Answers are from the Indiana certified crop adviser panel: Betsy Bower, Purdue Extension senior research associate, West Lafayette; Abby Horlacher, Nickel Plate Consulting, Frankfort; Brian Mitchem, Farmer 1st Agronomy Consulting Services, Decatur; and Dan Quinn, Purdue Extension corn specialist.

 

Our agronomist informed me that some of our cornfields will take yield hits if I don’t spray once more for diseases. I have made one pass and don’t want to spend the extra money. Should I make an additional pass?

 

Bower: What is the disease? What is the corn growth stage? What stage did you spray the last time? Are there new disease lesions?

 

If your previous application was before tassel, it has been three weeks or more since your last application, tar spot is on ear leaves at less than 5% coverage, or conditions are conducive for disease development, another application likely will provide positive return on investment. Leave a couple of unapplied strips to understand the impact of the second application.

 

If corn has started to dent, the ROI for another application is a push, especially if environmental conditions do not support disease development. If the hybrid has some resistance to tar spot, even if you are seeing lesions, the ROI for a second application likely will not pay.

 

This discussion is mostly about tar spot. However, if southern rust is found at R4 or earlier growth stages and the hybrid is not resistant, a second application could easily provide a positive ROI.

 

Horlacher: Three key factors are needed for disease to thrive: susceptible host, presence of a pathogen and favorable environment. If your agronomist is recommending another spray, you likely already have a host and pathogen. Consider growth stage, time until harvest and expected weather conditions.

 

If the crop is at drydown, do not spray. Yield loss is unlikely unless corn goes down, which can’t be prevented now. If the corn is still at grain fill or milk stage and rain is forecast, consider spraying.

 

The deciding factor should be level of pathogen pressure. Don’t decide without walking the field. Get multiple professional opinions.

 

Mitchem: Before making any decisions, consider growth stage, diseases present, disease severity and cost of treatment. If there is good potential for return, and you trust your adviser’s expertise, formulate a plan.

 

Southern corn rust and tar spot can have a return with a two-pass program in corn if disease pressure is very high and the crop is at a growth stage that can still benefit from the second application. For other common corn diseases like northern corn leaf blight, common rust and gray leaf spot, it would be rare to see a return for a second pass.

 

Quinn: Only in severe foliar disease cases is an additional fungicide application warranted. For example, if tar spot is severe and corn is infected earlier in the growing season, then it may be warranted and profitable to include a two-pass fungicide system. However, this is rare. The most consistent fungicide program for corn is a single application at the R1 or R2 growth stage. But if disease severity is low, even this application may not provide any yield benefit.

 

If you use scouting and prediction tools, you can make a more informed fungicide application or potentially avoid a fungicide application altogether if disease severity remains low. Ask your agronomist their justification for this statement. You can always spray to spray, but if disease is not present or the risk for disease is not high, then a positive return likely will not occur.