Sensors help cut apple sunburn by determining when cooling is needed

By Renee Stern

Contributing Editor

Artificial apples may soon aid growers in preventing sunburn damage to their fruit.

A fruit surface temperature sensor that mimics an apple in appearance and simulates how the fruit diffuses heat is undergoing a second year of field tests in orchards from Washington to California.

The 2004 results were so positive that growers snapped up the 50 beta models available this year, says Larry Schrader, a horticulture professor at Washington State University's Tree Fruit Research and Extension Center in Wenatchee.

Schrader developed the sensor as the next step in his earlier research establishing fruit surface temperature thresholds for sunburn and heat stress disorders. Sensor temperatures closely match match readings from thermocouples placed in adjacent apples.

Evaporative cooling is the best way to manage heat stress and prevent sunburn, says Jim McFerson, manager of the Washington Tree Fruit Research Commission. Growers have had to rely on intuition and guesswork when deciding when and how long to run their overhead sprinklers?costly decisions not only in energy and scarce water resources, but also in culls when they err.

Outright sunburn is only one result from heat and light stress that creates culls in freshly picked apples as well as fruit held in storage. Other disorders may stem from overuse of evaporative cooling.

At the same time, increased use of the ethylene-blocking SmartFresh to extend storage life "gives more opportunity for disorders to show themselves," McFerson says.

"When you pick, haul and store culls, there's a huge cost from the warehouse all the way to the grower," says Bob Brammer, president of Crane & Crane Inc. in Brewster, Wash. Tighter control of sunburn offers growers the chance to pack and sell more fruit--perhaps 90 percent of their harvest rather than 75 percent. "This is going to return a lot of money to the industry," Brammer says.

More efficient water use

Using water more efficiently also pays off. Charlie de La Chapelle, a Yakima, Wash., tree-fruit grower, tested a sensor last year on a 22-acre block and reduced the amount of water applied for evaporative cooling by 25 percent. Before that, he estimated cooling needs from air temperature readings and pan evaporation.

He's added a second unit this year to maximize efficiency in a drought that's cut his irrigation allocation to a third of normal. Without the sensors, he'd have to forgo evaporative cooling until water restrictions ease, he says.

How the sensor works

The sensor is mounted on a stand or trellis beside a tree to get full exposure during prime sunburn hours, 11 a.m. to 4 p.m.

Test units cost $310 for sensor, transmitter and receiver. How many units an orchard needs is one of the questions for this year's field tests, but site uniformity plays a key role, Schrader says. Knowing an orchard's hot spots and how they differ from sensor locations helps growers tailor their decisions.

Growers can check temperature displays on the spot, but radio frequency transmitters add the convenience of remote readings. That technology also permits automating cooling controls, so that overhead sprinklers turn on when sensors register one threshold temperature and turn off when readings drop to a preset level.

The same infrastructure, while using different temperature gauges, also allows growers to automate frost protection, de La Chapelle says.

The fruit surface temperature sensor "reduces the risk by reducing the variability of guesswork and increases peace of mind," he says. Without it, "You worry if you're applying water when it's not needed, and you worry if you're not applying it when it is needed."

"A grower could be in his headquarters and monitor hundreds of acres for heat and light stress," McFerson says. "We're approaching the orchard of the future where you can measure and manage" more data and horticultural decisions.

Keeping your cool

Sunburn browning occurs at a fruit surface temperature that ranges from 115 degrees for particularly susceptible varieties, such as Cameo and Honeycrisp, to 120 degrees for more-resistant varieties, such as Pink Lady. More serious damage from sunburn necrosis sets in at 126 degrees.

"It's not uncommon for fruit surface temperatures to be 20 to 25 degrees Fahrenheit above air temperature," Schrader says. "On extreme days it can be as much as 30 degrees higher."

That can be a costly difference. Brammer cited a September loss of several thousands of dollars from heat damage because the air temperature--at around 80 degrees--didn't appear to call for evaporative cooling. A fruit-surface temperature sensor would have registered the hidden solar radiation damaging his apples.

Schrader recommends starting evaporative cooling at a fruit surface temperature of 104 degrees and stopping when the temperature drops to 95 degrees, though growers may adjust those thresholds based on experience with the sensor and site variability.

He also suggests short cycles--10 minutes per zone--to prevent too much heat build-up during the dry periods.

Sensor readings last year allowed one California grower to abandon his previous practice of running overhead sprinklers until dusk and instead cut them off at 6 p.m., he says. The sensor "lets you manage cooling based on the actual needs of the fruit instead of guesswork," Brammer says.