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When chopped for high performance dairies, corn silage is an especially valuable crop. Over recent years, corn silage producers in some areas of the country have seen significant reductions in silage tonnage and quality caused by leaf diseases such as northern corn leaf blight (NCLB), gray leaf spot (GLS), and other foliar diseases. Fields of corn have “fired up�? as disease lesions spread from bottom leaves and up the plants.
Leaf diseases can spread rapidly with relatively cool summer temperatures, high humidity, and a wetter than normal growing season. Silage producers, particularly those with fields in river valleys where dense fog and heavy dews are common, have experienced sudden and rapid development of leaf diseases.
Because they decrease the surface area of leaf tissue available to create plant sugars, leaf diseases can lead to reduced ear fill, lower grain quality, and lodging. University trials have shown that dry matter intake, energy levels, dry matter digestibility, digestible energy, and starch can be significantly lower in silage infected with leaf diseases. Altered fermentation quality has not generally been observed in controlled studies. Understanding how the diseases develop and following management recommendations can help silage producers reduce the effects of foliar diseases.
When conditions are favorable, fungal spores are produced in crop residues during the spring and early summer. Rain can splash the spores from the soil or crop residues onto young plant leaves and wind can blow the spores from plant to plant and field to field. Infection can occur when temperatures are between 65 oF and 90 oF, particularly when water is present in the leaf whorl for several hours. NCLB lesions develop on lower leaves and may eventually spread over entire plants and fields (Figure 2). GLS lesions begin as small, brown or tan spots and mature into rectangular, brown to gray necrotic regions running parallel to the leaf and spanning the spaces between leaf veins (Figure 1). Gray fungal spores are produced on the underside of leaves, beneath mature lesions.
Figure 1. Gray leaf spot lesions reduce silage quality.
Gray leaf spot (GLS) (Cercospora zeae-maydis) can develop when prolonged periods of high relative humidity (90% or greater) and warm temperatures (70 - 90 °F) are present. Long lasting fogs or heavy dews are particularly favorable for infection. Due to an extended latent period, initial symptoms of GLS typically develop 2 to 4 weeks after infection. Immature lesions appear as small, brown or tan spots on the leaf surface surrounded by a yellow halo and may resemble lesions caused by other pathogens such as eye spot, anthracnose leaf blight, or common rust. As the lesions mature, they elongate and develop into distinct symptoms, which are characterized by rectangular, brown to gray necrotic regions that run parallel to the leaf, spanning the spaces between the major leaf veins. Tufts of fuzzy, gray fungal spores are produced on the underside of leaves beneath mature lesions, which give GLS its name.
The use of corn products with improved resistance is the most effective strategy to preserve yield potential, especially in fields with a high probability of GLS occurrence. Controlling this disease with fungicides depends on the potential severity of GLS, the stage of crop development, and any harvest restrictions associated with the fungicide to be used.
Figure 2. Northern corn leaf blight resistant corn (left) compared to diseased, NCLB susceptible product (right).
Northern corn leaf blight (NCLB) (Exserohilum turcicum) infection can occur and become severe when water remains on leaf surfaces or in leaf whorls for 6 to 18 hours and the air temperature ranges between 65 to 80 °F. Grayish elliptical lesions begin to appear on the leaves within two weeks after infection and gradually turn tan colored and cigar shaped (Figure 3). The best form of NCLB management is planting seed products with above average resistance. Fungicides are not considered to be cost effective for silage production.1
Figure 3. Eyespot
Eyespot (Aureobasidium zeae) is more prevalent in north central and northeastern states, but is becoming more common in the Midwest. Free moisture on the leaves from any source provides a conducive environment for infection. Symptoms or lesions can begin to appear within 9 to 10 days after infection. Initial circular lesions are about 1/16 inch in diameter, water soaked, and usually appear on the lower leaves first. Lesions become chlorotic and necrotic as they increase in size to about 1/8 inch in diameter. The lesions have a tan center with a darker brown or purple margin and a surrounding yellow halo, which is visible when the leaf is held up to the light.
Resistant seed products should be used if the disease was prevalent in a prior year, reduced tillage is practiced, and continuous corn is the cropping sequence. Fungicide applications may be beneficial if a prior infected field is in continuous corn, a susceptible product is planted, reduced tillage is the management practice, and weather conditions are favorable for development.
Figure 4. Common rust
Common rust (Puccinia sorghi Schwein) overwinters in the southern United States and Mexico and moves northward in late spring and early summer when fungal spores are wind blown into the Corn Belt. Disease development is favored by moist conditions caused by rainfall, dew, or high relative humidity (95% or greater), and moderate temperatures between 60° and 77 °F. The spores in contact with leaves germinate and infect after 3 to 6 hours in moisture. Symptoms may appear first on youngest leaves as light yellow spots about the size of a pinhead. In about 7 days these chlorotic flecks develop into reddish brown pustules. Common rust pustules are found on the upper and lower leaf surfaces while the pustules for southern corn rust are mostly found on the upper leaf surface. Pustules rupture the leaf epidermis and contain small, cinnamon-brown, powdery spores. The pustules become darker brown to black later in the season. Pustules are often found in bands or patches, indicating that infection occurred while the leaf was in the whorl.
Foliar fungal diseases can be managed by selecting resistant corn products, crop rotation, plowing under crop residue, and possibly using foliar fungicides.
Results of field trials using fungicides to control foliar diseases in corn silage have been mixed. Most observations to date have been in low disease environments. Some university trials across the dairy crescent (Minnesota, Wisconsin, Michigan, Ohio, New York, Pennsylvania, Maryland, and New England) have not consistently demonstrated an economic return. However, some trials conducted in the northeast have shown more benefit and silage producers are increasingly adopting fungicides as a way to manage foliar disease in silage corn. Strobilurin and triazole fungicides are labeled in corn for protection against many fungal diseases. Fungicide use should be determined by the potential for disease development and potential for an economic loss of silage tonnage and quality resulting from leaf diseases. Depending on when infection occurs, a fungicide application may be justified. When selecting corn products for silage, producers should consider that corn products vary in their resistance to foliar diseases. Planting corn products with above average disease resistance is the best management practice.
1Darby, H. Northern corn leaf blight. The University of Vermont. http://www.uvm.edu. 2Isleib, J. and Chilvers, M. 2012. Plant diseases can impact corn grown for silage by lowering yield and causing molds and Mycotoxins. Michigan State University. http://msue.anr.msu.edu. Web sources verified 06/07/2016. 141024100545