Growing Magazine - May, 2014

COLUMNS

Orchard Management: Factors in Effective and Efficient Spraying

By Sally Colby


This nozzleless sprayer uses one-half to two-thirds less spray to achieve superior coverage and penetration of the tree canopy.
Photo by Rob Flynn.

The chemicals used in modern apple orchards can account for a significant portion of the cost of production. Do growers know whether the product they're spraying actually hits the target? And once it's on the target, can the plant use the chemical to its fullest advantage?

Dr. Steven McArtney, Southeast apple specialist and associate professor at North Carolina State University, says that factors such as air volume of the sprayer, velocity, wind speed, nozzle type, fan type and location, tree characteristics and spray droplet size all influence how much of the chemical actually ends up on the target.

"There's also drift," said McArtney. "How much spray drifts away from the orchard? When spray applications are made during low humidity, how much chemical remains on the tree after the water evaporates quickly? Spray deposition on the ground is also a factor."

McArtney referenced a New York study that showed losses of 4 to 6 percent simply from evaporation, and losses of 10 to 15 percent of total volume from drift. The biggest loss was spray that hit the ground or row middles - as much as 30 to 50 percent. "Only 20 to 56 percent of the spray is actually hitting the orchard," he noted. "When I look at the possible fates of spray droplets, it seems to me that the best way to improve deposition on the target is by reducing the amount of spray, which can be up to 50 percent landing on a row middle."

Planting density can have a significant influence on spray efficiency. Medium-density orchards with larger trees may have better spray interception than high-density orchards on dwarfing rootstock. In some cases, the outermost leaves of a dense canopy capture the majority of the spray blast, leaving insufficient product on inner leaves and fruit.

Research in a high-density apple orchard in the Netherlands examined spray interception at four different times throughout the growing season. "At dormant stage, 20 percent of the spray coming out of the sprayer actually ended up on the trees," said McArtney. "That means that for every $100 you spend on a spray oil, only $20 of it lands on the tree."

As the season progresses and the canopy develops, more and more spray sticks on the tree. By bloom, 40 percent of the spray applied sticks to the tree. By midseason, 70 percent sticks, and by full canopy, 80 percent ends up on the intended target. However, spray interception by the target doesn't indicate the efficiency of spray coverage throughout the canopy.

McArtney said alternate row spraying is a good practice, but the negative aspect is that a lot of spray can end up on the ground. "The point of spraying is to completely displace the air space within the canopy with pesticide-laden air," he explained. "You can do that on the row right next to the sprayer, but with the wind speed coming off the fan of the sprayer, by the time it gets to the next row over, it's barely enough to carry spray droplets into the middle of the canopy if you're dealing with a dense canopy. In order to get spray coverage on the row next to the sprayer and the next row over, you're going to have a lot more drift falling into row middles."

Assuming that the spray reaches its target, what happens to the active ingredient once it lands? McArtney explains that the cuticle on the outside of the fruit, which prevents desiccation, is comprised of waxes and can prevent chemicals from entering the fruit: "It's impermeable, and anything that lands on the fruit surface will have a hard time passing through the cuticle. It's the major barrier in plants for the movement of the chemicals you spray."



The air-curtain orchard sprayer uses multiple cross-flow fans to disperse pesticide to apple trees. Under some conditions, the smoother, gentler flow of air can reduce spray drift by half.
Photo by Keith Weller.

There are two pathways by which chemicals can pass through the cuticle. "Chemicals can enter through a lipophilic pathway, which allows some chemicals to pass through waxes. But most chemicals we spray are polar and have a charge separation. They interact with waxes and cannot pass through using the lipophilic pathway," McArtney said, adding that there is indirect evidence of a hydrophilic pathway through aqueous pores. The diameter of those channels is about one-thousandth the diameter of open stomata, and although they move through very slowly, a lot of commonly used chemicals can get through.

A complicating factor is that the thickness of the cuticle changes throughout the season. Early in the season, the cuticle is 2 to 3 microns thick. By the end of the season, it's 15 microns.

Trichomes, which are small hairs that cover the flowers and young fruit, are present on the underside of leaves and can inhibit spray penetration. These structures limit contact between a spray droplet and the cuticle, but spray adjuvants help reduce surface tension so that the ability of trichomes to inhibit penetration is reduced.

McArtney has researched three classes of spray adjuvants: humectants, penetrants and surface-active agents (surfactants). Humectants absorb and hold water and slow down the process of drying. A typical humectant is carboxymethylcellulose, a thickener. Other humectants include glycerol and sorbitol. Penetrants help the active ingredient move through the cuticle, and surfactants lower the surface tension of the liquid and help droplets spread.

In a study conducted in Spain, humectants were used to help get calcium into apples. "In that very dry climate, it slowed down droplet drying, and they got higher calcium levels in the apple skin and core samples," said McArtney. "They also reduced bitter pit in storage simply by slowing the rate of droplet drying."

When McArtney used a humectant in North Carolina, where annual rainfall is 60 inches, there was no benefit. "As long as there is moisture in the spray droplet, the chemical can move through the cuticle," he said. "As the droplet dries, the rate of movement slows down. Once the droplet is dry, there is only residue on the surface of the target. This is why contact period and drying time are critical. Anything we can do to increase contact area and reduce droplet drying will increase the uptake of the active ingredient by the plant."

The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania. Comment or question? Visit http://www.farmingforumsite.com and join in the discussions.