Causes of and practical strategies for reducing sweet cherry polycarpy

Rolando Martin

Polycarpy is a common physiological disorder of several Prunus species believed to be induced by high temperatures during floral bud differentiation. Polycarpy reduces sweet cherry (Prunus avium L.) crop value by increasing culled fruit. This research program sought to 1) document variability in timing of flower bud initiation and differentiation of several commercially important cultivars, 2) study the role of tissue temperature and stage of bud differentiation on susceptibility of 'Bing' to polycarpy, and 3) evaluate the efficacy of potential practical strategies for reducing polycarpy in a commercial setting. Analyses by scanning electron microscope revealed that floral organ initiation and development of cultivars with late-maturing fruit lagged up to five weeks behind cultivars with early-maturing fruit. Further, there was asynchrony of about two weeks in organ development among flowers within a bud that may explain partially why incidence of polycarpy is variable within buds. Using a forced convection cooling/heating device to manipulate tissue temperature in situ, I found that 'Bing' floral buds are susceptible to heat-induced polycarpy between late July and early September. Peak susceptibility occurs when petal primordia are first visible. At earlier (i.e., no floral initials) and later (i.e., initial carpel development) stages of differentiation, buds are not susceptible to polycarpy. In a separate trial in 2007 I showed that differentiating buds in 'Bing' sweet cherry exposed to high temperatures prior to being susceptible (i.e., late June, early July), acquire resistance to subsequent high temperature induced polycarpy when at a susceptible stage of bud development. Three experiments were conducted in commercial orchards and compared the efficacy of postharvest over-tree evaporative cooling, the sprayable reflectives Surround and Raynox, and 20% shade net at reducing polycarpy. In a 5th-leaf 'Tieton'/'Gisela5' orchard, natural doubling in 2007 was about 30%, on a whole-tree basis. Shade, Surround, and evaporative cooling reduced doubling by 37%, 45%, and 50%, respectively. By eliminating 50% of the polycarpic fruit, an additional 2960 kg of fruit was harvested per ha. No treatment eliminated polycarpy but over-tree evaporative cooling and Surround show promise for reducing cullage from polycarpy and improving returns to growers.

Causes of and practical strategies for reducing sweet cherry polycarpy

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