Utility of Parthenocarpic Interspecific Hybrids Between Vaccinium corymbosum and Vaccinium virgatum for Breeding Blueberry Cultivars Suitable for Cluster Harvesting

Blueberries are handpicked because of irregular maturation of fruits within clusters. Therefore, harvesting is labor intensive and results in a short shelf life of the product. Cluster harvesting, as performed for grapes, could solve these problems. Previously, we produced many interspecific hybrids between the highbush blueberry (HB; Vaccinium corymbosum L.) and rabbiteye blueberry (RB; Vaccinium virgatum Aiton) and found that some individuals were parthenocarpic and maturation of berries within fruit clusters was relatively uniform. In the present study, we investigated the degree of parthenocarpy and suitability of the hybrids for cluster harvesting and considered the use of parthenocarpic hybrids to breed cultivars for cluster harvesting. The degree of parthenocarpy was evaluated in 21 blueberry plants (hybrids and cultivars) based on the average fruit set without pollination. It was found that 2 hybrid individuals had the highest frequencies of parthenocarpy and their fruit set and weight were close to those of pollinated fruits; moreover, pollinated fruits of these hybrids were seedless. Thereafter, the uniformity of flowering and fruit maturation was evaluated, and the 2 hybrids showed a relatively uniform maturity of fruits in a cluster; furthermore, fruit dropping occurred later. In addition, correlation analysis was performed for 5 parameters related to parthenocarpy and suitability for cluster harvesting. For seeded fruits, it was found that the flowers that bloomed earlier in a cluster formed a fruit containing more seeds and matured earlier. Comparison of cluster and individual harvesting showed that the percentages of mature fruits within clusters in the 2 hybrids were markedly higher than those in the HB cultivars. Moreover, cluster harvesting of the hybrids greatly shortened the working time needed for harvest and subsequent fruit sorting to 59% of that of individual harvesting. Thus, the 2 hybrids that had a higher degree of parthenocarpy than that in the existing blueberry cultivars were assessed to be highly suitable for cluster harvesting. Factors underlying this suitability may be related to the seedlessness of interspecific hybrids. These results suggest that interspecific hybridization between HB and RB is useful as a breeding method to produce cultivars suitable for cluster harvesting.