In an attempt to increase yield, the use of foliar fungicides on field corn has increased dramatically in the last decade. Traditionally, applications of these fungicides have been made to corn at the tasseling stage of growth. However, in recent years some pesticide manufacturers have promoted early-season fungicide co-applications with post-emergence herbicide treatments. These applications typically occur around the V5/V6 stage of corn growth. The "rationale" of this practice is that an additional application of fungicide can be made in hopes of protecting and/or increasing corn yield without incurring additional application costs, since a post-emergence herbicide application will be made anyway. This use pattern is predominately thought of as a supplement to the tasseling fungicide application, not a replacement. Last year, we conducted the first year of a two-year experiment to investigate the effects of early-season co-applications of herbicides and fungicides on corn injury and yield.
In 2011 we applied Stratego Yld, Quilt Xcel and Headline AMP in combination with some of the most common herbicides used as post-emergence treatments in corn. Treatments containing Capreno and Realm Q did result in a 10 to 22% corn height reduction 7 days after treatment (Figure 1) and from 5 to 9% visual chlorosis on corn plants (Figure 2), but by two to three weeks after treatment the corn had regrown and no visible signs of chlorosis were evident. Additionally, there was no effect of these combinations on the level of weed control observed (data not shown). Based on this one year of data, our results indicate that adding a fungicide with your early-season (V5/V6) post-emergence herbicide application will not increase corn grain yields in comparison to herbicide treatments alone (Figure 3). It is also important to note that in this trial, disease incidence was very low (<5%), so these results should be interpreted as to the effects of these combinations on corn yield in the absence of any significant disease pressure. As many university plant pathologists have indicated, fungicides are most profitable in corn production when a combination of factors are present, such as disease presence, weather conditions favoring disease development, disease susceptible hybrids, and late-planted corn.
REVISED: October 1, 2015