Bitter pit is a physiological disorder of apple linked with Ca deficiency and characterized by dark lesions on the surface of fruit. The commercially important cultivar 'Honeycrisp' is highly susceptible to the disorder. In-planta Ca transport occurs exclusively in the xylem; therefore, Ca deficiency in fruit is primarily the result of gradual xylem dysfunction. Because auxin promotes xylogenesis, we hypothesized that early-season applications of both native and synthetic auxins and ABA would improve xylem longevity and functionality resulting in higher Ca concentrations in fruit and reduced bitter pit. Using several experimental approaches, we demonstrate that auxins and ABA significantly prolong xylem function and markedly reduce bitter pit incidence. A multiple regression model was also developed to correlate temporal climatic data with bitter pit incidence over multiple years and disparate sites throughout Michigan. Collectively, results suggest potential novel uses of plant growth regulators to mitigate bitter pit.
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