Reducing sweet cherry doubling
Overtree cooling and Surround show promise for preventing doubling.
Instance of cherry doubling ranges from 5 to 15% in the Pacific Northwest, depending on the year and the cultivar.
Sweet cherry acreage and production in the Pacific Northwest have increased twofold in the last ten years. This places great importance on producing superlative fruit and maximizing packout. A significant cause of cullage in several important cultivars is polycarpy (i.e., multiple pistils, or "doubling"). An informal survey of Washington cherry packing houses found that doubling ranges from between 5% and 15%, depending on the year and cultivar. However, our own research has shown that doubling may be as severe as 50% in a susceptible cultivar like Tieton. Clearly, the potential losses from doubling are economically significant. Moreover, anecdotal reports from cherry packing sheds indicate that doubling has been increasing in recent years.
A search for published research into causes of doubling reveals how little has been done to better understand causes of this malady. Even less research has been published on how to prevent doubling. Research from Japan showed a clear role of high temperature during the previous season.
At Washington State University in Prosser, the sweet cherry physiology program has been investigating the causes of multiple pistils and practical means for reducing this disorder. We began in 2005 with funding from the Washington Tree Fruit Research Commission. Trials are researching three distinct but related components of pistil doubling: microscopic evaluation of floral bud initiation and organ differentiation; critical temperatures and periods of susceptibility; and practical strategies for reducing doubling.
Evaluation of floral bud initiation and organ differentiation
This work was undertaken to determine the time of initiation of each of the floral organs for sweet cherry cultivars with a wide range of fruit maturity dates: Chelan, Tieton, Bing, Skeena, and Sweetheart. We intended to document the progression of floral organ differentiation over the course of the season and relate that to susceptibility to doubling. Timing of flower bud initiation is similar in all five cultivars. However, the timing of organ differentiation follows a similar trend to fruit maturity throughout the year.
Critical stages of organ differentiation and temperatures
We also evaluated critical temperatures and periods of susceptibility to doubling by heating (ambient tissue temperature plus 9°F) and by cooling (ambient minus 9°F) entire Bing spurs in the orchard throughout key stages of floral organ differentiation. We discovered that Bing buds are susceptible to high-temperature-induced doubling during late July and early September. Our attempts to induce doubling in June and early July were unsuccessful, suggesting that buds are not susceptible at early stages of development (not surprising because our microscopy images reveal no floral differentiation at that time of year).
In addition, once pistil growth progresses to the stage in Figure 1 (bottom photo), buds are no longer susceptible. Initial stages of pistil development appear to occur in mid-August for Bing. However, we documented variability (one to two weeks) in organ differentiation among individual flowers within a single bud (i.e., some flowers are more advanced in development than others). Therefore, not all flower buds in a bud may be at a susceptible/ resistant stage of development at the same time.
We discovered also that differentiating buds exposed to high temperatures prior to the susceptible period (i.e., late June, early July) acquire resistance to subsequent high temperatures during the susceptible stage of bud development. This discovery helps explain why the industry observed fewer doubles in 2007 than expected. A week of temperatures in the high 90s and low 100s in late June acclimated the buds, and, therefore, high tem--p--eratures in late July did not induce as much doubling as anticipated.
Practical strategies for reducing incidence of doubling
The third part of this program investigated potential practical strategies for reducing doubling. This research was cooperative with industry and took place in commercial orchards around the state. Our treatments compared the effect of overtree evaporative cooling, Surround, and, in one orchard, a 20% shade. The treatments were initiated from mid-July and terminated in mid-August of 2006. We assessed doubling on representative limbs at bloom, and the same limbs and entire tree at harvest in 2007. In each orchard, three applications of Surround were necessary to maintain coverage. Over-tree evaporative cooling was applied only when air temperatures exceeded 93°F. Cooling was applied for 20-minute intervals followed by 10 minutes without cooling for as long as temperatures were high enough.
In a fifth-leaf planting of Tieton on Gisela 5 rootstocks, natural doubling in 2007 was about 30%, on a whole-tree basis (see chart above). This high incidence of doubling is not unusual for young Tieton trees and reflects the potential severity of the disorder. However, each preventative treatment reduced doubling significantly, with overtree evaporative cooling being the most effective. Shade, Surround, and evaporative cooling reduced doubling by 37%, 45%, and 50%, respectively. By eliminating 50% of the doubles, an additional 2,400 pounds of fruit was salable (yield was 8 tons/acre).
We found similar reductions in doubling from evaporative cooling and Surround treatments in a mature Bing orchard on Mazzard rootstocks. The --Surround treatment in 2006 reduced doubling in 2007 by 52%, and cooling reduced doubling by 44%. Natural doubling in this Bing orchard was only 4%, however.
Overtree evaporative cooling and --Surround show promise for reducing cullage from multiple pistils. Our data suggest that preventative treatments should be targeted for late July and August when temperatures are high (mid-90s).
"Effect of high temperature exposure time during flower bud formation on the occurrence of double pistils in 'Satohnishiki' sweet cherry," by K. Beppu, T. Ikdea, and I. Kataoka. 2001. Scientia Horticulturae. 87:77-84.