A research project to find out whether Ethrel (ethephon) could be used as a postbloom thinner for cherries showed that the material can reduce the fruit load.

But, perplexingly, having fewer cherries on the tree didn’t always result in larger cherries, which is one of the main objectives of thinning.

Dr. Matt Whiting, cherry horticulturist with Washington State University in Prosser, said there’s a need to thin the crop on productive cherry varieties so they don’t produce small fruit, and postbloom thinning allows growers to assess fruit set before adjusting the crop. Currently, the only reliable way to thin cherries after bloom is by hand thinning.

Whiting conducted a two-year project in which he tested Ethrel at three rates (100, 200, and 300 parts per million) and four timings (shuck fall and one, two, and three weeks after shuck fall) on Sweetheart, Rainier, Lapins, Skeena, and Santina cherries. All the timings were before pit hardening. The tests were done in commercial and research orchards and funded by the Washington Tree Fruit Research Commission.

Ethrel was effective, particularly at higher rates and earlier timings, but Whiting said there seemed to be a disconnect between thinning efficacy and fruit size. For example, in tests with Sweetheart, shuck fall (the earliest) applications did thin the crop. But, whereas the 100 ppm rate increased fruit size by 6 percent, the 200 ppm rate had no effect on fruit size, and the 300 ppm rate reduced fruit size by 25 percent, despite reducing fruit set by 73 percent.

In Lapins, however, fruit size was improved by almost every treatment, despite the inability of most treatments to thin the fruit.

“Whiting also tested abscisic acid at 500 and 1,000 ppm, but found it to be generally ineffective as a thinner. In fact, later applications of ABA at 500 ppm in Sweetheart improved fruit set by roughly 10 to 14 percent.