L., Nowell, D. C. & Nutter, F. W. 1999. Gray leaf Spot: A Disease of Global Importance in Maize Production. Plant Disease, 83, 884-895.

Wise, K. 2010. Diseases of corn: gray leaf spot [Online]. Purdue University Extension. Available: https://www.extension.purdue.edu/extmedia/bp/bp-56-w.pdf.

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.