If we had to irrigate orchards by pulling tanks of water down the alleys, would we do it?
We do that now with pesticides and plant growth regulators, hauling loads of water with small amounts of chemicals in them, then blasting the mixtures into the trees.
Only about a year ago, researchers at land grant universities across the United States obtained funding to investigate the feasibility of using a fixed-in-place system of pipes and nozzles—like a solid-set irrigation system—instead of airblast sprayers. They call the system the Solid-Set Canopy Delivery System, or the SSCDS.
At a field day in July at Michigan State University’s fruit experiment station in Clarksville, Michigan, the Michigan contingent of researchers gathered to show growers the working model built for them to do their studies and to report on progress they have made in less than two years.
The words “proof of concept” were used several times—meaning the scientists think they have proved that several things work as hypothesized—but the system’s not ready for commercial application. (It is believed, however, that some growers are trying it in small blocks in their orchards.)
What are some of these things that work, these “proofs of concept”?
Misting to extend dormancy
This spring, Dr. Jim Flore was able to delay apple trees from emerging from dormancy by eight days by misting them to keep them cool longer as spring was arriving. In sweet cherry trees, emergence from dormancy was delayed by 11 days.
“The long-term goal is to develop an effective, environmentally friendly method to protect sweet cherry and apple buds and flowers from spring frost damage and to delay bloom by cooling the buds once dormancy has been broken,” he said.
Flore said research years ago showed that applying water and allowing it to evaporate would lower temperatures, and since bud development is directly related to temperature, keeping them wet keeps them dormant. The problem was, it took large amounts of water when applied by conventional overhead irrigation sprinklers.
In experiments in the 1970s, it took up to 36 inches of applied water—an amount that caused trees to tip over, saturated and eroded orchard soil, and led to poorer fruit set and disease.
This year, the cooling effect was achieved with the application of 2 to 3 inches of water over nearly six weeks from early April to mid-May.
New technology—computers, data loggers, thermocouples, temperature sensors, solenoid valves—allowed Flore to set up a system that monitored bud temperature and responded to it.
“Programming was not difficult,” Flore said.
When bud temperature rose above 37°F, the computer turned on the water mist.
The cooling effect lasted until all the water evaporated, when the bud temperature would again rise. The length of time the mist was on, and the length of time between mistings, depended on temperature and relative humidity. In his tests, Flore found that at 75˚F, the mist came on for 105 seconds and was off for only 3 minutes. But at 39˚F, the misting interval was 20 minutes.
The cooling effect was immediate. Bud temperatures dropped by 15˚F within 90 seconds.
With the cool spring this year, the system ran only 58 hours all spring, he said, and used about 1,000 gallons per tree. “If we had had this in place a year ago, we would have had a crop—not a full crop—but a crop,” he said. In 2012, a series of 21 spring freeze events reduced Michigan’s apple and cherry crops by 90 percent.
Research was conducted at three locations. There was only one significant freeze event in Michigan this year—the night of May 12-13. At one orchard location, the freeze killed 51 percent of the apple flowers in the control orchard, but only 17 percent in the mist-cooled orchard.
While cooling early in spring sets back emergence from dormancy, by mid-July the crop had caught up. Early cooling did not greatly reduce growing degree-day accumulation, Flore said.
Phil Schwallier, director of the Clarksville station with particular expertise in apple thinning, used the SSCD system to apply the apple thinners NAA and Sevin three times this spring.
Data had not been collected in July, but visitors could easily see that the control Honeycrisp trees were overloaded and those thinned with the airblast sprayer were not as well thinned as those under the SSCD treatment. Only mild hand thinning would be needed there.
Entomologist Dr. John Wise noted that the SSCD system could save growers money and be kinder to the environment in several ways: “Application of lower doses on a tighter interval could reduce total product use,” he said. The 14-day spray interval many growers use for insect control requires that enough product be applied to last 14 days. “There is a lot of waste in that,” he said.
Plant pathologist Dr. George Sundin was not there to present disease data, but Wise said the system was used to apply insecticides, several fungicides, and antibiotics for fireblight control. Entomologist Dr. Larry Gut is conducting experiments applying mating disruption pheromones.
Gut has found that codling moth is very choosy about when it is active and that timing sprays of insecticides or pheromones could be improved with an all-at-one-time application through SSCD compared to a long spray operation with an airblast sprayer.
In Wise’s tests this year, he found that uniformity of deposition with the SSCD still needs to be improved. Deposition of spray material tends to be less uniform on tops or bottoms of leaves and closer or further from emitters.
The air blast of sprayers tends to overcome any wind effect and agitate foliage, which keeps foliage from blocking spray from nozzles. Wise said care will have to be taken in locating SSCD emitters so they are not blocked.
The emitters release smaller spray droplets than do airblast sprayers, but because there is not much force behind the release, off-target drift is less. Emitters are, however, more likely to clog than are airblast spray nozzles.
The SSCD is being tested at two locations in Washington—Prosser and Wenatchee—and also at Cornell University in New York State. Michigan State University entomologist Dr. Matt Grieshop is overall project director; Dr. Jay Brunner is directing the Washington work and Dr. Art Agnello the New York work.
In the West, the researchers are working to refine the application technology and also researching use of the system for summer cooling and to reduce sunburn.
How the SSCD system works
The Solid-Set Canopy Delivery System for spray application is much more sophisticated than would be a similar solid-set irrigation system.
Michigan State University horticulturists Dr. Matt Grieshop, project director, and Dr. Ron Perry, who worked with irrigation expert John Nye at Trickl-eez Irrigation to lay out the experimental system, explained that there are several steps in the spraying system; it’s more than just turning on the water to irrigate.
The applicator consists of four major components—a pumping system, a tank for mixing sprays, an air compressor, and a loop of plastic line carrying spray material into the orchard and back to the mixing tank.
Spraying takes place in four steps:
Charging: The premixed spray material is pumped into the main application line at low pressure, less than 18 psi. This is called charging the system. It fills the line with spray material.
Spraying: The return line is closed and the pressure is increased in the spray line to about 30 psi. This opens the spray emitters and allows the application of the equivalent of 70 to 100 gallons per acre of spray material.
Recovery: The return line valve is opened and the air compressor blows any remaining fluid in the mainline back to the spray tank.
Cleaning: The return valve is closed and the air compressor bumps to a higher pressure to clear any remaining material out of the microsprayers.
The use of an air compressor was chosen because using water to clean the lines would wash off the spray materials that had just been applied to the foliage.