By Alyssa Collins and Paul D. Esker
With widespread drought conditions plaguing the first half of our growing season this year, crop development has been quite variable. But those corn and soybeans that are hitting critical growth periods are where we should be considering if foliar fungicide applications against our key foliar diseases are necessary (Figure 1). The big question before deciding to pull the trigger is if 2023 conditions warrant their application. Now that the faucet has turned on, we may begin to see some of the foliar disease that has been lacking so far.
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 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 2023 for both soybean and corn and provide the most up-to-date information available about the relative efficacy of common fungicides used 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 the planting date was no later than May 21, and there were 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 the 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.
What about tar spot this year?
Tar spot is now active in some states in the midwest, but has not yet been detected in Pennsylvania in 2023. Given that tar spot was well established across the western and southern counties of PA as of last year, we expect to see the disease again this year but want to remind everyone that we still have lots to learn about this disease and the threat it poses to Pennsylvania corn production. We recommend that scouting starts in any fields that were affected by tar spot last year or in 2021 by focusing on the lower canopy where the spores would be most likely to be splashed up from the ground first.
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.
If you detect tar spot, does that mean you should spray immediately? Not necessarily. Tar spot management is a marathon across a season. Spraying in early growth stages is unlikely to boost yield and likely to leave your crop vulnerable during the most critical time later on. Continue to monitor the progress of spots on the plant. For grain production, the critical need is to protect the ear leaf and above. But this must be balanced by the longevity of protection for the duration of the season. Our fungicides can only provide 3-4 weeks of activity, so early application may leave your plants unprotected through grain fill. Spraying before you see symptoms on the ear leaf is unlikely to be an economical choice. Once you do see symptoms on that leaf, consider your growth stage and the weather projections (will there be periods of intermittent wetness?).
For silage corn, the considerations are a bit different. Your fungicide choice here will be influenced by the potential for the disease to rapidly dry your biomass and the length of time anticipated until chopping. Watch the postharvest intervals on any fungicide you plan to apply, as these might run close to the limit.
Continue to watch Field Crop News, and we will share progress of this disease and updated management recommendations as we learn more and observe conditions this season.
Source : psu.edu