By Denny Wickham, Pioneer agronomist
The warm, humid July weather proved to be conducive to the development of leaf diseases in corn, especially gray leaf spot (GLS). While this disease is not new (it was first noted in Illinois in 1924), increased use of no-till and reduced tillage corn production practices have likely led to higher incidence of the disease. The fungal pathogen causing GLS, Cercospora zea maydis, overwinters on corn residue from the previous crop and higher levels of residue left on the soil surface allow for greater survival of the pathogen. In response to higher temperatures and humidity that occur in late spring, conidia (spores) begin to develop on the corn residue and are blown or splashed from the corn residue onto the current year’s corn plants. Infection typically begins in June, but disease symptoms may not show up until late July or early August. Earlier infection allows for greater spore build-up and more damage to the leaves.
The three factors that are necessary for GLS to occur are: susceptible host, high amounts of disease inoculum, and favorable weather conditions. This is the description of the typical disease triangle. GLS will not develop if one of the three sides of the disease triangle is missing, but most areas of Ohio have the pathogen present and many areas provide an excellent environment for the disease. Ideal conditions for infection by GLS include extended periods of leaf wetness and high humidity. These conditions typically occur most often in river bottom fields or areas where fogs tend to form, but GLS can infect plants in any field where the period of extended leaf wetness occurs. Above normal rainfall and heavy dews that cause leaves to remain wet can also enhance disease progression. Disease inoculum is directly related to the amount of infected corn residue remaining on the soil surface.
Disease development is favored by temperatures of 70 to 86 degrees F and high moisture conditions. Infections require leaf surfaces to be wet for 11 to 13 hours, and relative humidity in the leaf canopy to be at or above 90% for an uninterrupted period of 12 to 13 hours.
Delaying disease development is the key to managing GLS. Proper residue management can reduce the amount of GLS inoculum available to infect the growing crop. Like many residue borne diseases, GLS management involves handling residue to reduce the amount of inoculum in the field. Reducing the amount of inoculum retards the rate of disease development.
The traditional recommendation would be to bury the residue, but that is not a realistic practice under today’s mindset of conservation of soil resources. However, shallow incorporation of residue and breaking the residue down into smaller sized fractions can help. Residues can be managed through tillage, but crop rotation is an important practice as the amount of inoculum is significantly reduced when soybeans are grown in the field.
Hybrid selection is another important step in reducing the impact of GLS on yield potential and stalk quality. While it is true that there are no completely resistant hybrids, there are significant differences in tolerance to the disease. You can check with your seed supplier for data on disease tolerance, but it’s also a good idea to scout your fields right now to determine if the tolerance level of the hybrid package you’re planting.
In addition to residue management and hybrid selection, foliar fungicides are available to help manage GLS, but these need to be used as part of a management plan that includes scouting, knowledge of the hybrid’s tolerance to GLS and the characteristics of each field. Proper timing and application are very important for foliar fungicides to work best and it’s important to know the developmental stage of each field to best time the fungicide application. The best fungicide timing for management of GLS is after disease is noted in the lower canopy and before there is any significant lesion development on the ear leaf or upper canopy. Fungicides can be very useful, especially if a hybrid is known to be susceptible to GLS, yet still has significant yield potential. Be sure to read all the information you can on the proper rate, timing, application and adjuvant use for any fungicide you ma
y be using. As with many decisions you will make about managing your crop, protecting it from foliar diseases is a process that begins with educating yourself so you can make the most informed decision.