Two of our most common foliar diseases on corn are gray leaf spot and northern corn leaf blight. Both of these disease pathogens overwinter in corn stubble and the spores or inoculum build over the growing season. As a result, the disease pressure or severity increases later in the growing season. The extent to which these diseases can negatively affect yield depends upon weather conditions, the genetic susceptibility of the corn plant and the growth stage at which the plant is infected.
Yields are most likely to suffer when genetically susceptible corn is infected at a young growth stage and weather conditions are favorable for disease development. This allows the foliar disease to progress up the corn plant. After pollination, it is the leaves on the upper portion of the corn plant, above the ear leaf, that are crucial for grain fill. If these leaves lose photosynthetic area or activity due to disease infection, grain fill is reduced.
With all of the late-planted corn in our area, there is an increased risk for foliar leaf disease damage this year. Pierce Paul, OSU Extension corn disease specialist uses gray leaf spot as an example to explain. “In a “normal” year, although lesions may develop early in the season, this disease typically takes off and spreads after pollination (VT/R1) when the number of spores in the air is high and the weather becomes favorable for infection. Depending on where you are in the state, VT/R1 usually occurs sometime in mid-July. Planting late does not prevent spores from building up or conditions from becoming favorable for the gray leaf spot fungus to infect plant in mid-July, however, the primary difference it that instead of infecting plants at the VT/R1 growth state, the fungus will be infecting plants at a much earlier growth stage, V8-V12, for instance. If the hybrid is susceptible and conditions become favorable, high levels of infection at V8-V12 will result in greater and more rapid diseases development, and consequently, greater damage to the upper leaves before grain-fill is complete.”
Regarding fungicide timing, research work by Pierce Paul shows that applications made at silking (R1) or tasseling (VT) were the most effective in terms of foliar disease control and yield response. In addition, fungicides tend to be most profitable and the yield response most consistent when conditions are favorable for disease development and susceptible hybrids are planted. For more information about guidelines for making fungicide application decisions, contact the Wayne County Extension office at 330-264-8722.
Rory Lewandowski is an OSU Extension Agriculture & Natural Resources Educator and may be reached at 330-264-8722.
CFAES provides research and related educational programs to clientele on a nondiscriminatory basis. For more information, visit cfaesdiversity.osu.edu.