Tag: drone

  • Characterizing RPAS Coverage at Four Cardinal Points on a Vertical Plane: Practical implications for spraying wheat at T3

    Characterizing RPAS Coverage at Four Cardinal Points on a Vertical Plane: Practical implications for spraying wheat at T3

    Global research into Remotely Piloted Aerial Systems (RPAS) is producing pesticide residue, drift and efficacy data that is helping to inform federal regulatory policy. It is reasonable to assume that Canada will ultimately sanction the use of RPAS for agricultural spraying. The first registered products will likely be fungicides intended for broad acre crops such as soybean, corn, and wheat.

    Those considering RPAS for agricultural spraying have expressed interest ranging from general curiosity to high demand. Successful adoption will be contingent on expectation management, which in turn requires education on the functional differences between RPAS and conventional application technologies.

    Quadrotor RPAS design dominates the current commercial landscape, with typical models featuring four rotary atomizers and 40 L tanks. There have been improvements in recent years, but these designs continue to suffer from a low rate of productivity (by North American standards for broad acre crops). This is due, in part, to low volumetric capacity and limitations with rotary atomizer design, which result in a debatably short effective swath width. Broadly, “swath width” refers to the minimal span consistently sprayed by a single pass, while “effective” indicates a spray coverage (i.e. deposition pattern and threshold dose) sufficient to achieve the desired result.

    Our research efforts have focused on identifying and evaluating variables that influence effective swath width. These include operational settings such as altitude, travel speed, volume applied and nozzle settings. They also include environmental factors such as meteorological conditions, crop morphology, and planting architecture. Establishing a combination of settings that account for these factors will inform operator practices and optimize the balance between RPAS effectiveness and efficiency.

    Study Objective

    Fusarium head blight is a significant economic threat in wheat. Fungicide application takes place at the T3 stage of development, with the intention of providing panoramic coverage of the wheat head. RPAS is being considered to apply these fungicides.

    The pursuit of productivity tempts operators to push operational settings to the point that spray coverage is compromised. This study will use operational settings based on the results of previous work and assign flight speed as the independent variable.

    Coverage will be assessed using water sensitive paper (WSP) positioned at the top of the canopy and oriented vertically in four cardinal directions to emulate the circumference of the wheat head. Isolating the resultant coverage in each cardinal direction may provide insight into droplet behaviour within the RPAS spray cloud and perhaps better assess effective swath width.

    Coverage from a conventional field sprayer will also be characterized. This represents the current standard and it will provide a basis for comparison.

    Materials and Methods

    Site

    The experiment was conducted at 45939 John Wise Line, St Thomas, Ontario (42.7320746, -81.0879887) on June 1, 2025. Common seed wheat was planted on October 6th, 2024, at 1.8 million seeds/ac on a 19 cm (7.5 in) row spacing. At the time of spraying, wheat was at the T3 stage of development, approximately 0.7 m (2.5 ft) high.

    Holder Design

    3D-printed holders were designed in Autodesk Fusion. They feature tabs that create a pressure fit (for quick WSP loading and unloading) and a back support (to prevent WSP movement in a downwash) to vertically position 1×3” WSP facing out on four cardinal points (figure 1). 21 poles were positioned on a 1 m spacing, leaving the tops coplanar with the wheat heads for in situ swathing. The square cross-section of the poles corresponded to a square depression in the WSP holder, ensuring the samplers were correctly aligned (figure 2). The gcode file to 3D print your own holders can be downloaded here. We used PLA filament with 3 walls and a 15% rectilinear infill to print 16 holders at a time on a Bambu P1S.

    Figure 1. 3D-printed WSP holder slotted onto a pole in the wheat field.
    Figure 2. Four WSP positioned vertically, facing four cardinal points, at wheat-head depth.

    Wind direction is indicated by the direction the wind is coming from, not the direction it’s blowing to. Therefore, a wind blowing in a northern direction is referred to as a southern wind. The WSP holders were aligned east to west, perpendicular to the prevailing southern wind. We established a trampled path ~0.75 m on the downwind side. This left the wheat canopy surrounding the samplers intact while still permitting access to the WSP holders (figure 3).

    Figure 3. 21 WSP holders on 1 m spacing with access ~0.75 away.

    WSP were given unique serial numbers to identify their position, and pre-loaded into the holders. 170 loaded holders were stored on raised grids inside four shallow plastic bins, arranged in the order they would be used (figure 4). This greatly expedited placement and retrieval, allowing for more repetitions to take place while optimal weather conditions held.

    Figure 4. WSP holders arranged in sequence in a storage bin. This allowed the pre-loading of serial numbered WSP prior to the day of the experiment.

    Application Method

    A DJI Agras T50 RPAS maintained and calibrated by the cooperator was fitted with four rotary atomizers. Previous work has demonstrated that higher rates of flow can have a detrimental effect on the spray quality from rotary atomizers, so distributing the flow over four nozzles was intended to prevent this. The atomizers applied a 250 µm spray over a 7 m swath width (corresponding with route spacing), as selected on the flight controller. Altitude was 3 m above the wheat heads, and flight speed was either 6 or 10 m/s (three repetitions each). Application volume was 50 L/ha, anticipating this to be a future label requirement.

    The RPAS flew a racetrack pattern over the samplers (figure 5). It flew with a prevailing tailwind between samplers 14 and 15, and then back with a prevailing headwind between samplers 7 and 8. We employed a DJI RTK-1 base station, which claims 1 cm horizontal accuracy.

    Figure 5. Racetrack flight path relative to WSP holders and prevailing wind direction.

    The RPAS was given sufficient distance (~40 m before and after the samplers) to reach the target speed, which was confirmed with a screenshot from the flight controller. The RPAS tank (capacity 40 L) contained municipal water with 0.125% v/v of NIS (Ag-Surf II). The level of liquid in the RPAS tank was maintained at 40 L throughout the trial to eliminate the effect of a changing payload.

    The field sprayer, maintained and calibrated by the cooperator, was a New Holland SP 275s. It extended the left boom over the samplers from position 1 through to 12 and made a single pass with a prevailing tailwind (three repetitions). It traveled at 4.5 m/s (16 km/h or 10 mph) spraying 187 L/ha (20 gpa) and rough terrain caused the boom height to fluctuate between ~25 cm and ~50 cm above the wheat heads. The nozzles were Greenleaf Technologies TADF 06 (greys) on a 50 cm spacing operated at 50 psi to produce a Coarse spray quality. The tank contained municipal water with 0.125% v/v of NIS (Ag-Surf II).

    Trial Procedure

    WSP holders were placed just prior to spraying while the RPAS or field sprayer was positioned ~40 m beyond the samplers. When wind conditions were deemed appropriate, a signal was given to initiate spraying. On pass completion, one minute elapsed before initiating collection to permit complete deposition of the spray and drying of the droplets.

    Weather Data

    Weather data was collected using a Kestrel 3550AG weather meter (Kestrel Instruments) in a vane mount positioned roughly 1 m below RPAS altitude. Data was logged as the RPAS or field sprayer boom passed the samplers (table 1). In the case of the RPAS, there was very little difference between the two passes per repetition, so values were averaged.

    TimeRep and TreatmentTemperature (°C)Wind Speed (km/h)Wind Direction
    10:21:511. RPAS 6 m/s16.43.1S
    10:33:562. RPAS 6 m/s17.76.2S x SW
    10:45:503. RPAS 6 m/s18.84.0S x SW
    10:55:401. RPAS 10 m/s18.62.1SW
    11:06:242. RPAS 10 m/s20.00.5S
    11:16:003. RPAS 10 m/s19.25.8S x SW
    11:29:541. Field Sprayer21.10.0S
    11:41:122. Field Sprayer19.91.7S x SW
    11:48:313. Field Sprayer20.23.9S
    Table 1. Time and weather conditions for each repetition. Data was captured as the nozzles passed over the holders. Wind direction is indicated by the direction it is coming from.

    Digitization

    WSP (Spot-On) were scanned using a DropScopeTM (SprayX). The software reported droplet density and percent area coverage, but only deposit density is considered in this report.

    Results

    Comprehensive Observations

    When the average deposit density from each of the four WSP per holder is added, we have a measure of total panoramic coverage. The mean total panoramic coverage from three repetitions of each treatment is shown in figure 6. For the RPAS treatments, distinct coverage peaks typical of RPAS applications correspond to the flight passes through poles 14-15 and 7-8. There is a slight spray displacement due to an occasional shift to a west wind (i.e. overall coverage shifted towards pole 1). However, the entire spray swath appears to have fallen within the range of the samplers. The back-and-forth flight pattern produced higher coverage at the interface between passes (pole 10) compared the extremes (beyond poles 19 and 3) suggesting some overlap. Deposit density was higher for the slower RPAS flight speed, and RPAS produced a higher deposit density than the field sprayer. Field sprayer coverage data is included for perspective but is generally not referred to unless it has some bearing on the evaluation of the RPAS coverage.

    Figure 6. Mean sum deposit in count/cm2 for each treatment. RPAS 6 m/s deposited an average 700 drops/cm2. RPAS 10 m/s deposited an average 400 drops/cm2. The field sprayer deposited an average 280 drops/cm2. r=3. S.E. bars shown. Arrows indicate RPAS flight path and direction.

    Swath Width

    If the coverage at pole 10 represents the edge of each swath, then a swath width of ~6.0 m can be estimated based on similar coverage at poles 16-17 and 3-4. This is less than the programed value of 7.0 m.

    This inference is supported when these averaged values were entered in an Excel-based model that calculates swath width. The model sums deposits from adjacent swaths assuming a racetrack pattern. Threshold coverage is subjective but adhering to the objective of establishing a balance between over- and under-dosing with the lowest possible C.V., we calculated swaths between 5.0 and 6.5 m (table 2).

    TreatmentPole PositionThreshold Coverage (count/cm2)Under-dose (%)Over-dose (%)C.V. (%)Swath (m)
    RPAS 6 m/s21-1012010.45.018.05.5
    RPAS 6 m/s10-11205.714.126.05.0
    RPAS 10 m/s21-10809.111.425.46.0
    RPAS 10 m/s10-13517.214.143.46.5
    Table 2. Swath widths calculated from the average cumulative deposit density for each WSP holder.

    It was expected that the field sprayer would produce a somewhat trapezoidal coverage pattern, tapering up at pole 12 (boom extreme) and level to pole 1 (sprayer chassis). Instead, note the gradual increase in coverage from pole 12 to 1. This is likely the result of boom yaw, where the boom end rose higher than the point closest to the sprayer. If this degree of coverage represents the industry standard, it is notable that the average boom coverage is either on par with, or considerably less, than that of the RPAS.

    Deposit Density

    The RPAS was programmed to produce a 250 µm droplet size, while the field sprayer produced Coarse (~218-349 µm) droplets. Smaller stains were produced by the RPAS than the field sprayer (figures 7 and 8), and their circular/oval shape suggest both a smaller droplet volume and a somewhat perpendicular flight path. Droplets produced by the field sprayer left long streaks, which suggest higher droplet volumes and a more parallel flight path.

    Figure 7. Typical deposition pattern from a single WSP holder in the centre of the RPAS flight pattern. Considered from the perspective of the RPAS, WSP starting at the top and rotating clockwise are left side, retreat side, right side, and advance side.
    Figure 8. Typical deposition pattern from a single WSP holder nearer the field sprayer chassis. Considered from the perspective of the field sprayer, WSP starting at the top and rotating clockwise are left side, retreat side, right side, and advance side.

    When the overall average coverage is calculated the RPAS at 6 m/s deposited an average 700 drops/cm2 and an average 400 drops/cm2 at 10 m/s. The field sprayer deposited an average 280 drops/cm2. When volume remains constant, smaller droplet diameters produce a greater number of droplets than with larger droplet diameters, so more droplets would be expected from the RPAS. However, the RPAS applied only 50 L/ha while the field sprayer applied 182 L/ha. Therefore, the RPAS distributed a greater density of potentially higher-concentration droplets on each WSP holder compared to the field sprayer. Further, a slower flight speed deposited a higher density of stains than a faster flight speed.

    Effect of Travel Speed

    The 6 m/s treatment resulted in slightly smaller swath widths (1 m or 15% less) than the 10 m/s treatment. The positive relationship between swath width and flight speed has already been established. The positive relationship between flight speed and off target drift has also been established, which may account for the significantly fewer deposits (almost 50% fewer) in the swath at 10 m/s versus 6 m/s. When a higher deposit density is valued, such as in the case of contact fungicide application, the loss of productivity from a slightly smaller swath width is a reasonable compromise for the superior coverage within that swath.

    Coverage by Cardinal Point

    North Facing WSP

    This side of the WSP holder faced north, away from the prevailing south wind. The RPAS deposited far more on this face when traveling with the wind between poles 14-15, making this side face the RPAS retreat (figure 9). It suggests the RPAS blows down and back, even against a tail wind. This is supported when RPAS returned between poles 7-8 and deposited comparatively less on this face.

    Figure 9. Mean sum deposit in count/cm2 for each treatment on north facing WSP. r=3. S.E. bars shown. Arrows indicate RPAS flight path and direction.

    RPAS speed may have had an effect. With airblast sprayers, slower travel speeds produce greater dwell times, which increases the distance a droplet travels on a given trajectory. Logically, when flying away from the retreat face, higher speeds would impart a greater forward momentum on a droplet, cancelling out some of the backward momentum (watch a video here). If this were the case, there would be comparatively improved deposit density on the 14-15 pass for slower speeds and reduced coverage between poles 7-8 as it blew past the target. Figure 9 supports this hypothesis.

    South Facing WSP

    This side of the WSP holder faced south, into the prevailing south wind. Considering the 6 m/s treatment, we see more coverage on the 7-8 pass than the 14-15, because it represents the retreat side of the sprayer (figure 10). The differential is far less on this plane than the north facing (figure 9) because the prevailing wind likely blew spray into the WSP on the 14-15 pass. Nevertheless there is significantly more on 7-8.

    Figure 10. Mean sum deposit in count/cm2 for each treatment on south facing WSP. r=3. S.E. bars shown. Arrows indicate RPAS flight path and direction.

    This relationship is less clear for the faster 10 m/s treatment. There does tend to be higher deposit on the 14-15 pass as spray was blown into the collectors. However far more was expected on this face for the 7-8 pass as it represents both the retreat face and has the added benefit of wind. Further, there was far less coverage overall when compared to the slower flight speed. We have no explanation for the lack of coverage on the 7-8 pass and can only conclude that higher speeds left droplets airborne and were not conducive to coverage.

    West Facing WSP

    This side of the WSP holder faced west, into the slight west wind. We see coverage is almost exclusively on those WSP facing the drone (figure 11). In other words, as the RPAS passed between 14-15 and 7-8, coverage was positively skewed from this point. The skewed coverage was evident at both flight speeds, but overall coverage was higher for the slower speed. Once again, we cannot explain why there was significantly reduced coverage on the 10 m/s pass between poles 7-8 except to suggest the spray may have remained airborne.

    Figure 11. Mean sum deposit in count/cm2 for each treatment on west facing WSP. r=3. S.E. bars shown. Arrows indicate RPAS flight path and direction.

    East Facing WSP

    This side of the WSP holder faced east, away from a slight west wind. We see that coverage is almost exclusively on those WSP facing the drone in the upwind direction (figure 12). Overall coverage was slightly higher for the slower speed, but far less overall coverage compared to the west facing samplers (figure 11). This is likely because the light west wind caused spray to displace from pole 21 to pole 1, washing past the back (insensitive) sides of the WSP.

    Figure 12. Mean sum deposit in count/cm2 for each treatment on east facing WSP. r=3. S.E. bars shown. Arrows indicate RPAS flight path and direction.

    Summary

    RPAS can provide par or better panoramic wheat head coverage compared to a conventional ground rig when they are flown using reasonable operational settings in optimal environmental conditions. A moderate flight speed (~6-8 m/s), appropriate altitude (~3 m above wheat heads), and four rotary atomizers producing a Medium-Coarse (~250 um) droplet size can produce an in situ 6 m swath width at 50 L/ha. Higher flight speeds produce a marginally wider swath at the cost of reduced droplet density and increased drift potential.

    Assessing coverage using vertical WSP facing four cardinal points has provided further insight into the behaviour of spray from an RPAS. Droplets from any application technology tend to deposit with wind and gravity, but rotor downwash represents an additional variable unique to RPAS. That force, combined with a forward cant of the drone during flight, lead droplets to deposit on vertical surfaces that face the rear (retreat) of the sprayer as well as surfaces that face and intercept spray that radiates laterally from the flight path.

    Drone Spray Canada, Bayer Canada, volunteers Kurtis Pilkington and Natalie, and grower-cooperator Adam Pfeffer are gratefully acknowledged for their contributions to this study.

  • Drone Spraying in Canadian Agriculture (up to 2024)

    Drone Spraying in Canadian Agriculture (up to 2024)

    Note: There have been updates to Canadian regulations governing drones since this article was written. For updates, please refer to this article.

    Introduction

    In Canada, the use of drones for pesticide application, otherwise known as RPAS, is regulated by two Federal Departments: Transport Canada establishes regulations for safe operation and Health Canada for the registration and conditions of use of pest control products.

    Drones are already used in Canadian agriculture for crop surveillance and livestock management, and they’re being used to apply granular fertilizers, for pollination, for frost protection and greenhouse shade management. The use of drones for general spraying was cleared by Transport Canada in July 2017. In 2018, Health Canada stipulated that the use of RPAS for pesticide application is not allowed under the Pest Control Products Act (PCPA) without sufficient data to characterize the hazards or risks associated with this use. You can read the updated (as of June, 2023) Pest Management Regulatory Agency Information note on the subject of pesticides applied by drone, here.

    At this time this article was written, only three pest control products were registered for application by RPAS in Canada. These were restricted-use microbials (two granular and one liquid) intended for larval mosquito control. Their labels were expanded to include RPAS in the fall of 2022 but as of 2023 no province or territory has yet permitted their application. International RPAS working groups (e.g. the OECD working party on pesticides and drones) comprised of academics, the agrichemical industry, government regulatory agencies, and both drone manufacturers and operators, are working collaboratively to assemble the evidence-based information we need to inform the expansion of other pesticide labels. These studies include:

    • comparative evaluations of efficacy
    • operator and bystander exposure studies
    • drift studies
    • residue studies, and
    • technical evaluations of how environmental, topographical, and operational parameters affect the above

    In parallel, several groups are working to develop pesticide safety certification and training materials. Pesticide training and certification programs across Canada are based on the Standard for Pesticide Education, Training and Certification in Canada. Canadian provinces/territories are responsible for the training and certification of pesticide vendors and applicators based on these standards. A national core manual for RPAS operator is anticipated.

    A candid moment from one of the many Health Canada-authorized RPAS research programs happening in Canada.

    Registration and Certification

    Anticipating future pesticide label expansions, perhaps you’re planning to buy and fly a RPAS. Pilots must register their drone (online for a $5 fee) and display that number on the drone. For more information, the Canadian Aviations Regulations (CARs) covers drones here. It’s a massive document, so jump to the end to find the relevant information under Section IX.

    Transport Canada requires all pilots with RPAS over 250 g to obtain a Pilot Certificate, either for Basic Operations or for Advanced Operations. In some cases, pilots may also have to apply for Special Flight Operations Certification (SFOC), which must be approved before the mission can take place. See below for details.

    Basic Operations Pilot Certificate

    The Basic Operations certificate allows pilots to operate any drone from 250 g up to and including 25 kg. This allows the pilot to fly:

    • Outside controlled airspace
    • No closer than 100 ft laterally from bystanders
    • In VLOS (or in contact with someone in VLOS)
    • Over 1.8 km from heliports
    • Over 5.6 km from airports

    If you’d like to explore the requirements, Transport Canada has an online document called TP15263 which describes required knowledge for Basic Operation pilots of small RPAS. Personally, I took a $100 online course (from a Canada-based drone flight school) to help me prepare for my exam. A good course will supply you with what you need to know about the laws, the environment, the aircraft, and your responsibilities as a pilot.

    The $5 exam has 35 multiple choice questions. You have 90 minutes to complete it and you need a 65% to pass. It was a surprisingly challenging exam, so don’t be discouraged if you don’t pass on your first try. You can take another swing at it after 24 hours, and you’ll encounter new questions randomly drawn from their database.

    Advanced Operations Pilot Certificate

    The Advanced Operations certificate allows pilots to operate any approved RPAS over 25 kg in VLOS. This allows the pilot to fly:

    • Inside controlled airspace
    • No closer than 16.4 ft laterally from bystanders
    • Under 16.4 ft above bystanders (essentially directly overhead)
    • In VLOS (or in contact with someone in VLOS)
    • Within 1.8 km from heliports
    • Within 5.6 km from airports

    There are two parts to this certification. The $10 exam requires an 80% to pass, covers more topics than the Basic Operations exam, and has the same 24h wait to retry. You must also undergo a Flight Review with a certified trainer, who changes about $200 for the service. Once the exam and flight review are successfully completed, there’s a $25 issuing cost. Be aware that flying a drone over 25 kg will also require an SFOC.

    Special Flight Operations Certificate

    In some cases pilots will have to apply for a Special Flight Operations Certificate (SFOC). Based on the CARs part IX regulatory structure, examples include operating a drone over 25 kg, flying at an advertised event, operating with foreign credentials (i.e. not a Canadian pilot) or operating outside the rules for Basic or Advanced operations, such as Beyond Visual Line-Of-Sight (BVLOS). Note, the requirements surrounding SFOC’s are under review as Transport Canada reassesses weight classes and streamlines the process, which can take weeks or months to complete. The new requirements are anticipated in late 2023 or early 2024 in Canada Gazette 2 (the 2019 update to Gazette 1 can be found here).

    Recency

    Once you have your Certificate(s) you must carry a copy with you while flying. Technically, Pilot Certification doesn’t expire, but you still have to maintain it. According to CARs 901.56, pilots cannot operate a drone unless they have successfully completed the following within 24 months preceding the flight:

    • Testing / Issue of their pilot certificate (Basic or Advanced).
    • A Flight Review
    • Any of the recurrent training activities set out in section 921.04 of Standard 921. This is an online questionnaire that has the answers posted right after each question. Don’t ask… just comply. Be sure to print it out after you complete it because it doesn’t save the answers.

    Just like the Certificate, the pilot must have Proof of Recency with them at all times. Unlike certification, it’s free.

    Learning from one another at a 2022 RPAS workshop in Southern Ontario.

    Records

    Every owner of a remotely piloted aircraft has to keep certain records. They need to be with you while flying for a certain period of time and all records must be transferred with the system if you sell or give it to a new owner.

    • The name of the pilot(s) and crew involved with each flight, noting time and date (keep with you while flying for 12 months).
    • Any maintenance, modification or repair of the RPAS, including precisely who did what and when. This should detail the instructions used to complete the work (keep with you while flying for 24 months).

    Fines and Enforcement

    Fines for contravening regulations range from a maximum of $3,000 for an individual to a maximum of $25,000 for a corporation. The RCMP and local police are part of the enforcement team.

    Crop Protection Products Registered in Canada

    This list is subject to change. To date, there are no pest control products registered for use in agriculture in Canada. There is no distinction between personal (i.e. home farm) and custom application. It is currently illegal to spray a registered agricultural product by drone. There are, however, two liquid formulation pesticides registered for non-agricultural use:

    • VectoBac 1200L – A biological larvicide intended for black flies and mosquitoes that must be applied over water sites. Label expanded in 2023.
    • Garlon XRT – A herbicide registered for industrial applications (e.g. controlling woody plants and vegetation in non-crop settings, such as around power lines and other utilities). Label expanded in 2024.

    Final Thoughts

    Get certified before you buy your RPAS, and do your research before you commit to a system. Rotor-based RPAS design is changing rapidly as manufacturers adapt to the demands of North American and European applicators. We once thought a swarm of lightweight, nozzled drones would be the path to success, and now the industry is leaning towards larger solitary drones with payloads over 40L and rotary atomizers instead of conventional nozzles.

    Be sure you understand what they can and what they cannot do. Only buy from a reputable dealer with practical spraying experience, and not someone with slick advertising that over-promises RPAS work rate, swath width, reduced drift or improved coverage potential. Ask to see data and remember, at the time this article was written: In Canada, it is currently illegal to spray pesticides in agriculture from a drone, whether it is on your property or not.

  • Stan and Blade’s Drone Emporium

    Stan and Blade’s Drone Emporium

    In this parody of SCTV’s Tex and Edna Boil we have some tongue-in-cheek fun while reminding people to maintain a healthy skepticism when reading RPAS marketing materials. Always be sure to ask questions and see the data before you believe what might be too good to be true.

    Special thanks to Jason Strove for his masterful post-editing magic.

  • RPAS Swathing in Broad Acre Crop Canopies

    RPAS Swathing in Broad Acre Crop Canopies

    This work was performed with contributions from Adrian Rivard (Drone Spray Canada) and Adam Pfeffer (Bayer Crop Science – funding partner). Dr. Tom Wolf is gratefully acknowledged for his editorial support and assistance interpreting the results.

    Introduction

    This research is part of a continuing effort to identify best practices for broad acre crop protection using remote piloted aerial systems (RPAS). Previous work in wheat, corn and soybean has provided insight into how RPAS operational settings and environmental factors affect drift potential, effective swath width and spray coverage. This information, paired with advancements in RPAS design, has helped operators to improve spray deposit accuracy.

    However, RPAS still produce what has traditionally been considered poor (or at least sporadic) broad acre coverage. Many studies have illustrated these shortcomings using herbicides or fluorescent tracers. Contributing factors include inappropriate operational settings, low application volumes (20-50 L/ha) paired with coarser spray qualities, and inaccurate swath widths. In light of these issues, we struggle to reconcile claims of acceptable disease control, which is arguably the greatest challenge in a spray-based crop protection paradigm.

    Tar Spot

    One real-world example of intermittent disease control from aerial applications (not just RPAS) is the case of tar spot in corn. Tar spot is a fungal disease caused by Phyllachora maydis and it is becoming a significant economic concern in Ontario. Left unchecked the disease causes rapid, premature leaf senescence. This reduces photosynthetic capacity, and ultimately, yield. Depending on spray timing, crop variety, environmental stressors, and the product applied, protection should last for up to three weeks.

    In the last few years there have been several reports (both in Ontario and in corn-producing US states) of tar spot “striping” following aerial sprays. Crops seem well protected directly beneath the flight path (green and healthy), but efficacy tapers to failure towards the edges of the swath (brown and desiccated). Fundamentally, this is likely due to inadequate spray coverage caused by an overestimation of the effective swath width.

    Figure 1 Tar spot striping in Western Illinois following two applications from a fixed wing sprayer (2023).
    Figure 2 Tar spot striping from RPAS volume trials. A brown strip can be seen between two passes in each RPAS treatment of 30 and 50 L/ha. The top is an application by a 100 foot horizontal boom. Each treatment is separated by an unsprayed check. (2023).
    Figure 3 – Tar spot striping in Ontario corn following fungicide application by helicopter (2024).

    Effective Swath Width (ESW)

    The measured swath width presents the lowest variability (as indicated by the coefficient of variability, CV) while minimizing the degree of over- and under-dosing. As a matter of operational productivity, wider swaths mean wider route spacing, which is attractive because it means fewer passes and faster applications. Once the agronomics are considered, the effective swath width is that portion of the swath that gives the desired biological result. It may equal, or only be a fraction of, the measured swath width. It is plausible that inappropriate effective swath widths from aerial applications are common, but have not always been detected, because:

    • Generally, fungicides are weakly systemic and give modest yield increases from disease suppression and their “stay green” properties. Until tar spot, a sub lethal dose of fungicide did not lead to rapid and acute crop failure.
    • Most growers do not intentionally leave unsprayed checks, or the check locations do not coincide with disease presence.
    • The applied product rate is sufficiently high to cover regions of under-application.
    • Taken together, deficiencies are often too subtle for passive detection.

    This is not to suggest that pilots intentionally inflate swath widths. Swaths are evaluated during fly-in calibration sessions using established protocols (e.g., Operation S.A.F.E.), and RPAS swath evaluation has emulated these practices. Calibrations take place on bare ground or stubble/grass using two-dimensional samplers (i.e., continuous samplers like string or bond paper, or discreet samplers like water sensitive paper). However, this protocol does not account for any physical interference from the crop canopy itself. This may have negative implications, particularly given the unique nature of the RPAS swath.

    RPAS tend to produce swaths with a very narrow span and a steep profile. To a certain extent, their swath widths share a direct relationship with altitude and headwind speed, and coarser sprays result in narrower swaths (with Dr. Michael Reinke, MSU). The outer edges of the RPAS swath represent the least amount of spray volume along the width, and this coincides with the turbulent dispersion zone of the downwash. Therefore, those extremes should contain a higher proportion of low-energy droplets moving in multiple directions relative the centre of the swath.

    While crop morphology and planting architecture are contributing factors (i.e. part of the agronomic use case), it is generally accepted that the degree of spray penetration falls off exponentially with canopy depth. It follows that this should also be the case for any lateral movement, resulting in a significantly shorter swath in-canopy versus on bare ground.

    Materials and Methods

    Spray Sampling

    Spray deposition was sampled using a 15.8 m (52 ft) Speed Track (Application Insight LLC) loaded with 3-inch bond paper (Staples Canada). The spray mix was 0.3% v/v FD&C Blue #1 Liquid. Bond papers were analyzed using a Swath Gobbler (2nd gen software – Application Insight LLC) at 100 mm sampling rate (i.e., ~150 discreet images per sample). Hue: 32-180. Saturation 17-60. Value: 156-255.

    The Swath Gobbler produces a complete, correlated and ordered record of the cross-section of a swath. For each discreet image, it reports the number of individual droplet stains on the sampler per area. It also reports percent area covered by measuring the total number of pixels with dye divided the total number of pixels in the image.

    The device deliberately does not calculate a Droplet Size Distribution (DSD) of the stains. This is because any DSD calculated from paper collectors relies on assumptions that cannot be validated, such as the fact that all droplets are captured and detected, spread factors are known for that application condition and similar for all stain sizes, there are no multiple hits, etc.

    RPAS

    The sprayer was a DJI T40, calibrated according to the pilot’s standard operating procedure (Drone Spray Canada). Certain operational settings varied with treatment and will be detailed later in this section.

    The flight path was perpendicular to the sampler, aligned with the centre using pin flags as references for the pilot. Spraying began approximately 20 m prior to the sampler to ensure the RPAS was at target speed and continued some 20 m past the sampler.

    Figure 4. DJI T40 approaching sampler on bare ground. Sampler was later moved into the adjacent wheat field (left).

    Defining Coverage

    Swath width will be calculated from two different coverage metrics.

    Percent Area Covered describes the amount of surface area covered by deposit. Given the variable degree of stain diameter (a function of sampler material, spray mix, and droplet velocity) this value can only be used as a relative index (i.e., can only be compared to itself). No conclusions can be drawn about how spray interacts with plant tissue, but generally more coverage correlates to improved crop protection.

    Deposit Density describes the number of individual droplet stains on the sampler per area. Higher densities can imply more uniform distribution over the plant surface, which is particularly important for contact materials.

    Previous studies (with Dr. Tom Wolf, Agrimetrix Research and Training, data not shown) indicate a higher correlation between deposit density and swath width at lower versus higher spray volumes. Lower volumes are typically comprised of finer droplets, which are more accurately resolved using deposit counts. Swath widths determined by deposit density also tend to be longer than those determined using percent coverage, better aligning with real-world observations of efficacy.

    Wheat

    R40 wheat was planted on October 9th, 2023, at 808,000 seeds/ha (2 million seeds/ac). Wheat height at the time of the trial was 60 cm (25 in). The location was 45180 Fruit Ridge Line, St. Thomas, Ontario. Deposition trials took place on May 23rd. Wheat stubble swath testing also took place at this location on May 15th.

    The RPAS was programmed to apply 50 L/ha using a 260 µm droplet diameter according to the DJI software. Air speed was 5 m/s and the flow rate was 11-12 L/min as it passed over the sampler. Swath was programmed at 8 m.

    Coverage was evaluated for water (control) and for a spray mix containing 0.15% v/v Interlock (a drift mitigating adjuvant – Winfield United) and 0.15% v/v Interlock + 0.125% v/v Activate Plus (a spreader adjuvant – Winfield United). For bare ground, each treatment had three passes (n=3) except for water, which had four (n=4).

    The wheat canopy was only sprayed with water three times (n=3). Limited passes were made because it served as a proof of principle. Any indication of relevant differences in the swath width would justify later trials in corn and soybean. These first passes revealed issues with the experimental design that were later corrected:

    • The RPAS spray tank level was not held constant. The RPAS weight affects the intensity of the downwash. The volume dropped from 30 L to ~20 L over the course of the experiment. In future trials, a tank volume of 20 L was maintained from a premixed source.
    • The wind direction occasionally shifted from a direct headwind to a partial cross wind from the RPAS’s right. In future experiments, we waited for an optimal wind direction before starting each pass.
    • The RPAS altitude was set to 3 m above bare ground. We assumed it would climb to account for the height of the wheat, but the canopy did not register with the RPAS sensors. As a result, spray was released ~60 cm closer to the wheat heads than to the ground in bare ground swathing. In future experiments, we confirmed that the RPAS was 3 m from the top of the crop canopy.
    • Despite best efforts, moving the sampler into the wheat parted and distorted the canopy. As a result, the sampler was not as obscured as it should have been. We developed strategies to minimize canopy distortion in corn and soybean that will be described later.
    Figure 5. Top-down view of sampler in wheat canopy. Note that the canopy did not close over the sampler as intended.

    Corn

    Corn was planted on May 15th, 2024, at 13,300 seeds/ha (33,000 seeds/ac). The sampler was erected in the field on July 3 to allow the canopy to grow up and around it. Deposition trials took place on July 26 and every effort was made to leave the canopy undisturbed around the sampler. Corn measured 2.4 m (9 ft) at the tassel and 1.2 m (4 ft) at the silks. The sampler height corresponded to the ears. The location was 42°40’52.1″N 81°04’45.9″W near 5277 Quaker Road, Sparta, Ontario.

    Figure 6 Sampler erected to 4 ft. Crop grew around the sampler to minimize any canopy disturbance.
    Figure 7 Sampler position relative to ears during sampling.
    Figure 8 Installing Speed Track for swath testing in wheat stubble.

    Soybean

    Soybean was planted on June 30th, 2024, at 80,800 seeds/ha (200,000 seeds/ac) on 38 cm (15 in) centres. Deposition trials took place the morning of August 14. While the densest area was selected for the trials, the field was patchy with crop height spanning 20-25 cm (8-14 in). Each section of the Speed Track was inserted under the canopy separately to avoid disturbing or damaging the plants. The track was elevated ~10 cm off the ground. The location was at 42°46’50.4″N 81°08’20.8″W near 43900 Talbot Line, Central Elgin, Ontario.

    Figure 9 Sampler in soybean.

    Corn and Soybean Treatments

    The following treatments were repeated three times in-canopy (n=3) (Table 1). The actual flow rate (recorded as the RPAS passed over the sampler) was always ~1.5 L/min less than programmed.

    Treatment #Droplet Diameter (µm)Programmed Swath (m)Volume (L/ha)Rate (L/min)Flight Speed (m/s)Spray Mix
    1320102010.510water
    232083010.58.3water
    332085010.55water
    43208305.75water
    550085010.55water
    63208505.750.5% Masterlock
    732083010.58.30.5% Masterlock
    Table 1 RPAS operational settings for corn and soybean treatments

    The following treatments were repeated three times on wheat stubble (n=3) (Table 2). Once again, the actual flow rate (recorded as the RPAS passed over the sampler) was always ~1.5 L/min less than programmed.

    Treatment #Droplet Diameter (µm)Programmed Swath (m)Volume (L/ha)Rate (L/min)Flight Speed (m/s)Spray Mix
    1320102010.510water
    232083010.58.3water
    332085010.55water
    43208305.75water
    Table 2 RPAS operational settings for wheat stubble treatments

    Weather Data

    The RPAS flight path was into the prevailing wind, but minor variations occurred throughout sampling. Weather was recorded as the RPAS passed over the sampler using a Kestrel 3550AG weather meter in a vane mount positioned on a tripod 2 m above ground (Table 3).

    TerrainWind Speed (km/h)Direction Relative to Flight PathTemperature (°C)Cloud Cover (%)RH (%)
    Bare Ground3-5Headwind +/- 25° from starboard20-21060
    Wheat Canopy5-7Headwind +/- 25° from starboard21-22060
    Corn Canopy2-4Headwind +/- 15° from starboard23-26<1075
    Wheat Stubble4-7Headwind +/- 15° from starboard26-28<1065
    Soybean3-4Headwind +/- 15° from starboard22055
    Table 3 Average weather conditions during trials.

    Results

    Raw Coverage Expressed as Percent Coverage or Deposit Density

    Coverage can be presented as raw data plotted by swath position. This is a qualitative means for assessing the swath. The bare ground data has been presented (using both coverage metrics) as an example (Figures 10 and 11).

    Figure 10 Swath coverage data for water on bare ground expressed as percent area covered. All four passes are plotted.
    Figure 11 Swath coverage data for water on bare ground expressed as deposit density. All four passes are plotted.

    Repetitions were similar enough to imply that environmental conditions were consistent during sampling. By averaging the repetitions, coverage in-canopy can be more easily compared to that on bare ground Figures 12 and 13).

    Figure 12 Average swath coverage data expressed as percent area covered. Bare ground (n=10). Wheat canopy (n=3).
    Figure 13 Average swath coverage data expressed as deposit density. Bare ground (n=10). Wheat canopy (n=3).

    The magnitude of coverage on bare ground exceeded that in-canopy, tapering to similitude and near-zero at the edges of the pattern. It can therefore be concluded that the entire swath was captured, and that spray was filtered by the canopy before reaching the sampler within.

    The difference between bare ground and the wheat canopy was greater when the data were presented as percent area versus deposit density. Differences in the number of deposits from finer sprays were more accurately resolved using deposit density than percent coverage. Since it can be expected that smaller droplets penetrate a canopy better than coarser droplets, it may be more appropriate to use deposit density to document their presence. We also saw indications of wider swaths when data were presented as deposit density, as well as a bimodal distribution that reflected the positions of the two rotary atomizers.

    While informative, this raw coverage format did not allow empirical comparisons. Each pass must be converted to a swath width.

    Converting to Swath Width

    Each pass was transformed by averaging Swath Gobbler data to a single value every 0.5 m. Data were then entered into the www.sprayers101.com swath width calculator and the SW was manually determined for each pass. Criteria was the lowest overdose, lowest underdose and lowest CV for an idealized threshold coverage of 90% that of the highest value in the swath. In the following histogram, the SW from all treatments have been averaged for ground and canopy terrains (Figure 14).

    There was a significant reduction in swath width in a wheat canopy compared to stubble or bare ground. There was a 41.2% reduction in swath width in a canopy when measured as percent area covered and a 26.6% reduction when expressed as deposit density. As previously stated, deposit density better reflects the contribution of finer deposits, which tend to penetrate deepest into crop canopies.

    Figure 14 Average effective swath width for all treatments on all terrains. Swaths expressed from both percent coverage and deposit density metrics. Standard error bars presented. Canopy (n=45). Ground (n=22).

    When the data is considered by terrain and by crop, we see that swathing on bare ground or in wheat stubble doesn’t have a significant impact. This justifies combining those data as “Ground” in subsequent analyses.

    Another observation that supports the use of deposit densities is the difference between the intended (i.e., programmed) swath width and the detected swath width on ground (Figure 15). The SW on ground was closer to the intended 8 or 10 m swath width when expressed as deposit density. It was approximately half the desired width when expressed as percent coverage, which is considerably less than common practice.

    Figure 15 Average effective swath width for each crop and terrain. Swaths expressed from both percent coverage and deposit density metrics. Standard error bars presented. Ground 8 m swath (n=19). Ground 10 m swath (n=3). Canopy 8 m swath (n=39). Canopy 10 m swath (n=6).

    Canopy Effect

    By percent area, corn had the biggest reduction in swath width compared to bare ground, then soybean, then wheat (Table 4 and Figure 16). This suggests the SW shares an inverse relationship with the canopy depth. However, the relationship reversed when SW was expressed as deposit density. The relationship between droplet size, crop physiology, planting architecture and canopy penetration is complicated, and no conclusions can be drawn beyond a reduction in SW in-canopy.

    Crop% Reduction in SW (% area)% Reduction in SW (deposits/cm2)
    Corn44.020.6
    Soybean32.228.3
    Wheat21.731.5
    Table 4 Reduction in average effective swath width in-canopy by crop compared to on ground. Swaths expressed from both percent coverage and deposit density metrics.
    Figure 16 Average effective swath width for each terrain. Swaths expressed from both percent coverage and deposit density metrics. Standard error bars presented. Bare ground (n=10). Wheat Stubble (n=12). Corn Canopy (n=21). Soybean Canopy (n=21). Wheat Canopy (n=3).

    Effect of Volume on SW

    The effect of spray volume on swath width is not immediately clear. When the data were expressed as deposit density, volume shared an inverse relationship with SW in canopy (Figure 17). There appeared to be no effect when expressed as percent coverage. The inverse relationship is weakly expressed, if at all, for both metrics on bare ground.

    Figure 17 Average effective swath width by volume and terrain. Swaths expressed from both percent coverage and deposit density metrics. Standard error bars presented. Canopy 20 L/ha (n=6). Canopy 30 L/ha (n=18). Canopy 50 L/ha (n=21). Ground 20 L/ha (n=6). Ground 30 L/ha (n=3). Ground 50 L/ha (n=13).

    Effect of Speed on SW

    For most RPAS designs, lower volumes are applied at higher flight speed (Table 5). Previous work demonstrated that higher flight speeds tended to result in wider swaths and an increase in drift. Do higher speeds cause wider swaths in-canopy, despite lower volumes?

    Volume Applied (L/ha)5 m/s Flight Speed8.3 m/s Flight Speed10 m/s Flight Speed
    20–3 treatments9 treatments
    309 treatments12 treatments
    5034 treatments––
    Table 5 – Number of treatments for each flight speed by volume applied.

    Flight speed had a clearer impact on swath width than spray volume did (Figure 18). There was a positive relationship between flight speed and swath width as measured by deposit density in canopy and on bare ground.

    Figure 18 Average effective swath width by speed and terrain. Swaths expressed from both percent coverage and deposit density metrics. Standard error bars presented. Canopy 5 m/s (n=27). Canopy 8.3 m/s (n=12). Canopy 10 m/s (n=6). Ground 5 m/s (n=16). Ground 8.3 m/s (n=3). Ground 10 m/s (n=3).

    Just as with volume, there appeared to be no significant effect on swath width in either canopy when expressed using percent coverage. This was likely because finer sprays were better able to penetrate a canopy and deposit density is better able to resolve their presence.

    Conclusions

    There was no difference in SW between stubble and bare ground. The SW on-ground was far closer to the programmed 8 or 10 m swath width when expressed as deposit density.

    There appears to be a significant reduction of SW in-canopy versus on-ground. A crop canopy created a 26.6% reduction when expressed as deposit density. Specifically, corn was -20.6%, soybean was -28.3%, and wheat was -31.5%. Previous work has demonstrated diminishing coverage with canopy depth in corn, but it is difficult to make comparisons between agronomic use cases (e.g. different planting architectures and plant physiologies).

    When the data were expressed as deposit density, spray volume shared an inverse relationship with SW in-canopy, but the effect on SW on-ground was less clear. However, RPAS speed had a clear inverse relationship with SW in-canopy and strong trend on-ground.

    It is understood that finer spray is better able to penetrate canopies. One reason is because finer droplets are able to become entrained the downwash. Another is simply mathematical advantage, given that finer sprays are comprised of exponentially higher numbers of droplets than coarser sprays, increasing the odds of deposition. Conversely, coarser droplets (which have the greatest influence on percent area covered), are more likely to impinge on the canopy structure before reaching the sampler. Deposit density appears to be the more accurate metric for calculating SW both on-ground and in-canopy.

    The reduced SW in-canopy versus on-ground explains, in part, why striping is occurring in aerial corn fungicide applications. The route spacing reflects on-ground swath width, where it should reflect the shorter, ESW.

  • Exploring Spray Drones in Soybean

    Exploring Spray Drones in Soybean

    White mould is caused by the fungus Sclerotinia sclerotiorum and it’s an annual threat to soybean when cool, wet conditions correspond with flowering. Variety selection (e.g. high tolerance) and cultural control (e.g. crop rotation and wider row width) are important management tools, but ultimately the application of a crop protection product between R1 and R2 is required for high-risk fields. (Learn more here).

    This article describes the results of an experiment exploring soybean canopy coverage and fungicide efficacy from a rotary spray drone. All work was performed under PMRA research authorization. There are currently no labels to apply crop protection products in Canada.

    Experimental design

    For the spray coverage trials, two locations were selected in southern Ontario (one south of Sparta and one west of Talbotville). This was a full field-scale trial with a single application made at R1.5 on July 18 (Sparta area) and July 22 (Talbotville area), 2023. There were two replications in each field and treatments were laid out parallel with the planting direction in a randomized design. Four other locations in Ontario and Quebec were also used in the larger efficacy/yield study. All locations had some level of white mould infection.

    1. Untreated Check
    2. *DJI Agras T30 – 20 L/ha (6.8 m/s, 2.5 m above canopy, TJ TT110015)
    3. DJI Agras T30 – 30 L/ha (5.7 m/s, 2.5 m above canopy, TJ TT110015)
    4. DJI Agras T30 – 50 L/ha (3.3 m/s, 2.5 m above canopy, TJ TT110015)
    5. New Holland 345 – 150 L/ha (TeeJet XR11006 nozzles on 50 cm spacing)
      *Not included in spray coverage trial

    We established an effective swath width of approximately 4 m (13.1 ft). The drone made three passes to cover the 12 m (40’)-wide treatment area, corresponding to the widths of the 9 m (30’) or 12 m (40’) headers later used to harvest in each field. Buffers were left on either side the treatment area. Fungicide was applied at label rate plus 0.125% Activate.

    Target placement and retrieval

    Soybeans were planted on 38 cm (15”) row spacing. The coverage sampling area was positioned in the middle of the treatment area. A length of rebar was positioned in-row and sheathed in PVC tubing. Two SpotOn brand water sensitive papers (WSP) from the same production run were secured face-up approximately 1/3 and 2/3 deep in the canopy. A block of six such samplers were positioned in a 3 x 2 grid (every third row and approximately 2 m apart in row). This block was then repeated 10 meters (33’) further into the block for a total 24 water sensitive papers per replicated treatment (see below).

    The papers were retrieved and temporarily placed on clipboards to dry before they were placed in paper bags for short term storage. They were digitized using a SprayX DropScope within 48 hours of retrieval on the “ground sprayer” setting, measured as percent surface covered (% area), and deposit density (# deposits/cm2).

    Weather during coverage trials

    Weather data was monitored using a Kestrel 3550AG weather meter (Kestrel Instruments) in a vane mount positioned 1.5 m (5 ft) above the ground. Wind speed fluctuated during the treatments, but wind direction remained relatively stable at 90 degrees to the flight path. The Sparta location averaged 6.4 km/h (4 mph) while the Talbotville location was considerably higher at 14.4 km/h (9 mph). Nevertheless, targets remained within the swath, despite any offset, as indicated by visual confirmation as well as the consistent coverage observed on the windward WSP compared to other, downwind samplers in each pass. Cloud cover was high at both locations.

    Results

    Coverage

    The coverage recorded from each WSP was averaged by canopy position (bottom 1/3 or top 1/3 of canopy) and presented in the following histograms with standard error. There were some spoiled collectors, primarily in the lowest canopy position, ruined by high humidity and physical contact with the plant. However, the lowest n for any treatment was 31 collectors and the highest was the full 48. Coverage is presented both as % area covered and as deposit density in counts per cm2.

    Efficacy and yield

    Three phytotoxicity ratings were performed 7, 14 and 21 days after treatment. White mould was rated at harvest and final crop yield reported in bu/ac.

    Observations and Considerations

    As expected, both water volume and canopy depth share direct relationships with percent-area covered (i.e. lower water and lower canopy depths mean lower coverage). Water volume also shares a direct relationship with deposit density for a given droplet size, but canopy depth is more complicated as smaller droplets tend to penetrate more deeply into canopies and low water volumes tend to produce smaller droplets. However, as a general observation, less water translates to less coverage no matter the metric for coverage, and this has been shown to reduce product efficacy.

    How, then, can we reconcile the claims of efficacy from low-volume drone applications? It’s typical that the % area covered from a 50 L/ha drone application is ¼ or less than that of “conventional” field drop systems which in North America tend to employ 150-200 L/ha. In speaking with Mark Ledebuhr (Application Insight LLC) about how low volumes could possibly be efficacious, he explained that in sugarcane production in Guatemala, the condensing humidity is likely the reason why their 1 gallon/acre applications are working. The droplet survivability, and the re-hydration and secondary movement of the deposits were a good thing.

    In the case of contact fungicides in North America, it may be humidity as well, but also the deposit density, combined with higher concentrations of active ingredient, that explain the similar efficacy and yields as seen here between the 50 L/ha (drone) treatment and the 150 L/ha (field sprayer) treatment. This would concentrate both the active ingredient (possibly increasing uptake rate, or residue persistence, depending on the product mode of action and the target’s physiology) as well as the adjuvant load (possibly improving sticking/spreading of deposits).

    Another consideration surrounds how deposit spread is analyzed. Water sensitive paper underestimates the spreading effect that can occur on plant surfaces (especially where surfactants are used). This is why WSP tends to be used as a relative index, meaning that papers should only be compared to other papers. Perhaps deposits are spreading more on the plant surfaces in the low-volume drone application (again, given the higher concentration of formulated adjuvants) than the water sensitive paper is indicating, and that is improving efficacy.

    This concept of how low-volume applications might affect coverage and subsequent efficacy, and the potentially positive impact of re-formulating products to include higher adjuvant loads, is well-described in this precis by Dr. Andrew Chapple and Malcolm Faers. Currently, accepting that the amount of control provided by the drone application falls short of that provided by a field sprayer, this study indicates that drones have the potential to produce acceptable results in fungicide applications if conditions are suitable, timing is optimal and water volumes are sufficiently high.

    This study was a collaborative effort with Bayer Canada and Drone Spray Canada.