Profitability of tree fruit producers is dependent upon consistent, high yields of quality fruit. Over the last decade, sweet cherry producers in the Pacific northwestern United States (PNW) have been transitioning to high-density plantings based on detailed studies showing increased production (Perry et al., 1998; Whiting and Lang, 2004; Whiting et al., 2005b) and the commercial availability of semidwarfing, precocious rootstocks (Kappel and Lang, 2008; Lang, 2000). These rootstocks greatly improve labor efficiency, increase production, and hasten net returns; however, the majority of sweet cherry acreage is planted on highly vigorous rootstocks at low to moderate densities. There has been little need to manage crop loads in these orchards due to the favorable balance of leaves to fruit resulting from the interaction between low scion precocity and high rootstock vigor. Despite this scenario, high yields from mature, low-density ‘Bing’ trees adversely impacted fruit quality (Proebsting and Mills, 1981; Spayd et al., 1986). Indeed an inverse relationship between sweet cherry crop load and fruit quality has been documented (Facteau et al., 1983; Proebsting, 1990; Roper and Loescher, 1987; Whiting and Lang, 2004).
Crop load management strategies to maintain fruit size via some form of pre or postbloom flower/fruit thinning has been the subject of recent investigation (Schoedl et al., 2009; Whiting and Ophardt, 2005; Whiting et al., 2006). To date, inconsistent response of fruit to chemical thinning strategies has limited commercial adoption. Presently, the industry largely relies on pruning for crop load management (L. Long, personal communication). Ultimately, the decision to impose a crop load management strategy, and the level to which it is carried out, will depend upon the value of the crop. The potential gross income of sweet cherry production is reliant upon yield, number of fruit per size category, and the price paid per size class.
The self-fertile cultivar Sweetheart is valued for its late-season harvest timing, precocity, and high productivity (Lane and MacDonald, 1996). However, ‘Sweetheart’ has medium to small fruit size (Lane and MacDonald, 1996) and propensity for surface pitting (B. Bailey, personal communication). Surface pitting of sweet cherry fruit is characterized by collapsed hypodermal cells resulting in sunken, irregular depressions on the surface of the fruit (Porritt et al., 1971). Symptoms are attributed to impact injuries sustained during harvest and postharvest handling operations (Facteau and Rowe, 1979; Thompson et al., 1997), but crop load has been positively correlated with surface pitting of ‘Sweetheart’ (J.P. Zoffoli, personal communication), as was observed for ‘Bing’ (Spayd et al., 1986).
When used in combination with dwarfing rootstocks, heavy fruit set can severely limit both vegetative development and fruit quality of sweet cherry (Edin et al., 1996); a scenario that would likely be amplified given the precocity and high productivity of ‘Sweetheart’. Hence, a large portion of ‘Sweetheart’ acreage in the PNW has been planted at moderate tree densities with vigorous, nonprecocious ‘Mazzard’ rootstock. An interesting horticultural question is therefore raised: Does the vigorous rootstock ‘Mazzard’ effectively balance the high productivity and precocity of ‘Sweetheart’, or could fruit quality, and importantly, crop value be improved by imposing a crop load management strategy? The following study was designed to address this question and determine the effects of crop load on vegetative and reproductive development of ‘Sweetheart’/‘Mazzard’ trees.
Ayala, M. 2004 Carbon partitioning in sweet cherry (Prunus avium L.) on dwarfing, precocious rootstocks, during fruit development Michigan State Univ. East Lansing PhD Diss.
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Whiting, M.D., Ophart, D., Lenahan, O. & Elfving, D. 2005a Managing sweet cherry crop load: New strategies for a new problem Compact Fruit Tree. 38 52 58
Whiting, M.D., Lang, G. & Ophart, D. 2005b Rootstock and training system affect sweet cherry growth, yield, and fruit quality HortScience 40 582 586
Whiting, M.D., McFerson, J.R. & Ophardt, D. 2006 Chemical blossom thinners vary in their effect on sweet cherry fruit set, yield, fruit quality, and crop value HortTechnology 16 66 70
Wy'East RC&D 2009 Integrated fruit production IFPNet. 16 Apr. 2011. <http://www.integratedfruitproduction.com/IFPNET/tabid/92/Default.aspx>.