As you travel over your field with your herbicide this season, you may be considering adding a fungicide to that pass. If so, there are a few things to think about. In general, early fungicide applications on corn (V5-V7) have not been shown to provide much yield benefit over no treatment (average is +1 bushel per acre). In repeated trials across the country, we see the best opportunity for yield response with a treatment at tassel (average: +8 bushels per acre). However, there are some situations in which an early treatment may be more economical.
When foliar diseases attack in the V5/V6 stage, yield can be reduced. But oftentimes, we don’t see disease developing until well after V7. Many of the fungicides available have some systemic activity allowing the product to move through the treated leaf, however, the chemical will not be able to translocate into tissue that is not there during the time of application, so it can only protect the leaves that exist at application. Also, most of the products will provide between 7 and 21 days of control. This means that an early application will not give protection against the late-season foliage diseases that tend to be far more problematic in our area.
So a scenario in which you could benefit from an early application might look like this: You are using a hybrid with little foliar disease resistance (1), on a no-till (2), continuous corn (3) field, already see grey leaf spot developing (4) with a humid forecast (5) and have forward contracted your grain at a good price (6).
If you decide to make an application at herbicide timing, we would suggest the use of a product containing two modes of action. This can help delay the development of fungicide-resistant pathogens. Any of these products will run you about $12 to $24 per application, not including application costs. With today’s delivery of corn hovering just above $4 per bushel, you will want to make sure you take cost into account. Check here for a chart of corn fungicides which will tell you how well they work on a given disease and the PHI. This will also be handy for you as you consider application later in the season.