This research was performed with Dennis Van Dyk (@Dennis_VanDyk), vegetable specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs.
Prior to 2017, Syngenta introduced the UK to the Defy 3D nozzle, which is a 100° flat fan, designed to run alternating 38° forward or backward along the boom. They prescribed a boom height of 50 to 75 cm, 30-40 psi, and travel speeds of 10 to 14 km/h in cereals and vegetables. Compared to a conventional flat fan, they claimed that the angle and Medium-Coarse droplets promise less drift and improved coverage.
In 2017, Hypro and John Deere began distributing the Defy 3D in North America. Our goal was to explore coverage from the 3D in vegetable crops. We compared the nozzle’s performance to common grower practices in onion, potato and carrot in the Holland Marsh area of Ontario.
Experiment
We used a technique called fluorimetry. A fluorescent dye (Rhodamine WT) was sprayed at 2 mL / L from a calibrated sprayer based on protocols generously provided by Dr. Tom Wolf.
Tissue samples from the top, middle and bottom of the canopy were collected from random plants.
The samples were rinsed with a volume of dH2O and this rinsate was then tested to determine how much dye was recovered.
The tissues collected were dried and weighed to normalize the samples to µL of dye per gram dry weight to allow for comparison.
In addition, we used water-sensitive paper as a check in key locations in the canopy to provide laminar and panoramic coverage. Papers were digitized and coverage determined as a percentage of the surface covered.
In carrot and onion, we compared a hollowcone, an air-induction flatfan, and alternating 03 3D’s at 500 L/ha (~40 cm boom height, ~3 km/h travel speed, ~27ºC, 3-9 km/h crosswind, ~65% RH).
In potato we compared the alternating 05 3D’s to a hollowcone at 200 L/ha (~55 cm boom height, ~10.5 km/h travel speed, ~22ºC, 6-8 km/h crosswind, ~65% RH).
Water-sensitive papers were originally intended as a coverage check, and not as a source of analysis, but their use revealed interesting information. The following images are the papers recovered a single pass in each crop.
Carrot
Onion
Potato
Results
The following table represents the percent coverage of these paper targets. Papers were digitized using a WordCard Pro business card scanner and analysis made using DepositScan software. This table is small, but you can zoom in for a quick comparison. The following three histograms show the same data graphically for carrot, onion and potato, respectively. Remember, this only represents a single pass, so don’t draw any conclusions about coverage yet.
Carrot
Onion
Potato
It was interesting to note differences in coverage observed on the papers versus the results of the fluorimetric analysis. It was anticipated that while water-sensitive paper serves for rough approximation of deposition, fluorimetry would be far more accurate. This is because of the droplet spread on the paper, and the evaporation and concentration of a spray droplet en route to the target. Again, here is a small table, and again, the next three histograms show the same data graphically for carrot, onion and potato, respectively.
Carrot
Onion
Potato
Observations
While water-sensitive paper is an excellent diagnostic tool for coverage, fluorimetry allows for greater resolution. The high variability in coverage meant little or no statistical significance, however the means suggested the following:
In carrot, the 3D deposited more spray at the top of the canopy.
In onion, the hollowcone spray had a higher average deposit, and penetrated more deeply into the canopy.
In potato, the hollowcone deposited more spray at the top, with little or no difference mid-canopy.
Each nozzle performed well at the top of the canopy, which is quite easy to hit. Certainly they exceeded any threshold for pest control. With the possible exception of hollowcone in onion, nozzle choice had only minor impact on mid-bottom canopy coverage. And so, if coverage is not a factor for distinguishing between these nozzles, we should consider drift potential. Due to the comparably smaller droplet spray quality, the hollowcone is far more prone to off target movement. This leads us to select the AI flat fan or the 3D as the more drift-conscious alternatives.
Future analysis would benefit from a larger sample size to reduce variability, and the inclusion of an air-assist boom to better direct spray into the canopy.
Applitech Canada (Hypro / SHURflo) is gratefully acknowledged for the 3D nozzles. Thanks to Kevin D Vander Kooi (U of G Muck Crops Station) and Paul Lynch (Producer). Assistance from Will Short, Brittany Lacasse and Laura Riches is gratefully acknowledged. Research made possible through funding from Horticultural Crops Ontario.
In Part One of this article, we showed that approximately 40% of minor use label expansions and registrant submissions rely on data from hand booms and guns. We also showed that a hydraulic backpack or knapsack will not give the same coverage as an airblast sprayer, and we concluded by suggesting that small plot researchers use spray equipment that reflects grower practices.
Unfortunately, practical logistics prevent most researchers from using a full-size airblast sprayer. They may not have access to such a sprayer, and if they do, it takes considerable time to mix and clean between treatments. Further, treatments are often only a single row, or even a single plant. It takes too much pesticide, too much time, and too much plot space to justify using a full-sized airblast sprayer, even if the relevance of the results are questionable.
Would another method of application better emulate an airblast application but retain the convenience of a hand boom or gun?
The motorized backpack mistblower
Using the same methods used to compare airblast to hand boom spray coverage in the previous article, we compared airblast sprayer coverage to that of a motorized backpack mistblower in grape, raspberry and peach (July, 2013). Once again, coverage was analyzed as overall percent coverage (see first graph) and droplet density (average droplets per square centimeter – see second graph).
Comparison of average % coverage in peach, raspberry and grape using a mistblower and air blast sprayer emitting he same volumeComparison of droplets per square centimetre in peach, raspberry and grape using a mistblower and air blast sprayer emitting the same volume
Results and Observations
The mistblower met, or in the case of droplet density, exceeded the coverage obtained using an air blast sprayer in most crops. The results led to a few observations:
The significantly-higher droplet density is a function of the Finer spray quality produced by the mistblower (see water sensitive papers below). This may still represent a confound between small plot work and large scale airblast applications.
Drift between proximal treatments may be an issue given how far the mist was blown. This should be considered when planning plots.
While not shown here, spray coverage was more consistent throughout each canopy, of each crop, when using the mistblower. This is likely because the operator was able to aim the output as they swept the spray over the canopy, thereby ensuring all surfaces were hit from multiple angles.
While we always try to be brand-neutral, it should be noted that we’ve used multiple Solo mistblowers over the years, and all of them required significant maintenance (no matter how they were cleaned and stored). It was very difficult to find brand parts and repair expertise in Ontario. The Stihl brand currently has far more dealers, and more accessible parts, and has not caused us any difficulties (yet).
Always use the highest grade gasoline in two-stroke engines to avoid ethanol gumming up the carburetors!
Always calibrate mistblowers by volume because raising and lowering the boom will affect the flow rate.
Conclusion
Hand booms, and likely hand guns, are not appropriate for testing agrichemical products intended for use with an airblast sprayer. Data derived from these methods should be questioned. An airblast sprayer is the best choice for any such research, but a mistblower is a viable alternative. Transparent, standardized operating protocols for testing products intended for use in airblast sprayers should be required.
Thanks to Vaughan Agricultural Research Services Ltd. for their assistance in the research performed for this article.
Peer-reviewed journal publications claim there is a significant difference in spray coverage and deposition patterns when an agrichemical product is applied using an airblast sprayer versus a hydraulic hand boom. An airblast sprayer creates Fine droplets that shear in entraining air and are carried into a plant canopy. Properly calibrated, the air opens the canopy to expose all target surfaces to the spray. By comparison, a hand boom relies on pressure to propel fine droplets into a canopy, and while there is some air-entrainment surrounding the spray, it cannot travel as far or displace as much canopy. As a result, most of it impacts on the outer surfaces of the canopy.
Knowing this, it is surprising that so many products intended for use with airblast sprayers are applied by researchers and consultants using hand booms or the high-pressure arborist-style handgun (see ‘Survey of Submissions’).
Survey of Submissions This graph represents a random selection of 150 minor use label expansion studies and registrant submissions from Canada and the USA spanning 1990 to 2011. It shows the application method by crop.
In 2012, we performed some research with the following goals:
To demonstrate the difference between spray deposition and coverage when using a hand boom versus an airblast sprayer.
To create a sound basis for questioning and potentially improving how agrichemical products for orchard, bush, and vine are tested in Canada.
Using water-sensitive paper to diagnose spray coverage, airblast sprayer application was compared to hand boom application in highbush blueberry, apple and grape.
Target locations in highbush blueberry.Target locations in apple.Target locations in grape panel.
Sprayers were calibrated to emit the same volume per planted area via hollow-cone nozzles. Volumes selected were based on typical application volumes for Pristine or Captan (commonly sprayed in Canada). While there is no standardized protocol (and there should be) we followed typical practices of 500L/ha for grapes, blueberry and apples until plant growth warrants higher carrier volumes. At that point, many researchers go up to 1,000 L/ha. Coverage was quantified by collecting and digitally scanning water-sensitive papers to calculate overall percent coverage (see graph) and droplet density (average droplets per square centimeter – see graph).
Overall percent coverageDroplet density
Conclusion
In all cases, airblast applications deposit > %50 more spray than a hand boom. In the case of grape, you’ll note there are three bars. This is because spraying 1,000 L/ha with the airblast sprayer drenched the targets (it was late in the season and the canopy was sparse), making it impossible to discern droplet density. When we reduced the output to 375 L/ha, we were able to register droplet density, which was still significantly higher than that produced by the hand boom at 1,000 L/ha. This raises significant questions about the validity of efficacy and residue studies performed with hand booms when growers apply the same products using airblast sprayers.
When this data was shared at extension conferences, it was sometimes noted that many researchers choose to spray the target until it is drenched, ensuring the dose administered to the crop reflects what was intended. This does not, however, invalidate the fact that a growers spray equipment and practices are significantly different, and the dose and spray distribution they achieve will not reflect the original research.
The recommendation is that researchers use the same equipment to test products as the growers use to apply them. But, recognizing the difficulties associated with performing small plot experiments with full-sized airblast sprayers, an alternative is needed. That topic will be addressed in part two of this article.
Horticultural Crops Ontario, the grower co-operators and former OMAFRA summer student Carly Decker are gratefully acknowledged for making this research possible.
Managing the canopy of any perennial crop (e.g. pruning, hedging, leaf stripping, etc.) is an important consideration. The benefits are manifold: It affects the health of the plant, the quantity and the quality of the yield. It allows light and air to circulate and it keeps the crop manageable. From the perspective of an airblast sprayer operator, the reason for canopy management is quite simple:
If you can’t see it, odds are you can’t spray it.
Picture this: It’s late April, and an apple grower and I are calibrating his sprayer. We achieve excellent spray coverage in the target block, shake hands and part ways. In late May I get a phone call from the grower. I assume it’s time to adjust his settings to match the growing canopy, but no… he had called to say he suspected apple scab in one of his blocks. Since I was the last person to adjust his sprayer, the unspoken implication was that I’d better come fix matters.
As I drove back out to his orchard, I considered what the problem might be:
Bad product choice?
Poor application timing?
Spraying in inclement weather?
Cutting rates?
Resistance? (a long shot)
Maybe it was ego, but I couldn’t believe it would be the calibration. We left ample volume to provide sufficient coverage to get the grower to petal fall. We ensured the spray swath was higher than top of the tallest tree, accounting for wind and an uneven alley. We did everything right to match the sprayer to the canopy and leave enough buffer to get to petal fall.
When I arrived, he took me to a block I hadn’t seen before. We didn’t calibrate the sprayer to match this particular group of trees, but he figured since they were about the same height, the sprayer would do its job. It was immediately obvious to me what the problem was, but I knew if I simply told him outright, the lesson might not stick. And so, with respect to that old proverb, I taught him to fish rather than give him one. We spent the next few hours trying to fix our alleged calibration problem by exploring:
Slower ground speed
Higher fan gear
Higher rpms to increase fan speed
Changes to deflector settings
Air induction nozzles in top positions
Higher sprayer output
Of course, none of these adjustments had any great impact on coverage because the problem was that the alley had grown so tight that branches were brushing the cab of the tractor (see picture).
If the canopy is brushing against the tractor, it may intercept spray before it expands fully. Essentially, it temporarily blocks nozzles.Closed canopies and tight alleys will almost always compromise spray coverage.
The canopy was so dense you couldn’t see the trunk! I asked the grower to move the sprayer down the row to a tree I saw that was far less dense that the others. We returned the sprayer to our original calibration settings and achieved excellent coverage once again. The only solution was to prune the trees, and once his workers did this, coverage improved considerably. An airblast sprayer can only do so much. Sometimes it comes down to canopy management.
An orchardist taught me this trick: If you want to know if spray will penetrate a canopy, you should be able to see the trunk.An orchardist taught me this trick: If you want to know if spray will penetrate a canopy, you should be able to see sunlight through the shadow at high noon.
Row Spacing in Specialty Crops
Canopy management isn’t just an orchard issue. For high bush blueberry crops, coverage problems may stem from insufficient pruning. How can spray reach the lower, inner portion of a mature bush to control spotted-wing drosophila if the canopy is too thick?
Sometimes it’s not the canopy, but the plant and/or row spacing. Many nurseries arrange container crops, shrubs, whips and cedars as tightly as possible. This may optimize how many plants will fit on a given area, but it compromises sprayer access (due to the reduced number of alleys) and may cause plants to block one another from the spray. Nursery sprayer operators often use cannon sprayers to throw spray over and through all those rows of plants, but cannon sprayers produce excessive coverage at the beginning of the swath and increasingly erratic coverage as a function of distance.
A cannon sprayer attempting five rows of cedars. This sprayer will eventually spray in from the other side, but experience has shown that coverage will be compromised in the centre rows and excessive in the outer rows. Spraying multiple rows may save time, but coverage is almost always erratic.Calibrating a cannon sprayer can greatly improve coverage consistency. Before calibration (above) the sprayer was equipped with full cone nozzles in the upper boom positions and excessive air was employed in an attempt to force spray through the canopy. Although the sprayer would eventually pass down the far side of the five rows, only the water-sensitive papers in the tops of the trees indicated suitable coverage, and a great deal of spray simply blew away. After calibration (below) considerably less air and spray was used, and coverage on water-sensitive papers placed lower in the trees and facing the sprayer was more consistent. Remember, the sprayer would eventually pass down the far side, resulting in similar coverage on the remaining papers. Don’t bite off more than your cannon sprayer can chew – the further spray travels from the sprayer, the harder it is to achieve consistent coverage.
Coverage can be improved by reducing the distance the spray has to travel (i.e. leaving more alleys and reducing the density of planted rows).
A Jacto cannon sprayer in a nursery. Many nursery and berry operations elect to spray multiple rows in one pass, but be aware that spray coverage suffers the farther away from the sprayer it goes. Independent research has shown that coverage is not reliable at half the distance typically claimed by many cannon sprayer manufacturers. This is a function of canopy density and weather. Always confirm coverage with water-sensitive paper. Photo Credit – M. Lanthier, British Columbia.The results of a cannon sprayer calibration in a container crop nursery. The cannon sprayed 1,000 L/ha and tried to cover too many rows in a pass. The water-sensitive paper showed insufficient and inconsistent coverage. When it was recalibrated to spray 550 L/ha, but drive more rows, the water-sensitive paper showed considerable improvement.
I also suspect that staggering plant spacing from row to row to reduce mutual shading might allow spray to penetrate more easily. As I write this, we’re planning to explore this concept in cedars.
This is speculative, but the when nursery shrubs, trees and container crops are planted in perfect grids, mutual shading probably prevents spray from penetrating deeply into the planting. By staggering the spacing, spray may be able to penetrate more easily between rows. This can be accomplished without reducing the number of plants per hectare significantly.
In the end, try to see the spray target from the droplet’s point of view. If you can easily see where you want the spray to go, you’ll do well. If you can’t see the target, it’s far more challenging.
Note: While there’s nothing wrong with this article, a more recent article on this subject can be found here.
It’s nearing the end of a long morning of spraying and you just want to get it done. As the tank empties and you watch the last of the spray cloud waft through the row, you’re thinking about rinsing out and moving on… but did the spray land where you wanted?
How do you really know if you hit the target?
Maybe you’re content with the occasional “shoulder checks” you made from the cab while spraying. Perhaps you stop at the end of the row and get out of the tractor to look for wet foliage during. Maybe you plan to return once the product is dry and look for white residue.
This early morning “shoulder check” was photographed by the operator using his smartphone. You can’t see coverage, but gaps in the spray will show if nozzles are plugged. You can also check to see if you are overshooting or blowing through the target. Photo Credit – C. Hedges, ON.
These are all good feedback practices, but a more accurate method is the use of water-sensitive paper, which turns from yellow to blue wherever spray touches it. You can easily see the distribution of the spray and the overall area covered, and it can be quantified so you can compare one sprayer set-up to another, or see the impact of weather, or even the effects of nozzle choice, pressure and water volume.
Water- (and oil-) sensitive paper: Cheap, simple and available on-line or in person from your favourite sprayer equipment store.
Draw a map
Begin by creating a simple drawing of the tree, cane, bush,vine, etc. you wish to spray. Label the drawing with unique numbers that correspond to where you are going to place the papers. Write the numbers on the back of each paper so you can see where they came from after they are collected. You should also note the pass number, so you can differentiate between each sprayer setup and corresponding pass. You might make a change and want to see how it affects coverage, and it’s very easy to mix up the papers if you haven’t record everything clearly. Plan to do this for at least two plants upwind from the sprayer to ensure you will get an accurate representation of average coverage. Be sure to wear disposable gloves and avoid dew so the papers don’t react prematurely.
Create a simple drawing of the target. Number positions on the drawing that correspond to where you plan to place the papers.
Distribute the papers
It is critical to distribute the papers evenly throughout each target canopy. They should be placed in key locations where pest damage has been an issue in the past (e.g. scab at the top of a tree, or spotted-wing drosophila at the bottom-centre of highbush blueberry), or anywhere coverage is notoriously difficult. Our preference is to place them at the top, centre and bottom of a tree canopy as well as laterally from the outer edge of the canopy beside the sprayer moving in towards the trunk.
Number positions on the drawing that correspond to where you plan to place the papers. Label the papers as well so you know where they came from. Consider writing the pass number and the position (e.g. 1-1 would be Pass 1, Position 1) so you can evaluate the changes to the sprayer settings from pass to pass. Later, all the information from the calibration can be entered into your spray records, like in this mock-up.
We use spring-back paper clips attached to alligator clips at 90 degrees to attach the papers to small branches. You can also staple them to the upper or lower face of the leaves (as long as they don’t cause leaf to droop). You can wrap them around stems for panoramic coverage or to monitor drenches. They can be stapled the trunk to show if spray is aimed into the canopy or being wasted. You can even skewer to the ground using wire flags to to illustrate poor lower-nozzle positioning and/or canopy run-off. Put them wherever you want to know about spray coverage!
This home-made double-ended alligator clip holds papers at right angles. One end for the paper, the other end to a twig or wire flag.
We typically orient them facing the alleys so their sensitive faces are square to the sprayer as it passes. We often use two in each location, oriented back-to-back facing each alley so you can resolve coverage from both sides. The important part is to ensure you are consistent. Mark the location in the canopy with some colourful flagging tape so you can find the papers after you spray, and if you wish to replace them with fresh papers to evaluate another pass, orient them the same way to make the comparison fair.
Water-sensitive papers located in five positions in an Empire apple tree. Two papers were pinned back-to-back in each position, distributed evenly throughout the canopy, facing the alleys. One paper was located at the lowest branch to determine if the lowest nozzle position needed to be on. Another paper was pinned to the ground face-up under the tree to show any excessive waste. Be consistent from pass to pass.
Spray, check and spray again
Once the papers are in place, pass by on one side with both booms open (as you would normally spray). Be sure to start spraying well before passing the target, and keep spraying afterwards to ensure the resultant coverage represents an actual application. It is very informative to get out of the cab and examine the papers before passing by on the other side. You can learn a lot about how the wind is affecting the spay.
Once papers are in place, pass by spraying with both booms open to emulate a typical spray day. Be sure to start spraying well before passing the target, and keep spraying afterwards to ensure the resultant coverage represents an actual application. It can be very informative to examine coverage at this point to see how wind is affecting the spray. Then, pass by on the other side to complete the application.An example of the coverage obtained on water-sensitive papers placed throughout an apple tree canopy, and on the ground beneath it.
Interpret the patterns
You might notice the outer portions of larger canopies receive more spray than the inside. This is hardly surprising given that spray must pass through the outside to get to the inside. As a result, inner papers often receive proportionally less spray and should be the basis for determining if you have sufficient spray coverage. This is also why the label recommendation of “spraying to the point of runoff” is unhelpful: the outer portion of wide, dense canopies often begin to drip before the inner portion receives sufficient coverage. Further, how do you spray to the point of runoff? How do you know when to stop before it’s too late? Label language can be frustrating…
When water-sensitive paper is sprayed to the point of run-off, the blue dye will drip. This is fine for a drench (dilute) application, but excessive for a typical concentrated application like foliar fungicides and insecticides.
When assessing coverage, don’t follow the droplet counts in the small guide that comes with the paper sensitive paper kit – they haven’t been updated for a very long time and are more appropriate for field crop applications – not airblast applications. Research and experience suggest that 85 discrete fine/medium-sized droplets per square centimetre and a total coverage of 10-15% should be sufficient for most foliar insecticides and fungicides. Remember, this is only a suggested threshold and in the case of coarser sprays, focus more on even distribution and the 10-15% coverage.
It’s debatable, but 85 Fine/Medium-sized drops per square centimetre and about 15% total surface covered on a minimum 80% of all papers represents adequate airblast spray coverage for most foliar applications. It is less applicable for applications made with Coarse/Very Coarse droplets, because there are fewer of them and they generally cover more area. In this case, focus more on the even distribution of spray and the 15% coverage. An extreme example of this is a drench (dilute) application of oil where total saturation is the goal. Conversely, ultra-low volume applications employ Very Fine droplets and a better metric is uniform, high droplet density rather than area covered.
Make a change and try again
There’s no easy way to define a threshold between sufficient and insufficient spray coverage. When you retrieve and examine the papers, think about how the product is intended to work: “Is it a contact, trans-laminar or locally systemic pesticide? What are the odds that an insect or spore will come in contact with residue? Will I be spraying again soon (e.g. fungicide) and will the spray already on the leaves have residual activity?” Regarding that last thought, protectant fungicide applications are often layered, so what one spray misses, the next will catch. Quite often, “sufficient coverage” is less than most sprayer operators think.
If you are content with the coverage, record your sprayer settings to use them again in that block (in similar weather, and assuming the crop canopy doesn’t change significantly before the next spray day). If you are not content, make a change to the sprayer to improve matters, reset the papers, and go again. It can take time and some effort to get it right, but improved coverage and reduced waste are ample financial reward for your efforts.
Other methods of evaluating coverage
It should be noted that while water-sensitive paper is versatile, cheap and easy to use, it has its shortcomings. Placement and orientation of the paper is very important; it’s easy to hit papers on the outside of the canopy with the sensitive-side facing the sprayer. It’s considerably harder when they are at the very centre of the canopy, or hiding behind fruit. When the thin edge of the paper is oriented to the spray (i.e. oriented facing the ground), it presents very little surface and can be difficult to hit.
Use enough air to only just ruffle the leaves. This exposes all surfaces, however briefly, to the spray. Too much air will align leaves with the spray, exposing only their thin edge and making coverage difficult. Too much air may also cause leaves to shingle (overlap), and create shadows like on the grape leaves shown here.
Further, the papers won’t show the finest droplets (<50 µm), so there may be spray even though you can’t see it. Taken collectively with the product’s mode of action (i.e. contact or locally systemic), and any possible re-distribution by rain or dew, spray coverage becomes a good indicator for protection, but it isn’t definitive. While coverage is a good indicator, improved coverage does not always mean improved efficacy.
Some sprayer operators use other methods to confirm their coverage. Kaolin clay is an inert compound that leaves white residue when dry. Red, yellow or green water-soluble, food-grade dyes will also indicate coverage. Even fluorescent dyes such as phosphorus can be sprayed at night and illuminated under black lights.
Kaolin clay and fluorescent dies sprayed into fruit canopies give a lot of information about sprayer coverage, but are relatively inconvenient compared to water-sensitive paper.Red food-grade dye sprayed from a horizontal boom to demonstrate downwind drift onto a white target. This was a messy experiment and my hands, and the sprayer, were pink for a long time afterwards. Photo Credit – J. McDougall, Ontario.
Take home
These methods give the sprayer operator a lot of information because they land on the actual target, not a piece of paper hung in the canopy. But, they require a lot of time and effort and are typically out of reach for most operators. Further, they do not allow multiple applications on the same canopy to compare the effect of sprayer settings on coverage – once the target is sprayed, it’s sprayed.
No matter which method you choose to use, understanding how changes to you sprayer, or the impact of weather, affect coverage is a critical piece of information. Operators should make an effort to evaluate spray coverage. Here are a few videos describing the process:
Using water-sensitive paper for airblast coverage diagnostics – thanks to Penn State, Univ. New Hampshire and Chazzbo Media (2014).
Checking water-sensitive paper in an orchard. Tower is spraying only water during a calibration run (2013).
