2018 ASHS Annual Conference

‘Honeycrisp’ apple is very susceptible to bitter pit, a physiological fruit disorder related to calcium deficiency. The main strategy for mitigating the problem has been increasing root Ca uptake and subsequent partitioning to fruit, but how rootstocks affect the balance of Ca with other nutrients and bitter pit incidence has not been examined in detail. The objectives of this work are 1) to compare fruit Ca level and its balance with other nutrients in flesh and peel tissues between ‘Honeycrisp’ and a bitter pit resistance cultivar ‘Gala’ across rootstocks; and 2) to determine rootstock effects on bitter pit incidence of ‘Honeycrisp’ in relation to tissue Ca level and its balance with other nutrients. By using mature trees of both ‘Honeycrisp’ and ‘Gala’ on four rootstocks (M.9, B.9, G.11 and G.41) in an existing field trial, we found that ‘Honeycrisp’ had much higher ratios of K/Ca, Mg/Ca, (K+Mg)/Ca and P/Ca than ‘Gala’ in both fruit flesh and peel tissues at harvest. The higher ratios in the flesh were primarily caused by lower Ca concentrations in ‘Honeycrisp’ but those in the peel also resulted from significantly higher concentrations of K and P in ‘Honeycrisp’. Among the four rootstocks evaluated, ‘Honeycrisp’ trees on B.9 had the lowest bitter pit incidence whereas those on G.11 had the highest incidence, with trees on M.9 and G.41 being in the middle. The incidence of bitter pit in ‘Honeycrisp’ was not correlated with tissue nutrient concentrations, but corresponded well to the ratio of (K+Mg+P)/Ca in the peel. These findings indicate that the imbalance of Ca with K, Mg and P is closely associated with bitter development and rootstock has a significant impact on bitter pit incidence.