For peanut growers, timely and effective fungicide applications are crucial for managing the crop and protecting yields from diseases like white mold and leaf spot. Considering where we are in the season, most peanut growers across the Southeast, including in Alabama, have initiated their fungicide program by now (at 30 to 40 days after planting) and are hopefully staying on top of the spray applications. In peanuts, adequate spray deposition within the peanut canopy is important to ensure effective fungicide applications, as disease and pest incidence are typically higher in the lower canopies. This becomes challenging as peanut canopies become denser later in the season and the upper canopy starts intercepting more spray particles, hindering them from reaching the lower canopy. While the choice of fungicide program and timeliness play an essential role in effective disease management in peanut, other factors related to spray applications (e.g. spray volume, nozzle type, etc.) are equally important and require careful consideration. Over the last few years, we have investigated the effect of several of these application variables, particularly on spray deposition within the canopy, as they can be controlled and adjusted accordingly to improve fungicide applications. Below are some general recommendations based on the findings from these studies:
Spray Volume: Using a higher spray volume is one of the most effective ways to enhance spray coverage. Most pesticide labels, including those for peanut fungicides, typically have a minimum volume requirement of 10 gallons per acre (GPA) for ground-based applications, but we have also heard of some growers using lower volumes than that. The minimum spray volume works when everything else is perfect, which is usually not the case. We have observed a strong correlation between spray volume and coverage in most of our peanut studies, where the highest spray volume of 20 GPA had the greatest coverage in the lower canopies, followed by 15 and 10 GPA. Based on those findings, though a higher volume of 20 GPA would be preferred (and used by some growers), it may reduce spraying efficiency due to the increased number of refills. In most cases, a spray volume of 15 GPA is usually ideal, as it provides sufficient coverage within the peanut canopy while also serving as an insurance in cases where other factors, such as ground speed or boom height, may impact the actual applied volume. On that note, it is important to emphasize that there is a big difference between thinking (or assuming) what your sprayer is putting out and knowing (by measuring) the actual spray output. Therefore, it is always good to calibrate the sprayer properly and verify the actual applied spray volume by checking it on multiple nozzles across the boom.
Nozzle Type/Droplet Size: The nozzle type is another important variable, as it influences droplet size, which in turn impacts the amount of spray particles deposited into the peanut canopy. The standard flat-fan (XR/XRC) or other similar nozzles, which produce fine to medium droplets, are most widely used for fungicide applications by peanut growers. While these nozzles work well in conditions when there is little to no wind, we found that the nozzles that produce larger droplets (coarse to very coarse), such as TeeJet AIXR, provide comparable or better coverage in nominal spraying conditions (low to moderate wind speeds, 3-7 mph) due to the lower amount of finer, driftable spray particles. A spray quality analysis – evaluating different droplet sizes contained within the spray – indicated that approximately 30% of the finer particles produced by XR/XRC nozzles don’t reach the target, whereas this number was only 10% for the AIXR nozzles, resulting in better coverage. This can also be viewed and understood in terms of applied spray volume. A target spray volume of 10 GPA applied with XR/XRC nozzles would result in an applied rate of 7 GPA only, whereas it would be 9 GPA for AIXR nozzles. It is important to mention that only a few fungicide labels list the actual droplet size requirements, so it is essential to utilize a combination of nozzle type and spray pressure (preferably within 30 to 50 PSI) that produces the medium (if standard flat-fan or XR/XRC is still your preferred nozzle) to coarser droplets to maximize spray deposition and product efficacy. Nozzles producing ultra-coarse droplets, such as TTI or dicamba tips, should be avoided (if possible at all) for fungicide applications, as they have lower coverage as well as reduced canopy penetration.
Ground Speed: The sprayer’s ground speed is another variable that affects spray coverage and efficacy, as it is directly related to the volume of spray applied per unit area. Sprayer calibration is performed assuming a constant ground speed to attain the desired spray volume (GPA). Whereas in reality, fluctuations in ground speed, due to field topography or conditions at spraying, are common and result in areas of under- and over-application within a field. For a sprayer (with no rate control capabilities) calibrated at 8 mph, the graph below illustrates how spray coverage reduces significantly with an increase in speed, while it increases when the sprayer is operating at a speed lower than the calibrated speed. This is also the reason why peanuts ( and disease control) sometimes appear better at the field edges (where the sprayer is usually slower) than in the middle of the field (where the sprayer is most likely operating at a higher speed). We have also noticed (and heard about) similar observations for large self-propelled sprayers equipped with rate controllers, where the sprayer can compensate (by adjusting pressure) for speed changes, but that also only works well within a specific ground speed range, based on the selected nozzle size. While ground speed changes within the field are unavoidable, the best way to ensure consistent or improved coverage is to maintain or stay below the ground speed for which the sprayer was calibrated. In most cases, avoid applying fungicides at ground speeds above 8 mph for three-point mounted sprayers and above 12 mph for self-propelled sprayers. Higher ground speeds also induce boom bounce, which again affects spray deposition and uniformity.
Boom Height: Another important, yet often overlooked, variable is sprayer boom height. The commonly used flat-fan nozzles have a tapered spray pattern, which means they require overlap from adjacent nozzles to provide uniform spray application within the spray swath. Maintaining a proper boom height (recommendation is 20” to 30”) from the target – peanut canopy in this case – is important to achieve this adequate overlap and attain uniform application. However, it is not hard to find sprayers applying fungicides with a boom set too high and observing where the spray is landing behind them (or in the neighboring field). Higher boom heights also result in greater spray drift, as most of the finer spray particles now have an increased potential for off-target movement, thereby leading to reduced spray coverage. In peanuts, a lower boom height is even more critical (and preferred) to push the spray particles into the peanut canopy and maximize spray deposition. One of the most common mistakes to avoid is setting up the boom height by just eyeballing it; instead, use a tape measure to set the correct height from the peanut canopy. While doing this, another important point to consider is to level the boom (different sections across the boom) as that again affects spray uniformity within the swath.
To conclude, it is worth mentioning that a reduction in spray deposition doesn’t always translate to poor disease control and/or reduced yield, but in some cases, it can. There are other factors, such as timing, weather, fungicide program, and crop rotation, that also play a role in the fungicide efficacy equation. Some of these factors we can control precisely, while some are beyond our control. Just as we say for everything else in farming, the best thing we can do is to control what we can and not worry much about the uncontrollable factors. The same applies here, where the spray variables mentioned above are among the easy-to-control factors that peanut growers can manage to ensure effective fungicide applications.