A scanning electron microscopy photo taken on July 31, 2006, shows that Bing (left) was further advanced than the Chelan variety (right).
Cherry or pistil doubling is significant in some years, affecting up to 15 percent of the Pacific Northwest sweet cherry crop and resulting in losses estimated to cost growers up to $45 million, says a Washington State University horticulturist.
Researchers are beginning to discover some of the triggers behind doubling with the hope of developing grower strategies to minimize the disorder.
Dr. Matt Whiting, WSU horticulturist, has found doubling as high as 30 percent in some blocks, adding that cherry doubling was particularly troublesome in 2004 and 2005.
Two years ago, Whiting and WSU graduate student Rolando Martin began studying the problem to learn more about the relationship between doubling and temperatures, timing, and critical periods of susceptibility. Whiting recently shared preliminary results of the cherry doubling research with growers attending the annual Cherry Institute meeting in Yakima, Washington.
The causes of pistil doubling are not a complete mystery. Scientists know that high temperatures can induce doubling, Whiting said. "We also know doubling is unrelated to water deficit, that buds are susceptible in the primary year of bud differentiation, and that there is significant variability within varieties."
However, there are several things they don’t know. "What are the temperatures that we should worry about?" he asked. "When are the critical periods of susceptibility? When are the floral buds initiated? Is there genetic resistance?" And most important to growers, how effective are preventative treatments?
Martin is using scanning electron microscopy in the project to reveal the seasonal trends for floral buds and organ differentiation. He has looked at the bud development of five cherry varieties—Tieton, Skeena, Sweetheart, Chelan, and Bing. By using electrons instead of light to form images, the microscope has provided high resolution, highly magnified pictures of floral bud and organ development.
The specialized microscope showed that flower bud initiation began in May. But Whiting said they also learned that buds do not differentiate at the same time.
Organ differentiation did not start until the end of June or early July, he said. "That’s when you start to see significant developmental differences between varieties."
For example, on July 31, they could see two flowers developing in a Chelan bud. On the same date, however, dissection of a Bing bud showed the flower development to be more advanced.
"We hope to be able to relate these periods of organ differentiation to varieties being more or less susceptible to pistil doubling," he said.
In addition to tracking bud development, the research is also studying buds in the field, subjecting them to different temperatures to learn more about the influence of temperatures on doubling. To be able to control temperatures in the study’s field component, the researchers used the "squid," a heating and cooling device developed by Dr. Julie Tarara, at the U.S. Department of Agriculture.
The squid, with different blower hoses that look like squid arms, blew hot and cool air on the cherry buds for two weeks at a time at different periods during the season. The squid allowed the researchers to expose buds in the orchard to temperatures that varied by 18°F. Fruit was evaluated the following year for doubling.
Whiting and Martin found 4 percent doubling from buds exposed to ambient temperatures from the squid. However, when the buds were exposed to hot temperatures for a two-week period in late July, doubling increased to 10 percent. Cherry doubling incidence rose even higher from hot treatments given in August.
The data helped the researchers identify a preliminary critical tissue temperature of 99.1°F. Temperatures above 99°F resulted in doubling, but researchers found no increase in doubling in temperatures below 99°F. The maximum tissue temperature measured in the study was 100°F.
"It’s not so much what the maximum temperature was, but how long the temperatures stayed there," Whiting said. They found only 5 percent doubling in cherries exposed to 100 hours of 86 to 95°F temperatures.
When the temperatures shifted higher from 96 to 100°F, it only took 37 hours to result in 5 percent doubling. At 101 to 104°F, it took 10 hours to achieve 5 percent doubling, but only 3 hours to result in 5% doubling at temperatures of 104°F or higher.
The researchers established grower trials last year to compare different strategies to minimize doubling. Strategies compared included whole tree shade from shade cloth, undertree sprinklers, overtree sprinklers (evaporative cooling), Surround (kaolin particle film), and Raynox, a sunburn protective material.
Although results from the treatments will not be available until the 2007 crop, Whiting and Martin found that the overtree evaporative cooling was the most effective in reducing the canopy temperature, both the canopy in the shade and canopy in full sun exposure. The protective films also helped reduce temperatures, although not as much as the overtree cooling.
The control trees were more than 9°F warmer in the afternoon than those with overtree cooling, Whiting noted. "The overtree sprinklers were even more effective in cooling than the squid device which blows out cool air."
He sees great potential for utilizing overtree cooling in cherry orchards to minimize pistil doubling. After collection of data this season, Whiting hopes to establish more precise thresholds for temperatures and timing of critical bud development periods. In the long-term, he believes that development of genetic markers indicating susceptibility may become part of some cherry breeding programs.