Making informed foliar fungicide decisions = scout before tasseling time

Foliar fungicides in corn - a historical perspective. From the early 1970's through the mid-2000's, when prices averaged close to $2 per bushel and corn was considered a lower value crop, producers worked to minimize all but the most essential inputs. Between 2010 and 2012, when corn prices reached historic highs, producers may have considered additional inputs. While many production costs remain high, corn prices have since fallen and are projected to average below $3.75 per bushel for the 2017 crop marketing year. All input decisions become more urgent during these leaner years – to be profitable, each input must increase yield enough to at least pay for itself.

Foliar fungicides - making an informed decision. Making an informed decision about whether to apply a foliar fungicide can be difficult and involves gathering information about each aspect of the disease triangle: the pathogen, the plant and the environment. Most of the foliage-infecting fungi responsible for economic yield loss in Illinois corn are favored by wet and humid weather and survive in residue from previously-infected crops. Consequently, disease risk is higher when planting into a field with a lot of corn residue or a history of disease. On the plant side, seed companies typically provide a numeric rating of the level of genetic disease resistance in each hybrid and foliar fungicides are generally not recommended on resistant hybrids. In addition to monitoring weather conditions, scouting fields for disease provides valuable information about the fungal disease pressure in any given field.

Scouting for disease is an essential component to making an informed decision regarding a foliar fungicide.

A "profitable" fungicide application = breaking even. A profitable fungicide application is one that increases yield enough to at least pay for itself, and includes the costs of both fungicide active ingredient(s) and application. For those that need to hire this service out, the per-acre cost may range between approximately $31 and $46, and largely depends on whether fungicides are still under patent-protection. A break-even yield response, therefore, varies based upon both application costs and grain prices. In order to break-even under these fungicide and corn prices, a foliar fungicide would need to increase yield by more than 8 and up to 12 bushels per acre or more.

Between 2008 and 2014, in multiple locations throughout the state, former University of Illinois Extension Plant Pathologist Dr. Carl Bradley and his team conducted 45 different replicated corn foliar fungicide trials. The data collected in these 45 location-year environments can provide indications as to when foliar fungicides are most likely to be profitable.

In these experiments, fungicides were applied between VT and R1 growth stages and disease severity and yield data were collected from both fungicide-treated and un-treated control plots. In 28 of the environments, disease pressure was considered to be low, with less than 10 percent final disease severity in the untreated control plots. The remaining 17 environments had moderate to high disease pressure, with final disease severity 10 percent or higher on untreated plants.

Regardless of the yield response needed to break even with a foliar fungicide, the chance of doing so was higher in moderate to high disease pressure environments.

Fungicides in the absence of disease and pesticide resistance. While in some instances foliar fungicides can increase yield in the absence of disease, these physiological responses are both difficult to predict and inconsistent. Of additional concern, with each fungicide application, a selection pressure is exerted on pathogen populations. Those individuals that are insensitive to the fungicide can survive to reproduce, while those that are sensitive will die without producing offspring. This is how, over time and repeated cycles of selection, pathogen populations can shift from fungicide-sensitive to resistant. Reserving fungicides for those instances that are warranted from a disease management standpoint will serve to prolong their effectiveness.

Additional resources

Fungicide Efficacy for Control of Corn Diseases

Corn Foliar Diseases: Identification and Management Field Guide