Integrating Crop Load Management with Disease Control in an Organically Managed Apple Orchard

The lack of effective crop load and disease management practices are major barriers to increased production of organic apples (Malus x domestica Borkh.) in the Eastern U.S. Labeled rates of lime sulfur or Regalia^®, a Giant Knotweed [Reynoutria sachalinensis (F. Schmidt) Nakai syn. Polygonum sachalinense F. Schmidt] extract, were applied as flower thinning agents in an organically managed ‘Honeycrisp’/‘M.M.111’ orchard. Application timing was based on a ‘Honeycrisp’-specific, temperature-based pollen tube growth model. There were four flower thinning treatments, each of which consisted of two separate applications—lime sulfur followed by lime sulfur or Regalia, and Regalia followed by lime sulfur or Regalia. There was also an untreated control, a hand-thinned control, and a “grower’s standard” control of lime sulfur applied when fruitlet size was on average 10 mm dia. These treatments were also evaluated for their early season disease control of powdery mildew [Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon] and cedar-apple rust (Gymnosporangium juniperi-virginianae Schwein.). All thinning treatments reduced crop load compared to the non-treated control, and after one application of lime sulfur or Regalia^®, the number of fertilized king blooms was reduced and zero percent of side blooms had been fertilized. All chemical thinning treatments resulted in fruit russeting. Russeting was more severe when lime sulfur was used as one of the bloom thinning applications. All treatments had fewer leaves with cedar-apple rust infection than the untreated control. These results suggest that bloom thinning sprays of lime sulfur and Regalia^® could reduce crop loads and decrease cedar-apple rust infection. Additionally, bloom thinning applications could take the place of one or more fungicide sprays, leading to an overall reduction in the amount of fungicides used for organic apple production.