Fruit Thinning Improves Fruit Weight and Return Bloom in Cranberry ( Vaccinium macrocarpon ) in High Tunnel Soilless Cultivation

Limited information is available on cranberry crop load management. Earlier research has primarily examined flower thinning in cranberry, while the impact of fruit thinning has not yet been investigated. This study evaluated the effects of fruit thinning during early fruit development on fruit quality, return bloom, and chlorophyll fluorescence in two cranberry (Vaccinium macrocarpon) cultivars, “Pilgrim” and “Stevens.” Treatments included: (0F) removing all fruits on the upright, (1F) removing all but the lowermost fruit, (2F) removing all but the two lowermost fruits, and (C) no fruit removal (control). The majority of the non-treated uprights produced two fruits in “Pilgrim” and three fruits in “Stevens.” In “Pilgrim,” 1F resulted in heavier fruit with higher anthocyanin concentration but lower concentrations of total soluble solids (TSS) and titratable acidity (TA). In “Stevens” 1F and 2F resulted in heavier fruit but had little effect on TSS and TA, while 1F decreased fruit anthocyanin. The effects on fruit surface color were negligible. In both cultivars, return bloom was enhanced by removing all fruit on the upright indicating a strong biennial bearing tendency of individual uprights. Fruit production strongly suppressed vegetative growth of uprights in the following year, especially in “Pilgrim.” Chlorophyll fluorescence kinetic parameters decreased in 0F, indicating lower photosynthetic efficiency in the absence of fruit. The results suggest that fruit thinning is an effective approach for increasing the individual fruit weight of cranberry crop and that cranberry fruit are strong sinks that effectively suppress generative and vegetative growth of the uprights in the subsequent year.