2019 ASHS Annual Conference

In northern highbush blueberry (Vaccinium corymbosum), bloom to fruit maturity is a period of rapid growth and nitrogen (N) uptake. Growers are advised to ensure that sufficient N is available to plants during this period of growth and often accomplish this through fertilizer applications from bloom through fruit development. In northern climates like Washington State, postharvest applications of N fertilizer is not recommended in highbush blueberries because it may stimulate excessive vegetative growth, reduce floral bud set, and increase the risk of winter injury through delayed acclimation. However, postharvest N applications in perennial crops like blueberry may positively contribute to the N storage pool, which could be used later and re-allocated according to plant demand in subsequent years. Early-fruiting cultivars with the potential for an extended growing season after harvest may particularly benefit from postharvest N applications, as the additional N may promote shoot and root growth that could support fruit production in future years while still allowing plants to form floral buds and acclimate to winter temperatures. The objective of this study is to assess the impact of postharvest N applications in early-fruiting ‘Duke’ blueberry on plant vegetative growth, nutrient status, yield, fruit quality, floral bud set, and cold hardiness. Four treatments varying in timing of N application were evaluated using a single fertilizer rate of 129 kg·ha^-1 N in 2018. Treatments were arranged in a randomized complete block design with four replications and included: 1) Control (100% of N applied pre-harvest); 2) 80/20 (80% pre-harvest, 20% post-harvest); 3) 70/30 (70% pre-harvest, post-harvest); and 4) 60/40 (60% pre-harvest and 40% post-harvest). Overall, no significant differences were observed in any of our measured variables. Leaf tissue N concentrations were at 1.40-1.69%, which is below the recommended range of 1.76-2.0%. However, no indications of N deficiency were observed in the plants and this range is common in the region where the study was conducted. Plants receiving the control treatment tended to acclimate sooner, but no differences in winter injury was observed in the field. The overall lack of statistical differences was expected as woody perennials with stored nutrient pools take time to respond to fertilizer treatments and this was the first year of the study. While this study is in progress and will be repeated in 2019, the results will optimize nutrient management practices and guide growers on optimal timing of N applications for early-fruiting blueberry cultivars.