By ALYSSA COLLINS and PAUL D. ESKER
Currently, corn and soybean growth stages are hitting the critical growth periods for considering foliar fungicide applications against our key foliar diseases (Figure 1). The big question before deciding to pull the trigger is if 2022 conditions warrant their application. The growing season has seen a little bit of everything, although we have seen increased rainfall in multiple locations over the past couple of weeks.
Figure 1. Symptoms of (left) frogeye leaf spot in soybean, (center) northern corn leaf blight in corn, and (right) gray leaf spot in corn. (Photo credit. Paul Esker, Penn State Extension)
The best approach for deciding about the need for a foliar fungicide application in field crops is based on using integrated pest management approach that considers cultural practices (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 would like to recommend that you look at the e-book Fungicide Use in Field Crops, available at the Crop Protection Network to help understand the factors that drive fungicide use and need. Additionally, foliar fungicide efficacy guides are updated for 2022 for both soybean and corn and provide the most up-to-date information available about the relative efficacy of common fungicides using in each crop.
What do we currently know about fungicide responses in soybean and corn?
In soybean, fungicide applications are currently recommended for foliar diseases when soybean is at the R3 growth stage. This is called "beginning pod" and means that the pod is 3/16 inch long at one of the four uppermost nodes on the main stem with 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 found when planting date was no later than May 21 and there was higher than historic averages of rainfall from planting to R3. Economically, lower profitability was expected when foliar disease was absent or at very low levels.
Most corn studies have found the greatest yield benefit from a fungicide application to be at the VT-R1 (tasseling to silking) growth stages. Intuitively, this makes sense since this is typically a time of high disease pressure and the tissues we want to protect most are present and active. The ear leaf and those leaves that are younger are the solar panels that generate the 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.
Recommendations.
This 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 can be seen anywhere from the third leaf below the ear leaf or higher on 50% of the inspected plants, or if frogeye leaf spot is seen 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, the planting of corn following corn and greater residue presence due to the use of no-till can favor pathogen survival and subsequent disease development. Even though current commodity prices are good, it is particularly important 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. In the end, the economics may not justify the need for a foliar fungicide if the risk is not there.
Do not forget the new kid in town.
Tar spot is currently active in states in the midwest, but has not yet been detected in Pennsylvania in 2022. Given that tar spot is established in Lancaster County and a few counties in Northwest PA as of last year, we want to remind everyone that we still have lots to learn about this disease and the threat it poses to Pennsylvania corn production. As you scout your corn fields, pay particular attention to any symptoms that appear to be tar spot. please work with your nearest Extension educator to obtain a positive identification and alert state specialists Alyssa Collins and Paul Esker.
Source : psu.edu