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“Spray to run-off” is the usual term.
We think of it as “a good douse,” every leaf surface wet. When the spray job is finished and the pump turned off, a few drips patter to the ground from the sprayed plants. Job done!
Sensibly, we don’t return until the residue is dry.
However, a plant canopy is very complicated terrain, and it’s harder to cover it than we think. Also, pests are notoriously smart about where they live—usually not fully exposed on the top surface of the leaf.
That’s where water-sensitive papers come into play—a way to see what is usually hard to assess.
We always mix our agrichemicals with water to deliver them to the plant surface. Folded over leaves to visualize both the underside and top side, the water sensitive papers let us see where the spray actually lands.
Let the citizen science begin!
Yellow water sensitive papers are folded over strawberry leaves so we can see where the spray lands, both tops and bottoms.
That’s just what we did one overcast morning at a strawberry farm near Christchurch. The grower uses a spray gun, a sensible modern spray applicator that works in well with the small scale tabletop system.
“Alright, what do you want me to do?” he asked, generously offering to be the guinea pig as we assess spray coverage.
I cracked out the bright yellow strips, trying not to mark them with my damp finger as I folded them in half and paper-clipped them over the leaf edges. We chose a somewhat vertical leaf as well as one held horizontally. The grower fired up the newly cleaned sprayer filled with water, and walked his typical spray pattern.
We checked the papers. The top surface and whatever surface of the vertical leaf that had gotten blown over by the force of the spray plume were liberally plastered, while the opposite sides were only slightly speckled.
“What do you want me to do next?” the grower asked.
“Well, now that we know where the spray lands and doesn’t land, the next steps are up to you. What do you want to try?”
We tried several things:
2. Change the spray gun air-intake setting—now leaves weren’t as drippy, but the dry undersides were unchanged.
No matter how much water we used, we couldn’t seem to get it on the under sides of the leaves.
3. Add AIR to help distribute the water droplets: we simulated air blast with a leaf blower pointed alongside the spray gun. Unfortunately, it gave a very similar pattern to the first time—horizontal leaf bottoms remained dry.
4. Try the leaf blower pushing air in the OPPOSITE direction to the spray gun. Now that was the ticket! Both leaf surfaces got waved around and covered.
We’re not suggesting that it’s practical to actually use a leaf-blower following just behind a sprayer gun! But using what he had on hand, the grower was able to demonstrate that it’s “spray plum complexity” or air moving in different directions that allows product to reach the leaf undersides.
How much coverage is enough?
The goal is to get all surfaces to glisten with tiny droplets, but not enough to cause drips to run off the leaf. A fully dark blue water sensitive paper indicates that we have too much moisture in our chemical delivery system.
For the gun-sprayer scenario, in order to reduce the spray volume which we determined was excessive, we planned to change the nozzle to a smaller aperture and to move over the crop faster.
Are you telling us we all need to go out and buy new sprayers?
No.
Some growers use air blast sprayers to get that “spray plume complexity” and make fine water droplets deposit on the undersides of leaves, but the exercise is still useful knowledge for those who don’t plant to upgrade to a fancier sprayer. Nozzles can be adjusted, and drive speeds varied. Also, just knowing the limitations of the spray coverage allows for better decision-making.
I once worked with a nursery using a boom sprayer for seedlings, as well as a fogging system for enclosed greenhouses. The knowledge about the dry leaf undersides for both systems meant that we stopped relying on contact insecticides, knowing we weren’t reaching the pests hiding on the undersides of leaves. From then on we focused on products that were translaminar or systemic.
As David Manktelow’s talk at the SGNZ winter conference demonstrated, getting spray coverage is complicated. The next article will touch on spray adjuvants and their effect on spray coverage, an angle which cannot be assessed with the water sensitive papers.