PROSSER, Wash.—A more accurate way to measure cold hardiness in apple and sweet cherry buds and blooms during early spring is under development by researchers at Washington State University Prosser Irrigated Agriculture Research and Extension Center (IAREC). The three-year project, funded by the Washington Tree Fruit Research Commission this year, will help Pacific Northwest growers better protect their orchards during frosts.
Led by AgWeatherNet director Gerrit Hoogenboom and research associate Melba Salazar-Gutierrez, the project ultimately will provide more updated information about how new apple and cherry varieties handle cold at different stages of growth.
“Historical cold hardiness data are based on research that was conducted more than 30 to 40 years ago and with older varieties,” Salazar-Gutierrez said. “These data are still being used today, even for new varieties. So far, little is known about the hardiness of new cultivars under local weather conditions.
“There is, therefore, a need to update this information using current varieties with new scientific methodologies,” she said.
Developmental stage a key factor
Frost in early spring often damages apple and sweet cherry buds and blooms, and crop resistance to freezing temperatures varies depending on the buds’ development, she explained. This variable susceptibility to cold makes it difficult for growers to know when to take measures to protect their orchards when the mercury plummets.
“From dormancy to fruit set, the flower bud undergoes a number of developmental changes that are associated with a progressive, increasing vulnerability of the pistil [the female reproductive part of a flower] to low temperatures,” Salazar-Gutierrez said. “In dormant flower buds, the effect of freezing temperatures is not uniform, with ice crystals formed in only some floral tissues. At full bloom, the damage may be more extensive, depending on the severity of the freeze. With cherry, flower buds are more susceptible to injury than vegetative buds.”
Temperature and time combinations tested
Previous cold hardiness measurements were not as specific as the new system. Researchers made observations and took samples of buds and flowers after naturally occurring freeze events to determine injury, Salazar-Gutierrez said. They also tested freeze tolerance using differential thermal analysis to come up with predictions of critical lethal temperatures, but this technique is only effective for early stages of bud development.
The new system more accurately determines lethal temperatures for later bud and bloom growth, she said. An automated freezer sampler, called the “vending machine,” exposes the buds to different durations and controlled cold temperature combinations.
Created by John Ferguson, an IAREC staff member who also came up with a cold-hardiness prediction model for grapes, the vending machine is a standard environmental chamber with a built-in slot at the bottom of the door, hence its nickname.
Four plastic racks inside the chamber hold perforated cylinders for samples. When samples reach a designated temperature, they are automatically released from the racks and fall through the door’s slot into a basket outside the chamber. The custom-modified freezer sampler can hold smaller cuttings of limbs and flowers and run samples overnight, thus processing more samples faster.
Spring samples analyzed
Hoogenboom, Salazar-Gutierrez and their AgWeatherNet research team studied Red Delicious, Gala and Fuji apples as well as Bing, Chelan and Sweetheart cherries beginning in February. They collected samples from the WSU Roza Research Farm and C & M Orchards near Prosser and tested temperatures ranging from –40 to 30 degrees Fahrenheit at varying times to learn when buds and flowers will die.
The team is still analyzing the results from last spring. Salazar-Gutierrez said the next step is to continue collecting samples on new orchards for two more seasons, which will depend on cooperation from growers willing to participate in the project. The researchers also want to study how apple and sweet cherry buds fare as orchards enter dormancy in fall and winter.
The researchers will then develop a model for growers with a range of early spring temperatures that buds at all stages of development will tolerate.
“The overall outcome of this project will be updated hardiness charts for apples and sweet cherries that include the critical temperatures for each of the different stages of spring bud development,” Salazar-Gutierrez said. “This is a very important tool for growers who are monitoring their individual apple and cherry orchards for appropriate crop management and activation of frost protection systems. This will allow for better planning to improve fruit quality, enhance yield and ultimately increase net returns.”