The 2010 ASHS Annual Conference

The strongly biennial apple variety ‘Honeycrisp’ will not bear fruit if the crop load was too high the previous season. Under Michigan conditions, the optimum crop load is 4-6 fruit per cm² trunk cross sectional area (TCA). However, adjustment to the optimum does not ensure adequate return bloom. We investigated the effect fruit per spur, fruit position within spur cluster, and seed number per fruit had on return bloom. Individual branches of ‘Honeycrisp’/M9 (10-year-old, 2008), and whole trees of ‘Honeycrisp’/Nic29 (3-year-old, 2009), were hand-thinned to 1 or 2 fruit per spur for <4, 4-6, and >6 fruit per cm2 branch (F/BCA, 2008) and tree (F/TCA, 2009) cross-sectional area, respectively. At harvest, both fruit and spur-of-origin were coded to relate individual fruit size, weight, seed number and occurrence of bitter pit, to number of return reproductive and vegetative buds. In spring 2009 (branch treatments), all vegetative (simple) and reproductive (mixed) buds per branch were counted on the tagged spurs as well as on the whole branch. 2008 branch treatment spurs with two fruit per cluster were 1) less likely to produce reproductive buds as the crop load increased, and 2) more likely to produce fruit with bitter pit than single-fruited spurs. Other interactions and differences will be discussed. Although the number of seeds per fruit for fruit of one fruit per cluster (spur) was not significantly different, from the values for two fruit per cluster, we suggest that seed number may decrease with increased crop load at a greater rate for solitary fruits than dual fruits. Single fruits were significantly heavier in weight than those grown as doubles at crop loads <4 and 4-6 fruit per TCA, but not at the higher crop load. Flowering data in the spring of 2010 will relate return bloom to previous crop size, fruit growth rate and seed number. Whole tree treatments will be evaluated in spring 2010 for return bloom.