About Gray Leaf Spot
Gray leaf spot is due to a fungus called Cercospora zeae-maydis, and can be economically damaging to corn crops. This fungus will overwinter on plant residue on the soil’s surface. When temperatures begin to increase during the spring, the fungus will produce spores. These spores can easily travel through water to young plants, often infecting the plants lower leaves first. The spores can also travel via wind. The spores will cause lesions to form; these lesions will create their own spores, which are likely to splash onto the upper leaves of the same or other plants. This makes it seem as though the fungus moves up the corn plant. Depending on environmental conditions and corn variety, symptoms may not appear until 14 days after initial infection. Damage is likely to be worse in corn fields that were planted late or if lesions occur on the plants ear leaf, especially 2 weeks prior and following tasseling. However, even if corn was planted on time, when conditions are favorable for Cercospora zeae-maydis, yield loss will still be high.
Gray Leaf Spot Identification and Habitat
After initial infection, the first symptom commonly appears as tiny lesions on leaves surrounded by a yellow halo. As time passes, the lesions will lengthen into rectangular and fairly narrow spots that are gray or brown in color. They elongate parallel to foliage veins and can grow up to 2 inches in length. However, sometimes symptoms vary by hybrid vulnerability. If the variety has some resistance, lesions may not expand beyond initial spot appearance. These spots may have more of a jagged and round appearance. When these spots are created, it reduces photosynthetic areas, which means the grain development will slow down. This disease is often confused with other foliage diseases such as common rust, eyespot, and leaf blight. Occasionally, this disease can also lead to lodging and stalk rot.
Infection is likely to occur when weather temperatures are warm for a long period of time, typically between 75 and 85 degrees Fahrenheit with a relative humidity percentage of at least 90. Symptoms will be most pronounced after an overcast day with heavy dew or in areas close to the woods where dew remains for a prolonged period. Without high humidity, development will often cease until humidity rises once again to the appropriate level.
Gray Leaf Spot Management and Control Methods
For cultural control of gray leaf spot, use resistant hybrids whenever possible. A reduction in tillage can promote more fungi to overwinter in plant residue; therefore, in these cases it is especially important to use a resistant variety. However, it is still recommended to use production practices which promote the decomposition of plant residue. Crop rotation with a non-susceptible plant type followed by tillage often helps manage gray leaf spot. If infection in the previous year was bad, leave corn out of rotation for 2 years, especially because the fungus can survive on residue for over a year.
There are in-season fungicides available. However, before application, always consider economic factors and whether the fungicide cost will outweigh the yield loss the disease might cost. Fungicides with strobilurin, or a mix of strobilurin and triazole have been effective against gray leaf disease. Other active fungicide ingredients include pyraclostrobin, or pyraclostrobin mixed with cyproconazole, metconazole, fluxapyroxad, azoxystrobin, or trifloxystrobin. Always be sure to carefully read the label for cautions and proper application before use.