Fungicide Considerations for Corn and Soybean

On June 24, we received our earliest report to date of tar spot of corn from a field in eastern Pennsylvania. Corn was at the V9-V10 growth stage, leading to much discussion about the timing and need for a foliar fungicide. Given the report and the fact that soybeans in some parts of Pennsylvania are entering the flowering stage, this is an excellent time to discuss risk factors that may influence the need to consider a fungicide spray in corn and soybeans. The decision to spray should always start with understanding what the key targets are in corn and soybeans (Figures 1 and 2). This follows from our philosophy of targeting the right organism with the right product at the right time.

Before pulling the trigger, it is important to spend some time understanding how the Crop Risk Tool helps us determine risk. Depending on your location, this decision will be quite different, and we will need to monitor changing conditions from this point forward into early August. The best approach for deciding about the need for a foliar fungicide application in field crops is based on using an integrated pest management approach that considers cultural practices (things like variety or hybrid selection, crop rotation, tillage, combined with crop scouting) and, when conditions are warranted, selecting the best product for the right target.

We also recommend that you review the e-book Fungicide Use in Field Crops, available online, to help you understand the factors that drive fungicide use and need. This coincides with a review of current knowledge on fungicide efficacy for corn and soybean. 

What Do We Currently Know About Fungicide Responses in Soybeans and Corn?

In soybeans, fungicide applications are recommended for foliar diseases at the R3 growth stage. R3 is called the "beginning pod" growth stage, meaning the pod is 3/16 inches long at one of the four uppermost nodes on the main stem and has a fully developed leaf. An article in the journal Phytopathology that examined fungicide trials made at R3 across nine states and Ontario, Canada, from 2005 to 2018 showed that the average response to foliar fungicides was 1.64 bushels per acre. Higher yield responses were observed when the planting date was no later than May 21, and rainfall averaged above historical levels from planting to R3. Economically, lower profitability was expected when foliar disease was absent or very low.

Most corn studies have found that the greatest yield benefit from fungicide application occurs at the VT-R1 (tasseling to silking) growth stages. This idea makes sense, since this is typically a time of high disease pressure and the tissues we most want to protect are present and active. The ear leaf and younger leaves are the solar panels that generate the sugars that then fill the grain. These later applications take us further into the season, protecting this foliage. Understanding the growth stage and fungicide timing can be critical for those growing for grain and silage. The downside of the VT fungicide timing is that it requires specialized equipment, such as a highboy or aerial application, which is more expensive and must 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 watch the pre-harvest interval restrictions, especially when chopping silage.

Recommendations

Early-to-mid July is a crucial time to scout corn and soybean fields and determine if diseases like gray leaf spot or northern corn leaf blight in corn are observed anywhere from the third leaf below the ear leaf or higher on 50% of the inspected plants, or if symptoms of frogeye leaf spot can be observed in the upper soybean canopy. Knowledge of the soybean variety or corn hybrid is also critical to determine the degree of resistance to the primary targets. In corn, planting corn following corn and greater residue presence due to no-till use can favor pathogen survival and subsequent disease development. Current commodity prices are lower than in previous years, so it is essential to consider the product + application cost, which can run as high as $30-40 per acre, to determine the realized yield response needed to cover those costs. Ultimately, the economics may not justify using a foliar fungicide if the risk is not there.

What do the risk models tell us so far - July 6, 2026?

We will monitor five diseases this year. In corn, this includes gray leaf spot, tar spot, and Gibberella ear rot/deoxynivalenol. In soybeans, we will monitor white mold and frogeye leaf spot. The conditions that favor each disease differ, and it is critical to review each model in relation to the growth stage and the timing of peak infection risk.

How are we monitoring risk this year? We have identified locations throughout Pennsylvania, with fields selected from the following counties: Butler, Centre, Franklin, Greene, Lackawanna, Lancaster, Lehigh, Mercer, and Tioga. In each case, we will report the model run date, the current risk, and a seven-day risk outlook. 

Corn: Current conditions indicate a high risk of gray leaf spot, which will remain high over the next seven days. Tar spot risk varies across Pennsylvania, while the current risk for Gibberella ear rot (and mycotoxin contamination) is very low. We recommend checking your local risk for these latter two diseases more closely.

Soybean: Current conditions for frogeye leaf spot remain low across much of Pennsylvania but are shifting to a moderate risk in the western portions of the state. White mold risk is currently lower in many areas, but will increase over the next seven days. It is especially important to check the models for your location and the current soybean growth stages across your fields.