Best Practices for Foliar Fungicide Applications with Spray Drones

Unmanned Aerial Systems (UAS), Unmanned Aerial Vehicles (UAVs), or Remotely Piloted Aerial Systems (RPAS), commonly referred to as drones, are widely used in field crop production for crop scouting, agricultural input applications (e.g., pesticides, fertilizers, seed), and other activities. Pesticide application with a drone offers several advantages over ground-driven spray equipment or fixed-wing or helicopter applications (manned aircraft), including the ability to access small, fragmented, or irregularly shaped fields, avoid obstacles surrounding the fields such as utility poles, hedgerows/shelterbelts, waterways, and treat localized areas instead of an entire field. However, spray drone application characteristics differ substantially from both ground sprayers and manned aircraft, requiring careful attention and adjustments to application parameters, particularly swath width. Current research suggests that spray drones can provide similar results as ground and manned aerial applications, but achieving comparable results requires the applicator to understand the capabilities and limitations of spray drones.

The following best management practices are intended to help applicators address common issues with spray drone applications and optimize the performance of foliar fungicide applications.

Understand Licensing and Application Laws in Your State

Spray drone applicators must comply with all Federal Aviation Administration (FAA) licensing and training requirements for aerial applicators and adhere to pesticide licensing requirements of their respective states. Some states may have additional licensing requirements for spray drone applicators. For example, many states may require an additional aerial pesticide application license in addition to a commercial pesticide application license for any drone applications.  Unlike the United States, where the FAA regulates drone operations, spray drone applications in Canada must comply with Transport Canada RPAS regulations, Health Canada Pesticides Regulatory Directorate (PRD – formerly PMRA) pesticide-use requirements, and applicable provincial pesticide legislation.

Always check the pesticide label before applying a product via spray drone and follow all label restrictions. Many pesticide labels either prohibit aerial application or do not specify if the product can be used with aerial application methods. Products that do not have an aerial application section on the label cannot be applied by a drone. It is important to note that while some pesticides are labeled for aerial applications, most labels were written for manned aircraft rather than for drones. As a result, labels may provide limited guidance on optimal drone operating parameters.

Factors Influencing Success of Drone Applications

Spray efficacy is influenced by a variety of factors, regardless of application method. Factors such as spray height, speed, wind direction and speed, nozzle type and droplet size, carrier volume, and weather all influence pesticide applications. The effects of many of these factors on spray pattern and distribution, and the importance of adequate coverage in fungicide applications are described in the Crop Protection Network Fungicides for Field Crops Web Book. For best results, review the Application Coverage chapter and the Why Fungicides Fail chapter of this book.

There are a few spray factors that may have a greater influence on spray distribution and pattern with drones compared to other application methods. These factors include swath width and off-target movement, or drift.

Swath Width

University research has demonstrated that the manufacturer-reported operational swath width of a spray drone, and the effective or agronomic swath width, which is the area uniformly covered by the drone application in the field, can vary, especially with variable operating conditions. Unlike a boom sprayer, where boom width largely determines the spray swath width, the drone swath width fluctuates with drone spray height, speed, weather conditions, and payload weight. These variables interact throughout the application event. For example, a change in flight height of only 2 to 3 feet can increase or decrease the effective swath width by several feet. Likewise, as the spray tank empties during flight, the drone becomes lighter, altering rotor downwash and potentially changing spray deposition. Small changes in operating conditions such as reduction in weight as the payload is sprayed can alter spray deposition and effective swath width in real time by up to 30% in a single pass. Similarly, swath width decreases during acceleration and deceleration, resulting in tapered edges and potential gaps in spray coverage. Operators should be aware of these and other factors that can affect drone spray application uniformity and coverage.

University research trials and field demonstrations have shown that the effective swath width achieved in swath testing on bare ground may differ from the manufacturer-reported swath width because of differences in flight height, speed, weather conditions, payload, and other operating variables. Furthermore, once applications are made over a crop canopy, the effective swath width is often reduced even further. Research has shown reductions of approximately 25% in soybean and up to 45% in corn compared with swath widths measured over bare ground because the crop canopy alters rotor downwash and spray deposition, among other factors (Figure 1).

Because swath width is not a fixed variable, drone applicators should calibrate every drone individually before use, at a range of common flight speeds and altitudes. Drones of the same make and model may produce different effective swath widths, so it is important to do this even if you have similar drones. The most effective way to do this is to use equipment designed for measuring coverage over large swath areas, such as the Swath Gobbler (Figure 2). Some Universities may offer free or reduced-cost testing of drone swaths using this equipment. Contact your local Extension Specialist to determine if there are University-led swath testing options available in your state. Other options include spreading water-sensitive spray cards (Figure 2) over a wide swath area or adding tracer dye to a tank of water and spraying over a wide gravel or cleared area to determine spray coverage.

A safe rule of thumb is that after the swath width has been measured over bare ground, reduce the effective swath width by approximately 25% to 30% for in-canopy applications. This additional overlap helps maintain more uniform fungicide coverage and minimizes skips or streaks in the field. Ultimately, this means that fields will take more time to spray, but pesticide efficacy should increase with improved swath adjustments.

Drone Applications are Prone to Drift

The goal of all pesticide applications is to maximize the amount of product in contact with the desired target area within the crop canopy, and minimize the amount of off-target movement, or drift. All standard drift management precautions apply with drone applications, but University research has demonstrated that drone fungicide applications may be more prone to altered spray pattern, deposition, and off-target movement, even at low wind speeds. Therefore, it is especially important to monitor weather conditions (i.e.: wind and inversions) prior to and during drone applications and maintain a flight height that balances swath width and drift reduction. Verify droplet size under your operating conditions whenever possible, as actual droplet size can differ substantially depending on flow rate, atomizer speed, and product formulation.

There are specific adjuvants that are marketed to maintain spray pattern and reduce drift during aerial applications. Using an aerial or drift-reducing adjuvant may not “solve the problem” of drift from drone applications, and may increase droplet size, impacting fungicide efficacy. University research is currently underway to examine how different drift-reducing aerial adjuvants impact spray pattern and fungicide efficacy with drone applications. 

The Evolving Technology of Spray Drones

Spray drones are an effective way to apply fungicides for foliar disease management in field crops, but they are not a “one-size fits all” application method. Their greatest value comes from matching the technology to situations where it provides clear agronomic or logistical advantages while following sound application practices. Successful fungicide applications with drones still depend on the same fundamentals that guide every pesticide application.

Spray drone technology is evolving rapidly. New aircraft designs, application systems, payloads, and operating recommendations continue to emerge, and research is expanding to determine optimal application parameters for improved efficacy. As with any new application technology, operators should seek research-based recommendations and receive appropriate training in both drone operation and pesticide application. Contact your local Extension Specialist to learn more about state-specific University/Provincial resources on drone pesticide applications.

Acknowledgements

Authors

Doug Houser, Iowa State University; Stith Wiggs, Iowa State University; Daren Mueller, Iowa State University; Luke Fuhrer, Iowa State University; Damon L. Smith, University of Wisconsin-Madison; Shawn Conley, University of Wisconsin-Madison; Kiersten Wise, University of Kentucky

Reviewers

Martin Chilvers, Michigan State University; Travis Faske, University of Arkansas; Andrew Friskop, North Dakota State University; Albert Tenuta, Ontario Ministry of Agriculture, Food and Agribusiness

Additional Resources

Sprayers 101

Using Spray Drones in Indiana - PPP-156 

Droplet Trajectories from RPAS Application- Implications for Swath Width Measurements

How to Calibrate a Drone – Swath Width Calculation

How to cite: Houser, D., Wiggs, S., Mueller, D., Fuhrer, L., Smith, D. L., Conley, S., Wise, K. 2026. Best Practices for Fungicide Applications with Spray Drones. Crop Protection Network. CPN-4013. https://cropprotectionnetwork.org/publications/best-practices-for-foliar-fungicide-applications-with-spray-drones.