Typical pesticide sprayer applications don’t often result in even, targeted coverage throughout the canopy. Sprayer calibration is crucial, and drift is a concern. Without adequate coverage on undersides of leaves, all sides of the tree and in the lower, upper and middle canopy, spray programs just aren’t fully effective. In high density orchards, trellised tree architecture presents a much different framework for chemical applications than do traditional orchard layouts.

With the need for thorough and effective spray coverage for high density orchards, Matt Griesop, Michigan State University entomologist, has been working with networks of microsprayers, typically used in irrigation systems, and utilizing them as spray applicators within trellised high density orchard rows. This permanent spray system uses the orchard trellis to support high flow rate microemitters within the tree rows, which are used to deliver protective sprays. Hydraulic and pneumatic pumps power the process, allowing precision management and timing of orchard spray applications.

“We’re taking a traditional pesticide delivery system and turning it on its head,” Griesop said of this system, known as the Solid-Set Canopy Delivery System (SSCDS). “These can indeed provide comparable pest management to air blast-type systems.”

Researching the Benefits

The benefits of the SSCDS include worker protection, the ability to spray when most beneficial no matter whether the orchard is muddy or wet without concern about soil compaction and ease of application.

In 2019, trials demonstrated that the prevalence of apple scab was not significantly different between air blast and SSCDS systems. Insect control efficacy was comparable to that achieved with air blast sprayers as well. But there were some problems with the original design of the system.

The microemitters were originally placed throughout the tree canopy, with some emitters in mid-canopy, some lower down and some placed above the canopy. Issues were found with inconsistent spray coverage. Tree growth would cause leaves to obscure the microemitters within the canopy, causing spots within the rows where sprays could not penetrate. The microemitters placed within the tree canopy were also prone to pruning damage.

The next experiments solved these issues by only arranging microemitters above the tree canopy. Trial plots of a quarter acre were sprayed either with an air blast sprayer, with microemitters throughout the canopy or with microemitters in an overhead system. Blocks of Honeycrisp, Gala and Fuji were treated and analyzed. Tree rows were spaced 11 feet apart, with three feet between trees.

The “traditional” microemitter system placed within the canopy provided more consistent coverage within all levels of the tree canopy, but very little spray was reaching the undersides of leaves. The overhead emitter SSCDS provided less coverage to the lower levels of the canopy, but was able to provide at least 20% coverage of the overall lower leaf surfaces. This coverage rate was found to be adequate to protect the crop.

All three spray systems were able to provide season-long scab control, and insect damage was not seen in any plot. But bitter pit was seen in Honeycrisp plots where the air blast sprayer was used. Fuji showed the most variability in apple scab and insect damage in the SSCDS system.

“We still have a little bit more work to do with coverage,” Griesop said. “Perhaps our most disease-prone varieties will have to be treated a little bit more carefully with this technology.”

Yield and fruit size differences were also found between the systems. In SSCDS plots, apples were smaller overall but tonnage was greater when compared to plots where the air blast sprayer was used, and there were more apples per tree. All thinning had been done through the SSCDS, with no hand thinning at all. They timed the thinning to Gala. Additionally, the thinning timing used with an air blast sprayer does not work well when using the SSCDS.

“We saw a definite thinning effect in Honeycrisp, which led to that incidence of bitter pit,” he explained.

The overhead SSCDS trial showed comparable pest management results to the traditional microemitter system spread throughout the canopy. This means that microemitters can be placed overhead only, eliminating the total number needed to provide effective spray coverage and reducing the cost of the system. The microemitters in the overhead system can be spaced every three linear feet, rather than the one per two linear feet used in the traditional system.

“I am confident in saying that we can probably move away from having nozzles within the canopy,” Griesop said. “That’s a big positive outcome.”

Future Progress

There is no cost analysis available for the SSCDS, as it is not commercially available. Researchers are working to get the custom components and microemitters needed for this system manufactured and made more available to growers. The system has main pneumatics supply lines, with smaller lines branching off to supply each high density tree row with microemitters.

Trials are now in progress examining winter decay in leaves and the role it may play in apple scab. The goal is to determine if the decay rate of leaves can be accelerated in order to decrease the amount of apple scab inoculant which is available in the orchard and thereby reduce primary scab infections the next season. Researchers used the same plots tested in the spray trials and treated the trees with either 5% urea or a biological microbial spray designed to break down field crops after the leaves fell from the trees. Control plots were sprayed with water.

Organic growers cannot use urea, so the potential for biological products to promote decay, and therefore reduce scab inoculum, would be beneficial.

“We know that we can accelerate leaf decay by applying urea at about a 5% concentration right about the time your leaves fall off the trees” and again in the spring, Greishop said. “For conventional folks, it just gives you another option. There might be a reason you don’t want to apply urea at those times.”

Leaf decay bags will be used to determine the coverage of leaves on the orchard floor and examine the amount of leaf scab pathogen. Bags were collected in December, and will also be collected in March and May. Scab will be evaluated for each variety of apple in July and September 2021. Eventually, a decay curve for each product will be able to be calculated.

“Next year I’ll be able to tell you if this is another tool we can put in the quiver for apple scab,” Griesop said.

Grieshop spoke at the 2020 Great Lakes Expo, which was held in a virtual format.