It's been hot and dry for most of the corn growing season so far. These conditions are highly unfavorable for foliar diseases to develop in field corn, yet several producers have expressed interest in applying foliar fungicides.
Gray leaf spot (GLS), the number one foliar disease of corn in the state, develops best at temperatures between 70 and 90 F, especially when conditions are consistently wet and humid. For GLS infection to occur the leaf surface needs to be wet for 11 to 13 hours and relative humidity in the canopy needs to be at or above 90% for an uninterrupted period of 12 to 13 hours.
Similarly, northern corn leaf blight (NCLB), a disease that has been increasingly prevalent in the state over the last few years, requires even cooler, wetter conditions. NCLB infection occurs when free water is present on the leaf surface for 6-18 hours and temperatures are between 66 and 80°F.
Therefore, temperatures this year, mostly in the upper 80s and upper 90s, and moisture in particular, have not been favorable for either GLS or NCLB to develop.
With the year’s crop already between late vegetative and early reproductive stages, it is highly unlikely that either GLS or NCLB will reach levels high enough to impact grain yield.
Questions and Answers:
Question 1. Do foliar fungicides increase yield in field corn?
Answer 1. Fungicides MAY indeed lead to an increase in yield in field corn, but the yield response is HIGHLY variable. The greatest and most consistent increases in yield are usually seen when foliar diseases are present and at high levels.
Question 2. What about yield increase in stressed corn?
Answer 2. I know of no research data showing conclusively that fungicides lead to higher yields in corn under stress conditions, other than when the stress is caused by diseases.
Comparisons made between fields, between hybrids, or even between years are not valid to drawing conclusions about fungicide effects on yield. For instance, you should not base your assessment and conclusion on a comparison between the field next to your house that was treated with a fungicide and the field five miles down the road that was not treated.
To conclusively say whether or not a given fungicide increases yield in stressed corn, you will need to compare treated and non-treated, stressed and non-stressed corn in the same field, planted with the same hybrid, all exposed to the same set of weather, soil, and crop production conditions. In addition, such a comparison will need to be made over multiple years and locations.
Question 3. Will the fungicide pay for itself?
Answer 3. It depends mainly on grain price, fungicide cost, application cost, hybrid susceptibility to disease, and the level of disease in the field. You are most likely to see a return on your fungicide investment when grain prices are high, fungicide and application costs are low, the hybrid is susceptible, and disease levels are high.
Question 4. Do fungicides affect respiration, photosynthesis and other physiological processes?
Answer 4. Yes, some fungicides do indeed affect crop physiology. However, most of these effects have been observed in the greenhouse, under controlled conditions. In the field where conditions are highly variable, a greening effect is sometimes seen when strobilurin fungicides are used, however, the association between this greening effect and grain yield is not clear or consistent. Remember, the biggest drivers of yield are hybrid genetics, weather conditions, and crop nutrition, not fungicide application in the absence of disease.
Summary results from 8 years of fungicide research - up to 172 trials
Fungicides tested: Headline, Stratego, Quilt and Quadris. All fungicides were applied at VT (tasseling) or at R1 (silking).
Average yield range: 164 to 197 bushels/acre.
Yield difference between fungicide-treated and untreated plots: In 28 to 48% of the trials, yield was higher in the untreated plots than in the fungicide-treated plots. Remember the piano graphs you see at meetings? These clearly show how highly variable yield responses are to fungicides.
Average yield difference between fungicide-treated and untreated plots: 3.6 to 6.2 bushels/acre.
Average yield difference between fungicide-treated and untreated plots when foliar disease levels are low (<5%): 1 to 5 bushels/acre.
Average yield difference between fungicide-treated and untreated plots when foliar disease levels are higher (> 5%): 7 to 10 bushels/acre.