By Pierce Paul

Corn is all over the board in terms of growth and development, ranging from V6 to R2. Late-planted fields are usually at greater risk for foliar disease development, and reports coming in indicate that gray leaf spot is already present on the lower leaves in some areas. 

The conditions we've had over the last few weeks have certainly been favorable for foliar diseases. Gray leaf spot (GLS), the number one foliar disease of corn in Ohio, develops best at temperatures between 70 F 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. 

On the other hand, northern corn leaf blight (NCLB), a disease that has been increasingly prevalent in the state over the last few years, requires wetter and slightly cooler conditions.

Q & A      

Here are answers to five frequently asked questions in regards to fungicide use in field corn.

Question 1. When really should I apply a fungicide for disease control in field corn?

Answer: You should first scout for foliar diseases.

Just prior to tassel emergence, plants should be examined for disease symptoms. For residue borne-disease, such as gray leaf spot and northern corn leaf blight, a foliar fungicide application should be considered under the following situations:

  • Susceptible hybrids: If disease symptoms are present on the third leaf below the ear or higher on 50% of the plants examined.
  • Intermediate hybrids: If disease symptoms are present on the third leaf below the ear or higher on 50% of the plants examined, AND the field is in an area with a history of foliar disease problems; the previous crop was corn and there is 35% or more surface residue; and the weather is warm and humid through July and August.
  • Resistant hybrids: Fungicide applications generally are not recommended. 

Question 2. Do foliar fungicides really increase yield in field corn when there is not disease?

Answer: 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 on susceptible hybrids. 

Question 3. What about yield increase in stressed corn?

Answer: We 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. 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 4. What about the economics? When will the fungicide pay for itself?

Answer: 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. 

For instance, if we assume that it cost between $20 and $36 per acre to put on a fungicide treatment (product plus application) and grain prices range from $7 to $9 per bushel, you would need at least a 3- to 4-bushel increase in yield for the fungicide to pay for itself. 

Over the last three years, our data have shown that when fungicides are applied in the absence of foliar diseases, the yield increase is greater than 3 bushels per acre 45% of the time. In other words, there is less than a 50% chance that the fungicide will pay for itself when applied in the absence of foliar diseases. As the price for grain decreases and/or application cost increases, you will need more than a 3-bushel increase in yield for a fungicide to pay for itself.         

Question 5. Do fungicides increase yield by affecting respiration, photosynthesis and other physiological processes?

Answer: 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. 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 per acre.

Average yield difference between fungicide-treated and untreated plots when foliar disease levels are low (<5%): 1 to 5 bushels per acre. 

Average yield difference between fungicide-treated and untreated plots when foliar disease levels are higher (> 5%): 7 to 10 bushels per acre.