OPTIMIZATION OF POLLINATION AND FRUIT SET IN NORTHERN HIGHBUSH BLUEBERRY

Thesis (Ph.D.), Horticulture, Washington State UniversityFruit set in northern highbush blueberry (Vaccinium corymbosum L.) can be low under certain climatic conditions, contributing to reduced yields in northwestern Washington. Little is known about the mechanisms of reduced fruit set and yields in this important production region, but it may be associated with inadequate pollination, and/or nutrient deficiencies. Enhanced nutrient availability during critical stages of flowering, ovule fertilization, and fruit development may lengthen the pollination window or improve pollen germination and tube growth. Calcium (Ca) and boron (B) are of particular interest for reproductive developmental processes. Additionally, increased honey bee (Apis mellifera) hive stocking densities and provision of supplemental bumble bees (Bombus vosnesenskii) may improve pollination. Commercially available fertilizers containing Ca and B were applied to ‘Draper’ and ‘Bluecrop’. In another experiment, the recommended honey bee stocking density of 10 hives/ha was compared to a high stocking density (20 hives/ha) in ‘Duke’ and to bumble bee supplemented pollination (10 honey bee hives + 8 colonies of bumble bee/ha) in ‘Draper’. Flower morphology and the impact of shade and Ca on fruit drop were also studied. No significant increases were observed for fruit set, estimated yield, fruit firmness or berry weight in the Ca and B study. Honey bee visitation was increased with high honey bee stocking densities. Number of seeds per berry, berry weight and estimated yield were also significantly increased with high honey bee stocking densities. No significant differences were realized with bumble bee supplementation and inconsistent sourcing prevented us from repeating the experiment for two years. Floral morphology differed across cultivars, with the new ‘Top Shelf’ having a split corolla that may impact pollination. Shade and foliar Ca increased and decreased ‘Draper’ fruit drop, respectively, demonstrating Ca may be a tool to reduce fruit drop. This thesis shows pollination and subsequent yields can be increased through modified hive densities and that, while B and Ca do not improve pollination and fruit set, Ca can reduce premature fruit drop that occurs in certain cultivars.Washington State University, Horticultur