Category: Coverage

All hort articles about achieving, confirming or correcting for spray coverage.

Spray Coverage in Field Tomato

Spraying field tomato is difficult – period.

In Ontario, early variety tomato canopies get very dense in July. The inner canopy is relatively still, humid, cool and a perfect environment for diseases such as late blight. It is challenging to deliver fungicides to the inner canopy and this can lead to inadequate disease control. Matters are slightly improved as the fruit grows and pulls the canopy open, and staked tomatoes might allow for the use of directed sprays, such as drop arms in staked peppers. But, there’s no getting around it – from a droplet’s perspective, it’s tough to get through the outer canopy.

DSCF0002 — Imagine you are a spray droplet trying to get inside this canopy.

Study 1 – Qualitative Observations

In August, 2011 we worked in a market garden operation in Bolton comparing the spray coverage from four different nozzle configurations. We used the growers typical spray parameters: a travel speed of 4.5 km/h (2.8 mph), an operating pressure of about 4 bar (60 psi), a boom height of 45 cm (18 in) above the ground, and a sprayer output of 550 L/ha (~60 gpa). To monitor spray coverage, water sensitive paper was placed face-up in the middle of the tomato canopy. This diagnostic tool turns from yellow to blue when contacted by spray.

Water-sensitive paper at top of tomato canopy - easy to hit. — Water-sensitive paper at top of tomato canopy – easy to hit.

This particular sprayer was equipped with an air assist sleeve that blew a curtain of air into the canopy at about 100 km/h (65 mph) as indicated by an air speed monitor placed at the air outlet. When properly adjusted, air-assist booms have a number of benefits:

They part the outer canopy giving spray access to the inner canopy.
They rustle leaves to expose all surfaces to spray.
They permit the use of smaller droplets, which are more numerous and adhere to vertical surfaces, by entraining them and reducing drift.
They extend the spray window by permitting the applicator to operate in slightly higher ambient wind speeds.

Boom sprayer with air assist sleeve operating.

We sprayed using the four different nozzle configurations, with and without air assist. Our goal was to make qualitative assessments (Good, Moderate, Poor), and here’s what we observed:

Nozzle Type / Sprayer Output	With Air Assist	Without Air Assist
80 degree flat fans /~550 L/ha (60 g/ac)	Good coverage in upper canopy Poor / Moderate canopy penetration Low drift	Good coverage in upper canopy Poor canopy penetration Moderate drift
80 degree air induction flat fans /~550 L/ha (60 g/ac)	Inconsistent upper canopy coverage Poor canopy penetration “No” drift	Inconsistent upper canopy coverage Poor canopy penetration “No”/Low drift
TwinJet dual 80 degree flat fans /~550 L/ha (60 g/ac)	Good coverage in upper canopy Poor / Moderate canopy penetration Moderate Drift	Good coverage in upper canopy Poor canopy penetration Moderate/High drift
Hollow cones /~750 L/ha (80 g/ac)	Good coverage in upper canopy Moderate canopy penetration Low drift	Good coverage in upper canopy Poor canopy penetration Very High drift

The air induction nozzles performed poorly. Their Coarse/Very Coarse droplets impacted on the outer canopy, created run-off and resulted in very little canopy penetration. Medium droplets produced by twin fans and conventional flat fans were both inconsistent with inner-canopy coverage, but some advantage may have been observed with air assist. The TwinJets contributed to higher drift (likely because they were too high off the canopy) but otherwise produced coverage similar to the conventional flat fans. From these observations, the convention that spray shape (e.g. cone, fan, twin) has little or no impact on broadleaf canopy penetration holds true.

Acceptable spray coverage deep in canopy (harder to hit) using hollow cone nozzles. — Acceptable spray coverage deep in canopy (harder to hit) using hollow cone nozzles and air assist.

After inspecting the papers deep in the canopy, we were surprised that air assist did not obviously improve canopy penetration. It did seem to help, but it wasn’t a slam-dunk. This may be because finer droplets (<50µm) are not easily seen on water sensitive paper. It might also be because we did not calibrate the air speed to the canopy: too little air and spray impacts on the outer canopy, while too much air forces leaves out of the way and spray is blown into the ground. It was obvious that drift was greatly reduced, so logically the spray had to have gone somewhere – we can only assume it entered the canopy.

The best results were achieved with hollow cones and air assist. Theoretically, smaller droplets should improve the potential for coverage by sheer number, but they slow quickly and are easily blown off course. Winds were only about 5 km/h (3 mph) during the trials. Had they been higher, the no-air-assist condition would have resulted in poorer canopy coverage. While we feel the air assist improved inner canopy coverage, we attribute much of the performance to the spray volume of 750 L/ha (80 gpa), which was significantly higher than we used with the other nozzles. When we attempted lower volumes using the hollow cones (not shown) the inner canopy coverage was greatly compromised. Higher volumes are a demonstrated means for improving canopy penetration, so this observation is consistent with what was expected.

The 2011 trial suggested that hollow cone tips used with high volume and air assist, improved canopy coverage and penetration. They are, however, very prone to drift and their use is not recommended without an air assist sleeve to counter the spray drift. Spray volumes over 500 L/ha are highly recommended.

Study 2 – Quantitative Observations

In July, 2016 we ran another study in Chatham-Kent. This operation was concerned about spray drift and recently changed from Hardi hollow cones on 25 cm (10″) centres to TeeJet Turbo TwinJets on 50 cm (20″) centres. They wanted to know if they had improved their coverage. We decided to test four nozzles at similar driving speeds and volumes.

Once again, we used water-sensitive paper. This time we placed two pieces back-to-back (face up and face down) about 1/3 down into the canopy. Then we placed two more in the same orientation about 2/3 down into the canopy. We did this for three plants for each pass. The next four images show the visual drift and weather conditions for each nozzle. Note that only one boom section was nozzled (indicated by a white line) in each condition.

Condition 1 – Turbo TwinJet (Coarse Spray Quality)

Condition 2 – Hollow Cones (10″ centres – Fine/Medium Spray Quality)

Condition 3 – XR 110° FlatFan (Fine Spray Quality)

Condition 4 – TeeJet 3070 (Coarse Spray Quality)

It was very humid, making it difficult to place and retrieve the papers without smearing them. This made it tricky to discern differences in coverage, and the blurring prevented us from quantifying droplet density (i.e. number of drops per unit area). Nevertheless, papers were scanned and the percent coverage was calculated using the DepositScan software developed by the USDA’s Dr. Heping Zhu. The average percent-coverage (± S.E. n=3) is shown in the image below.

Coverage on the upward-facing papers in the upper portion of the canopy showed excessive coverage for all nozzles but the 3070. Little or no coverage was detected on the downward-facing cards, but without air-assist or a directed application (e.g. drop arms), this was expected. It’s the deeper canopy that’s of particular interest. The only significant difference may lie in the XR flat fan which showed more coverage on the upward facing papers and some (however little) on the downward facing papers.

This came as something of a surprise given that the XR produced a Fine spray quality and there was no air assist to guide spray into the canopy. I believe the high humidity and low winds played a role in this outcome by reducing evaporation and off-target drift. On a drier, windier day, we likely would not have seen this level of inner canopy coverage for either the XR or the hollow cone. By comparison, the Turbo TwinJet with its Coarse spray quality not only reduces off target drift, but would be more resilient in drier and windier weather and may very well have produced the best coverage by comparison.

Take Home

Drawing from both studies:

Properly calibrated air assist will reduce drift and has promise to improve canopy penetration/coverage.
Spray shape (e.g. twin, hollow cone, flat fan) does not seem to play a role in canopy penetration.
Spray quality larger than Coarse may negatively impact canopy penetration in tomato.
Coarse spray quality is perhaps the most versatile option when volume is sufficient (>500 L/ha).
Fine-Medium spray quality is only a viable option in high humidity and light winds. However, air assist is critical to counter drift, and high spray volumes (>500 L/ha) are still required despite the higher droplet count.
Underleaf coverage is exceedingly difficult to achieve, even with finer spray quality and air assist.

This occurred in Ontario (date and location withheld). The sprayer missed the outer edge of the tomato field during a late blight application. An unintentional field check, and amazing to see the results.

August 4, 2022

Herbicides in Asparagus – A creative solution
In 2016, an asparagus grower in southern Ontario picked up a used De Cloet Hi-Boy originally used to spray tobacco. His vision was to create a three-row herbicide sprayer for asparagus and we were invited to participate. His concept was to design shrouds that would contain the herbicide, but not snag the asparagus or drag heavily on the ground. This article follows the development of the sprayer from concept to testing to final product.
The sprayer itself was a classic three-wheel, self-propelled affair. The asparagus was planted on four foot centres, leaving a three foot alley. While the goal was to hang three shrouds off the boom, we started with one to work out the bugs.
This operation uses 2,4-D to control weeds in the alleys and while a little can hit the asparagus stem up to 12 inches (where the branching starts), we wanted to avoid contact at all costs. That led us to the TeeJet AI 95° flat fan nozzle, which produces a Very Coarse to Extremely Coarse spray quality. A single nozzle could be suspended to span the 3 foot width of the alley.
The first version of the shroud was suspended off the boom from four anchorage points. A certain amount of of play was allowed so the shroud would find plumb (i.e. hang vertically), even when the sprayer boom yawed or pitched over uneven ground.
The shroud was constructed of sheet metal, angled to reduce the potential for contact with the asparagus branches, and terminated in stiff, nylon brush-style mud flaps commonly seen on trucks. These brushes were cut to a few inches in length to span the distance between the side of the shroud and the ground. This would create a “seal” to prevent spray from escaping, maintaining some degree of contact with uneven ground.
We tested the first version by placing water sensitive paper in two positions on the ground, just inside the reach of the brushes. We had to be careful not to run them over with the centre wheel of the sprayer. We also adhered two papers to the angled inner walls to see how much, if any, spray was hitting the inside of the shroud.
Our first pass on June 16th was at 9:00 am, 19.1 ºC (66.4 ºF) with a cross wind of 5 to 7 km/h (3.1 – 4.3 mph). relative humidity was high at 85% and travel speed was slow at 3.2 km/h (2 mph). We started with the .06 AI tip at 50 psi, but we drenched all the targets with excessive coverage because we were travelling so slow. We also found the stiff brushes were creating furrows in the soil, as shown below.
For our second pass, we tried the .04 tip and raised the shroud while dropping the tip to keep it suspended 15 inches over the ground. We were still drenching the targets and noticed the shroud was hitting the asparagus spears, causing physical damage. The damage is shown below – note the dark green on the bent spear.
This led to a decision to flare the side walls more aggressively, bringing them further into the centre of the alley and away from the spears (shown later in the article). This had the added benefit of angling the brushes as well to get a maximum span for weed control in the alley. For the final coverage pass we used the AI .03 tip, which gave more than 45% coverage on the ground, with even distribution, and there was no indication of spray on the papers adhered to the inside of the shroud. This coverage is more than is likely required, and the operator should be able to spray up to 6.5 km/h (4 mph) without compromising coverage.
Since the coverage tests, the grower added additional sheet metal fenders to the the existing fenders, encasing the wheels and creating a smooth transition for the shroud to gently deflect the asparagus. The fenders were needed because the grower found the asparagus was being pushed out by the wheel fender only to bounce back in front of the shroud, which snagged the fern and damaged it. The additional fenders keep the fern spread and prevent it getting caught in front of the shrouds.
The grower was very happy with the sprayer’s performance and planed to build another. Why be satisfied with the status quo when you can tap into your creative side and be innovative? If you don’t think you’re imaginative enough to try upgrading equipment on your farm, here’s a simple test to prove that it’s in you. It’s easy to see the bird in the image below, but with a little concentration you’ll be rewarded with a ski-jumping rabbit.
Thanks to TeeJet for donating the nozzles and water-sensitive paper and to Ray and Brad Vogel of Lingwood Farms for inviting me to participate.
Learn more about spraying asparagus here.
June 6, 2022
Drop Hoses Improve Coverage in Field Peppers
In early July 2016, a farm supplier contacted us on behalf of a client with a history of disease control issues in his field pepper operation. He wanted us to calibrate their sprayer and diagnose spray coverage to see if there was room for improvement. Improved coverage doesn’t necessarily mean improved efficacy, but generally it’s a reliable indicator. When we arrived at the field the winds were gusting over 15 km/h, which had the potential to create a massive drift issue. We were only spraying water, so it was decided that if we managed decent coverage in those conditions, there would be no need to worry on an acceptable spray day.
Field pepper in Southern Ontario in mid-July
The grower traditionally ran two different settings on his sprayer. They were relatively low volumes for a vegetable operation, but the crop was still small at this stage, so we did not propose raising the volume:
1. TeeJet AITX 11008’s on 50 cm (20″) centres at 11.25 kmh (7 mph) and 3.44 bar (50 psi). That’s 3.35 L/min (0.89 gpm) per nozzle for a total rate of 350 L/ha (37.5 gpa).
2. TeeJet ConeJet TXVK18’s on 50 cm (20″) centres at 7 kmh (4.5 mph) and 3.44 bar (80 psi). That’s 1.6 L/min (0.42 gpm) per nozzle for a total rate of 275 L/ha (29.5 gpa).
To test the coverage with these settings, we folded a piece of water-sensitive paper over a leaf to cover both surfaces, and wrapped one around a hollow tube to mimic a plant stem (see figure). Three plants were papered for each sprayer pass. Papers were collected, digitized and analysed for percent-coverage and droplet density. When diagnosing coverage for a horticultural crop, a distribution of 85 medium deposits/cm² and 10-15% coverage is a reasonable standard for most applications.
Location of water-sensitive papers in situ.
The first condition (the AITX tips) averaged 17% coverage on upper leaf surfaces (37 deposits/cm²). These were coarser droplets at relatively low volume, so it was no surprise that we didn’t achieve 85 deposit/cm² target. When using such large droplets, it is more important to achieve an even distribution and the 10-15% surface coverage (we achieved 17%). There were no deposits on the underside of the leaves (See figure 1), but that was also expected as coarser droplets tend to follow a downward vector that is not conductive to under-leaf coverage.
Figure 1 – Water-sensitive papers from three plants sprayed in Condition 1. Percent coverage and droplet density are calculated for the leaves, and a visual inspection is made of the stems.
The second condition (the ConeJets) provided better coverage. The fine droplets produced covered an average 17.5% coverage with a distribution of 99 deposits/cm² on upper surfaces, and 23% coverage with a distribution of 185 deposits/cm² on lower surfaces. Panoramic stem coverage was improved as well (see figure 2). This is excellent coverage, but the finer droplets were highly prone to drift (see below). With no form of drift control, this set up is undesirable.
Figure 2 – Water-sensitive papers from three plants sprayed in Condition 2. Percent coverage and droplet density are calculated for the leaves, and a visual inspection is made of the stems.
With no form of drift control, the finer droplets produced by hollow cones create unacceptable spray drift, even in moderate wind conditions.
This led us to propose a more directed boom arrangement: We set up a hollow cone over the row (the grower’s original ConeJet) and a drop hose suspended in each alley with two TeeJet XR 8004 flat fans positioned on an angle (i.e. not vertical or horizontal to ground). This gave sufficient height to span the canopy with as little direct waste on the ground as possible. As the crop grows, the nozzles would need to be twisted into a more vertical alignment.
ConeJet TXVK18’s alternating with drop hoses with TeeJet XR 8004’s.
We did not use an air induction fan to avoid the Very Coarse spray quality and we used 80° instead of 110° to ensure the spray did not overshoot or undershoot the plant. Here are the details of the third set up:
3. TeeJet ConeJet TXVK-18’s on 100 cm (40″) centres at 7 kmh (4.5 mph) and 3.44 bar (80 psi). That’s 1.6 L/min (0.42 gpm) per nozzle. Also, two TeeJet XR 8004’s per drop on 100 cm (40″) centres at 7 kmh (4.5 mph) and 3.44 bar (80 psi). That’s ~4.5 L/min (1.2 gpm) per drop hose. Together, set of nozzle for a total rate of 523 L/ha (56 gpa).
This set up raised the volume considerably and aimed spray directly at the sides of the plant. Coverage was excessive and in a few cases exceeded what the diagnostic software could reliably resolve (see figure 3). Since the plants were still small at this stage, it was decided we would let them “grow into the volume” and come back to check coverage once they were at full size.
Figure 3 – Water-sensitive papers from three plants sprayed in Condition 3. Percent coverage and droplet density are calculated for the leaves, and a visual inspection is made of the stems.
When we returned in mid-August the plants had reached full maturity. In this final coverage trial, we added a second water-sensitive paper to each plant to span the height of the crop canopy, which had grown considerably.
The same pepper plants ~5 weeks later had more than doubled in size.
Coverage was reduced compared to how we left things in July, but appeared to be sufficient on key surfaces (see figure 4). The papers showed upper leaf-surface coverage of 63%-to-offscale and deposit distribution of 137 deposits/cm²-to-offscale. Coverage on the lower leaf surfaces was greatly reduced to 4-4.5% and 36-90 deposits/cm². Panoramic stem coverage was present, but minimal. Applying higher volumes would likely have improved matters.
Figure 4 – Water-sensitive papers from three plants sprayed in Condition 3, ~5 weeks later. Percent coverage and deposit density are calculated for the leaves, and a visual inspection is made of the stems.
When asked about the drop hoses, the grower reported “They are a bit of a nuisance because they take extra time to put on, and they get caught in the bush at the back of the field. But if they increase our coverage, then they’re worth the extra effort.”
Final thoughts
Adding drop hoses to a vegetable sprayer may be unconventional, but if fungicide coverage is a concern, and the drops will fit between rows, they might be worth a try. Carefully consider the volumes you use because they should reflect the size of the plant canopy you are trying to protect. Finally, water-sensitive paper provides excellent feedback to help you decide if your field volume, nozzle rates and nozzle positions are providing acceptable coverage.
April 22, 2022
Pressure Changes Spray Angles
When we consult a nozzle catalogue we are interested in the flow and droplet sizes produced at a given pressure. Perhaps we should also consider the effect of pressure on spray angle. We have several articles discussing the collective impact of spray overlap, nozzle spacing and boom height on coverage uniformity (Check here and here for example). However, we don’t really address the fact that fan angle is not a constant. This may be more relevant with the growing adoption of spot sprayers.
To illustrate the potential for fan angle variation, we assembled a collection of red, flat fan nozzles (‘04s) from several manufacturers. We plugged each nozzle into a spray pattern table, set the regulator at a given pressure, and photographed the spray angle and flow distribution. This process was repeated for each nozzle at seven different pressures within the manufacturer’s approved range of 20-80 psi. After digitizing the photos, we measured the spray angle using a digital protractor.
We anticipated a concomitant increase in spray angle as the pressure increased. This is not news. Anyone who has operated a sprayer has seen the spray pattern open up as the boom fills and pressurizes. Bear in mind this was only performed once (i.e. n=1), so while it illustrates trends it shouldn’t be mistaken for a rigorous scientific comparison. Further, this demonstrates a static situation and not a dynamic one where travel speed, wind conditions and the vortices from the sprayer it self will influence matters.
We saw similar trends with nozzles other than 110˚ fans, but let’s focus on 110˚s due to their current popularity.
Fan angles for five common 110 degree AI flat fans over their manufacturer-recommended pressure range
The spray angle for 110˚ nozzles ranged from 75˚ at 20 psi to approximately 143˚ at 80 psi. One nozzle failed to reach 110˚ at any pressure. Conversely, there was another that was over 110˚ at nearly all pressures. Ideally, spray nozzles should be operated around the middle of their manufacturer-recommended operating range. Three of the nozzles tested came close to 110˚ at that median pressure, but only the TeeJet AIC110-04 measured 110˚ at the middle of its recommended range (~50 psi).
Using that nozzle as an example, let’s look at the pressure, spray angle and subsequent distribution of flow along the swath at three different pressures. At 20 psi, the spray angle was 85˚. The yellow balls are floats that reflect flow as a series of cross sections of the swath. We see that aside from the tapered edges (which illustrate the need for 100% overlap between neighbouring nozzles) the distribution was fairly even. One of the priorities in nozzle design is to ensure a low coefficient of variability over the operating pressure range. In other words, the length of the swath may change, but the spray quality and uniformity in that swath is still within spec. At 50 psi the nozzle produced the expected 110˚ fan, and the spray distribution remained even. At 80 psi, the angle spread out to 125˚, spanning a greater distance, but it started to produce a less-even distribution.
Photographs of spray angle and distribution for the TeeJet AIC110-04 at the extreme low, middle and highest pressures of its recommended pressure range.
When fan angle changes with pressure, it can have significant implications. Nozzle spacing on a boom varies from sprayer to sprayer. Generally 50 cm (20 inch) centres are the standard in North America, but we’ve seen 15″ and even 10″. Nozzle spacing and boom height collectively determine the degree of spray overlap. Excessive overlap isn’t a problem, although additional nozzles do mean added expense, cleaning time and potential for plugging. Conversely, gaps in the pattern could lead to sub-lethal applications or flat-out misses. For example, in this soybean demo plot (below) we sprayed a contact herbicide at low pressure to collapse the spray pattern. You can see the alternating stripes of hits and misses that resulted from an incomplete overlap of spray.
Soybean demo plot sprayed with a contact herbicide using 110˚ air induction flat fans at 20 psi. The collapsed spray pattern did not overlap sufficiently to burn the entire crop down, leaving a striped pattern and demonstrating the poor coverage.
Nozzle manufacturers generally recommend a 100% spray overlap for flat fans. This creates sufficient overlap when the boom sways low to the ground. It also increases the degree of droplet size homogeneity under the boom as coarser and fewer droplets are generally found at the “horns” or edges of the pattern compared to the centre. In order to ensure this degree of overlap, sprayer operators should observe and consider changes in fan angle over their typical pressure range. Otherwise, the cost of poor deposit uniformity under the boom could be high.
- Operate nozzles around the middle of the manufacturer-recommended pressure range. However, just because a nozzle is rated over a range of pressures does not mean the angle is constant.
- Lower pressures are a greater concern than higher pressures. 30 psi is the absolute lowest pressure for operating a 110˚ air induction flat fan; the ideal operating range for these nozzles is 50-70 psi.
- If nozzles are not maintaining the recommended 100% overlap at your preferred pressure range, then consider switching nozzle rates, and adjusting pressure and boom height.
This work was performed with Victoria Radaukas, 2015 OMAFRA application technology summer student.
February 1, 2022
Pesticide Redistribution: An Important Aspect of Synthetic Pesticides
If you’re a sprayer operator with some experience behind you, you may have applied mercury arsenate, nicotine, Paris green, or perhaps even DDT. All of these historical pesticides were effective, but they were also toxic to both the applicators and the environment. Fortunately, today’s agrochemical manufacturers produce pesticides that are effective while being far less hazardous.
One important aspect of modern synthetic pesticides that enhances their efficacy is their ability to redistribute. Pesticide redistribution is the movement of a pesticide from its initial point of deposition to a different spot on or in the plant. Pesticides that can redistribute can improve pest control compared to those that must contact the target pest but cannot innately redistribute. This is especially true when spraying hard-to-wet plant tissues, such as flower clusters or fruit. Even when the immediate coverage of these tissues is insufficient, the subsequent relocation beyond the initial spray deposit can result in a more effective protective barrier. When plants are rapidly growing, many of these products can translocate through the plant tissues to protect newly emerged tissue that did not receive a direct deposit.
Some of the most difficult and persistent pests are more effectively controlled by redistributing pesticides. Materials that move within the plant after application provide improved control of piercing-sucking insects such as aphids and psyllids, as well as pests that feed in difficult-to-spray areas such as under leaves. These products can absorb into plant tissue, increasing their resistance to wash-off by rain or irrigation.
Five Types of Pesticide Redistribution
There are five significant types of pesticide redistribution: translaminar, vapor, xylem, phloem and redistribution via precipitation
Translaminar Redistribution
Translaminar redistribution (Figure 1) in its most literal sense is a compound moving from the side of the leaf that received spray, to the unsprayed opposite side. This results in protection on both sides. However, translaminar redistribution also involves limited radial movement providing a “halo” of protection around the initial deposition. The extent of this area of influence is product-dependent.
Figure 1. Schematic of translaminar redistribution, with small round dots indicating deposition of pesticide, arrows indicating the direction of redistribution, and the shading indicating the area of the plant protected by the pesticide.
Vapor Redistribution
Vapor redistribution (Figure 2A) occurs when surface depositions volatilize and move laterally along a plant surface, re-adsorbing to the plant surface in new locations as they move. Again, the extent of vapor activity is product specific, but also condition specific requiring an optimal combination of temperature, relative humidity, wind, and solar radiation to facilitate volatilization. When pesticides are referred to as “locally systemic,” it often implies that they exhibit translaminar and/or vapor redistribution properties.
Figure 2. Pesticide redistribution type schematics (A) vapor, (B) xylem, (C) phloem. The small colored groups of dots indicate deposition of pesticide, while arrows indicate the direction of redistribution, with shading representing the area of the plant protected by the pesticide.
Xylem and Phloem Redistribution
Xylem redistribution (Figure 2B), also called xylem systemic, refers to the absorption of a pesticide and subsequent systemic movement of the pesticide through the xylem vessels of a plant. Xylem vessels move water and minerals in an upward and outward direction in plants. There is very little movement of water and nutrients downwards or backwards along branches or leaves in xylem vessels. Xylem redistribution can help protect growing tissues from damage by pests or diseases when the pesticide redistributes from the point of application to the newly developing tissues. Most systemic fungicides and insecticides redistribute via the xylem.
Phloem redistribution (Figure 2C), also called phloem systemic, is the bi-directional movement of pesticides in the phloem vessels of a plant. Phloem vessels transport sugars and other nutrients both to the roots of plants and upwards and outwards to shoots and fruits/seeds. Phloem systemic pesticides are sometimes called “true systemic,” because they can translocate throughout the entire plant.
Some pesticides that redistribute via the xylem or phloem can be applied to the soil substrate to be absorbed by the roots and redistributed throughout the plant. The process of plant nutrients or pesticides being transported from one place to another within the plant is called translocation.
Soil-Applied Systemics
Several factors affect a pesticide’s ability to redistribute. These factors affect the speed of uptake, the duration and extent of translocation, and the amount of accumulation in plant tissue relative to the initial dose. For pesticides labeled for soil application, their uptake by plant roots and redistribution via xylem or phloem can lead to long residual efficacy of the product; Up to eight weeks or more depending on the product, plant, and soil. This is in contrast to foliar-applied products, where good residual efficacy could be expected to last two to three weeks depending on the product. However, foliar-applied products tend to provide a more rapid kill of target pests and a more rapid absorption and translocation of active ingredients.
For soil-applied systemic pesticides, the composition of the soil substrate can affect the uptake of the pesticide by the plant. Growing media high in organic matter (>30% bark or peat moss) can bind pesticides, making it difficult for plants to absorb them through roots and subsequently translocate via the plants vascular system. Soil applications of systemic materials should take place one to six weeks prior to the onset of the insect pest or pathogen. This allows sufficient time for the pesticide to translocate to, and accumulate in, target tissues. The more water-soluble pesticides (e.g. Thiamethoxam) are taken up more rapidly than the less water-soluble pesticides (e.g. Imidacloprid).
Redistribution via Precipitation
In contrast to systemic pesticides, contact pesticides cannot redistribute on their own. However, rain or irrigation can spread the deposit to some degree, increasing coverage area. This effect should not be relied upon, as it depends on the product formulation, the intensity of the precipitation, and the interval following application. In the case of prolonged precipitation, the residual activity of contact products can be greatly reduced as they are diluted and washed off plant tissues.
Plant Morphology
The status of the plant to which they are being applied is a significant consideration when applying redistributing pesticides. Both soil-applied and foliar-applied pesticides are more rapidly absorbed and redistributed when applied to young plants or juvenile plant tissue. In general, when plants are actively growing, have a strong root system, or are actively transpiring, they tend to absorb and translocate pesticides more rapidly than when plants are growing slowly. In addition, plants with difficult to wet leaves or surfaces due to thick cuticles or waxy layers tend to not absorb pesticides as readily. Penetration into plants with difficult to wet surfaces can be improved by adding adjuvants such as surfactants to tank mixes.
Multiple Modes of Redistribution
The extent to which each product can redistribute can be thought of as a continuum. Generally, when a product exhibits some form of redistribution, it can also redistribute via a different method. A good example of this is xylem and translaminar redistribution. When a product can redistribute via the xylem it generally can move through the leaf via the translaminar pathway as well. Some products can redistribute via the xylem, translaminar, and vapor pathways all at the same time. Others, while technically able to redistribute via more than one mechanism, are only biologically effective via one mechanism.
Consult the Pesticide Label and Other Reputable Sources
The best way to determine how a pesticide product redistributes is to consult the manufacturer’s label, as well as technical information from reputable sources such as government or academia. If a manufacturer provides a technical information bulletin it is generally available on their website on the pesticide product page along with the label. However, because there are no standardized metrics to rate pesticide redistribution, there can be significant disparity between products. Some products that are advertised as being xylem systemic for example, are actually less systemic than products that are not even advertised as being systemic. Additional information on the efficacy and redistributing characteristics of specific products can be obtained from extension agents or crop consultants.
Conclusion
In summary, when selecting a pesticide remember to consider the four different pathways of redistribution (xylem, phloem, translaminar, and vapor) and how these methods may improve the efficacy of your application, allowing you to get more out of every drop.
January 25, 2022