Pennsylvania Corn: Early-Season Considerations for Foliar Fungicides

Vegetative corn field. Photo: Paul Esker, Pennsylvania State University

Corn growth and development in Pennsylvania is variable depending on the location. Considering that we are again above average with rainfall across the state, we are starting to receive questions about early season fungicide applications and if they would be of benefit.

This is a common question, especially since it is an attractive option because at that time the corn is short enough to be easily driven over with most equipment, and most farmers are used to applying an herbicide at this time as well. This means you do not need to deal with the hassle and expense of getting a custom applicator to make the application.

Given the questions focused on early-season applications, university plant pathologists across the country have studied this question for several years. In the majority of studies, we have found no significant yield benefit to a fungicide application at this early vegetative timing. Recently, there was an article published in the journal PLOS ONE by Wise et al. (2019) that used meta-analysis to look at the yield response due to fungicide use at V6 (sixth-leaf stage), VT (tasseling), and V6 + VT across a large area of the U.S.

Results indicated that the most profitable use for a fungicide was at VT or V6 + VT, which for the latter implies that the response to fungicide was due to the VT application. Furthermore, the likelihood of a return on investment with using foliar fungicides was also influenced by the cost of the fungicide program and the expected corn price and in general, it was well below 50% in the V6 cases, unless the price received was high with a low fungicide cost.

What are some the reasons that early season applications are less profitable? Mainly, the challenge at this time of year is that our primary yield-limiting diseases do not become problematic until the reproductive stages of development. The reasons for this include:

  1. The fungal leaf diseases that are most yield-robbing are favored by warm temperatures and high humidity. At the earlier growth stages like V5-V7, the crop canopy is very open with excellent air flow. It is not until later stages that the canopy closes over, trapping in heat and humidity. If you have ever scouted corn, you know exactly what we mean!
  2. The leaves that really do the work of grain fill (the ear leaf and above) are not yet formed at this time. Because of this, we cannot protect them with an early fungicide application. Even our most persistent fungicides only last for about three weeks in the plant tissue.
  3. The fungi that cause diseases like Grey Leaf Spot (GLS) and Northern Corn Leaf Blight (NCLB) survive in crop residue from previous years. It takes some time each season for conditions to become right for them to start sporulating, and then for those spores to leapfrog from the lowest leaves to the upper, more critical ones.

Given that the majority of research to date, including the recently published article, as well as our 2018 foliar fungicide work at Rock Springs and Landisville, have indicated that it is around VT when we see the most effective (yield and economically) applications, we summarize several factors that drive this response.

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Typically, at this time of year, there is higher foliar disease pressure and the tissues we want to protect most are present and functioning at this time. The ear leaf and those leaves that are younger are the solar panels that provide the plant the energy and sugars that then fill the grain.

These later applications take us further into the season with protection of this foliage. This can be critical not only for those growing for grain, but also silage producers.

The downside of the VT fungicide timing is that it requires specialized equipment like a highboy or aerial application, which is more expensive and needs to be scheduled with a custom applicator. However, if a farmer is only going to make one fungicide application, this should be the one, as it traditionally provides the biggest bang for the buck.

Some farmers have taken this further by following a VT application about two or three weeks later with a second treatment with good results in high disease pressure situations. Those considering this should be careful to watch the pre-harvest interval restrictions, especially when chopping silage.

To be clear, we are not saying that there is never a situation in which an early season fungicide treatment provides a yield advantage. We look at corn disease risk as the culmination of several factors. The more of these factors are at play in your field, the greater the benefit you can realize from a fungicide at any timing.

For example, if you have:

  • continuous corn
  • no-till production
  • greater than 30% corn residue
  • extended warm, humid weather
  • low lying field, where dew tends to persist
  • irrigation
  • a hybrid with a poor resistance rating for GLS or NCLB
  • high yield potential
  • disease already present at the time of scouting

…then you stand to gain the most from a fungicide treatment.

We have also found in previous work with crop consultants and farmers that the factors that drive successful corn production can be grouped into four areas (from “most” important to “least”):

  1. maximizing profit, maximizing yield, commodity prices, and plant populations;
  2. disease resistance and crop rotation;
  3. field history, tillage, and seed price; and
  4. new herbicides, foliar insecticides, and foliar fungicides.

There is no one silver-bullet that drives successful corn production and pesticides, while important, should be considered as part an integrated management approach.

Another drawback with early season applications? Given the most recent data, aside from the added expense of a fungicide with your herbicide application, there is a good chance we would be encouraging fungicide resistance among the fungi that are in that field.

Whether or not they are causing disease at the time, the pathogens are present in that residue, and any exposure to our most common active ingredients nudges their populations closer to becoming resistant to these products. Our current fungicides work well and are pretty safe for humans and the environment. We’d like to keep these tools around for our use as long as possible.

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