Walking into a field of corn midseason and finding spots or necrotic lesions on corn leaves can be an alarming experience. Factors that can cause spots or lesions include environmental stress, mechanical injury, chemicals and disease pathogens. Like any agronomic diagnosis, differentiating between these factors is key to making the proper diagnosis and implementing potential management decisions. When discussing midseason corn diseases many come to mind; however, we’ll focus on three economically important ones that a large portion of the Corn Belt deals with annually.
Gray leaf spot (GLS) is caused by the fungal pathogen Cercospora zeae-maydis and is found worldwide. If severe, GLS can cause substantial yield loss. The fungus overwinters on residue which makes no-till and minimum tillage acres more susceptible to this disease. High relative humidity and warm weather environments encourage infection and development of the disease. Lesions can appear as early as V8 (eight leaves) on the lower canopy and move up the plant as favorable conditions permit. Initial GLS lesions appear as small, necrotic spots with chlorotic halos that elongate into distinct rectangular blocks that run parallel on the leaf surface (Figure 1). Management for GLS starts with selecting genetics that offer tolerance and protection. In addition, crop rotation and the use of an in-season fungicide application can help protect yield potential. A fungicide application during VT-early silk is widely considered as the most crucial timing for application and protection for the remaining growing season when considering infection severity, favorable weather and economics of the application.
Northern corn leaf blight (NCLB) is caused by the fungal pathogen Exserohilum turcicum and is characterized by cigar shaped tan-gray lesions. Lesion color changes from pale green to tan as it matures, and can become darkened in areas due to fungal sporulation, especially after periods of high moisture (Figure 2). Lesions will elongate and appear in the upper canopy in susceptible corn products. This disease is primarily seen in post-tassel corn but scouting should begin at V14. Like most corn diseases, the pathogen is harbored and overwinters on crop residues. Favorable environments that increase the potential for NCLB infection are high humidity and moderate temperatures. Selecting a product with genetic tolerance should be the first line of defense; however, using a VT-R1 fungicide application has shown positive yield response. Diagnosis is important as NCLB lesions are often mistaken with Stewart’s and Goss’s wilt.
Goss’s bacterial wilt and leaf blight (GW), caused by the bacterium Clavibacter michiganensis subsp. nebraskensis, was traditionally considered a Western Corn Belt disease; however, in the past decade the pathogen’s footprint has expanded and the disease now occurs in large parts of the U.S. Corn Belt. The disease can cause a major impact on yield potential of susceptible products. Crop injury occurs when bacteria that overwintered on crop debris are splashed by rain or overhead irrigation onto leaves and enter the plant through wounds caused by wind and hail events. The disease can be systemic, in which vascular tissue becomes plugged with bacteria during vegetative growth and if severe can result in premature plant death or as a leaf blight (Figure 3). Lesions of GW are identified by their water-soaked look with wavy margins, black freckles inside the lesions and “ooze” seen on the leaf surface. The lesions can be misdiagnosed as stresses relating to drought stress, heat stress and fertility deficiencies. Current protection against GW infection is reliant on selecting products that have been developed and screened for their tolerance to the disease, as fungicides offer no activity on bacteria-based pathogens.
Selecting products that are tolerant to the pathogen of concern for your relative geography is the first place to start. Proper identification of the disease pathogen and understanding your environment and potential exposure are key when making management decisions on your farm. Application timing of fungicides for fungal diseases should be done with economics, yield potential, disease pressure and weather patterns in mind. Differentiating between fungal and bacterial pathogens is important in relation to economics and effectiveness of treatment options; spraying fungicides on bacterial diseases is a waste of money and resources. Working with your local Channel Seedsman and support team to select the right products can bring value to your operation.
Rees, J.M. and Jackson, T.A. 2008. Gray leaf spot of corn. G1902. NebGuide. University of Nebraska-Lincoln Extension.
Wise, K. 2010. Gray leaf spot. Diseases of Corn. BP-56-W. Purdue University.
Wise, K. 2011. Northern corn leaf blight. Diseases of Corn. BP-84-W. Purdue University.
Northern corn leaf blight. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/
Jackson, T.A., Harveson, R.M., and Vidaver, A.K. 2007. Goss’s Bacterial Wilt and Leaf Blight of Corn. G1675. NebGuide. University of Nebraska-Lincoln Extension.
JWise, K., Ruhl, G., and Creswell, T. 2010. Goss’s Bacterial Wilt and Leaf Blight. Diseases of Corn. BP-81-W.
Gray leaf spot in corn. Agronomy ADVICE. Channel.com. http://www.channel.com/agronomics/Documents/AgronomicContentPDF/GrayLeafSpotinCorn_Channel_Advice.pdf/
Identification and management of northern corn leaf blight. Agronomy ADVICE. Channel.com. http://www.channel.com/agronomics/Documents/AgronomicContentPDF/IdentificationandManagementofNorthernCornLeafBlight_CH.pdf/
Goss’s bacterial wilt and leaf blight in corn. Agronomy ADVICE. Channel.com.http://www.channel.com/agronomics/Documents/AgronomicContentPDF/Goss%27s%20Bacterial%20Wilt%20and%20Leaf%20Blight%20in%20Corn%20-%20Channel%20-%20Advice.pdf/