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Thinning will be important in highly efficient cherry systems like this Washington State University orchard that has angled fruiting walls comprised of vertical fruiting uprights.

Thinning will be important in highly efficient cherry systems like this Washington State University orchard that has angled fruiting walls comprised of vertical fruiting uprights.

Flavorful, firm, and large sweet cherries must (and can) be grown within highly efficient orchard systems for growers to be successful. The potential shortages, and increasing expense, of skilled harvest labor are forcing growers to make radical changes in production systems from the classic Bing/Mazzard system of old. New scion and rootstock genotypes are critical to this transition.

Irrespective of variety, the production economics associated with the precocity induced by the Gisela rootstocks are compelling. Moreover, with prudent management of canopy fruit-to-leaf-area ratios, high yields of high quality fruit can be produced on the Gisela series of rootstocks. Indeed, cumulative yields per tree (third to eighth leaf) of 10.5-row and larger Bing fruit on Gisela 5 and Gisela 6 were 2.5- to 3.5-fold greater than on Mazzard in a trial at Washington State University-Prosser. In addition, due to the vigor-controlling effects of these rootstocks (Gisela 5 and Gisela 6 were 58% and 74% the size of Mazzard), harvest efficiency was much improved when compared to Mazzard, although this wasn’t determined empirically.

Currently, canopy fruit and leaf populations are balanced most readily by pruning. Guidelines for managing crop load on Gisela rootstocks with pruning are based upon aggressive dormant tipping into new shoot growth and frequent renewal cuts. These techniques work reasonably well but do nothing to balance crop load, or, more relevantly, fruit-to-leaf ratios on the spurs that remain. Our previous work has shown that ideally, crop load must be balanced on an individual spur basis, such as thinning every spur, rather than treatments that balance overall tree source-sink relationship.

Additionally, the chaotic canopy structure that results from dormant tipping and heading is undesirable (pruning begets pruning), and pruning young trees limits orchard precocity. For these reasons, we have investigated chemical bloom and postbloom thinning alternatives. Such strategies are important for balancing fruit numbers on every spur and will be critical to adopting the highly efficient single-plane fruiting walls comprised of vertical fruiting uprights as highlighted in previous Good Fruit Grower articles.

Crop load strategies

At WSU-Prosser, with funding from, and in cooperation with the Washington Tree Fruit Research Commission, we have investigated potential crop-load management strategies that span the 15-month sweet cherry developmental timeline; from floral bud initiation, to hand-thinning before harvest. We have shown that rates of gibberellic acid as low as 50 parts per million can reduce Bing return bloom, reducing fruiting potential and need for thinning in the subsequent season.

In addition, GA3 is more effective at reducing flower bud initiation than GA4+7. Our current research is investigating the effects of GA on late-maturing, self-fertile varieties that may benefit from the delay in maturity of the crop in the season of application as well as the reduction in fruiting potential in season two.

Bloom thinning

Our work was focused initially on developing a reliable bloom-thinning program. We have investigated the impact of several potential thinners on fruit set, yield, and quality, as well as mode of action and the timing of the fruit abscission.

Over five years, the most consistent thinning efficacy was with 2% ammonium thiosulphate (ATS) or 2% fish oil plus 2.5% lime sulfur applied at about 20% and 80% of full bloom. These programs have reduced natural fruit set by some 18% across multiple scion-rootstock combinations. The ideal thinning program will reduce fruit set and increase the packout of the highest quality fruit. ATS has been the most consistent performer in this regard.

For example, in 2006, fruit from ATS-treated Skeena/Gisela 5 trees were 15% heavier (about a gram per fruit) than control fruit. In addition, ATS-treated trees yielded 93% 10.5-row and larger, compared to only 67% from control trees. At the density of the research orchard (580 trees per acre), this improvement in fruit quality translates into an additional 2 tons of 10.5-row and larger fruit per acre from ATS-treated trees in comparison to the untreated trees.

Postbloom thinning

Because of year-to-year variability in natural fruit set and the often unpredictable nature of bloom thinning, in 2005 we began investigating the potential for postbloom thinning. Having the opportunity to assess fruit set, and, therefore, the need for thinning, would be beneficial. A postbloom application of 2% lime sulfur was made in 2005 to investigate the potential for thinning via photosynthetic inhibition. Applications were made 14 days after full bloom to coincide with the switching from growth supplied by stored resources to being supplied by current season assimilates. In addition, this is a period of high fruit growth rates in early stage I, and therefore, high sink demand.

We hypothesize that by reducing assimilate supply at this stage, we may be able to induce resource limitations and fruit drop. Indeed, fruit set (# fruit/100 flowers) in 2005 was reduced significantly by lime sulfur applied 14 days after full bloom, to 27% from 39%. This response is likely a result of photosynthetic inhibition from the treatment because pollination/ fruit set had already taken place.

However, the postbloom lime sulfur application was less effective at reducing fruit set than the applications made during bloom. This is likely because postbloom applications were less phytotoxic compared to applications during bloom, and there was no interference with pollination and fruit set—a clear thinning mechanism of bloom applications of lime sulfur. We are currently investigating other potential postbloom thinning agents as well as the importance of rate and timing.


• High quality fruit can be grown on dwarfing, precocious rootstocks with prudent crop load management.

• Chemical blossom thinners fish oil plus lime sulfur and ammonium thiosulphate show greatest potential as bloom thinning agents.

• Lime sulfur and ATS applied at bloom interfere with fertilization, rather than photosynthetic inhibition, to affect thinning.

• Vegetable oil emulsions are not effective blossom thinners.

• Need to develop an effective postbloom thinning program for sweet cherry.

• Optimum timing of postbloom thinning appears to be between 2 and 4 weeks after full bloom, though thinning later in the season can help improve fruit size in severely overcropped trees.

• Lime sulfur shows efficacy as a postbloom thinning agent at 14 days past full bloom.

• Tergitol is not recommended for postbloom thinning.

• GA3 on Bing is more inhibiting to flower bud induction than GA4+7.

The next generation sweet cherry orchard must be precocious, consistently productive, bear excellent-quality fruit, and be designed to be labor efficient.

At WSU-Prosser, we will continue to design/refine efficient cherry orchard systems and develop the pragmatic strategies that will enable success. Clearly, the precocious and productive rootstocks that are critical to the efficiency and profitability of modern orchard systems will require rootstock-specific management strategies.

Further reading

Lenahan, O.M. and M.D. Whiting. 2006. Fish oil plus lime sulfur shows potential as a sweet cherry post-bloom thinning agent. HortScience. 41: 860-861.

Whiting, M.D., D. Ophardt, O. Lenahan, and D. Elfving. 2005. Managing sweet cherry crop load: new strategies for a new problem. Compact Fruit Tree 38:52-58.

Whiting, M.D. and D. Ophardt. 2005. Comparing novel sweet cherry crop load management strategies. HortScience. 40:1271-1275.

DISCLAIMER: Some of the pesticides discussed in this article were tested under an experimental use permit granted by Washington State Department of Agriculture. Application of a pesticide to a crop or site that is not on the label is a violation of pesticide law and may subject the applicator to civil penalties up to $7,500, according to information from the WSDA. In addition, such an application may also result in illegal residues that could subject the crop to seizure or embargo action by WSDA and/or the U.S. Food and Drug Administration. It is the grower’s responsibility to check the label before using the product to ensure lawful use and obtain all necessary permits in advance.