Managing Apple Crop Load and Diseases with Bloom Thinning Applications in an Organically Managed ‘Honeycrisp’/‘MM.111’ Orchard

Abstract

Although demand for organic produce continues to increase in the mid-Atlantic, few apple (Malus ×domestica Borkh.) growers in the region have adopted organic management practices due to the considerable disease, insect, and weed pressure, as well as the lack of effective crop load management tools. In this study, lime sulfur (LS) and Regalia® (R) were applied in different sequences (i.e., LS/LS, LS/R, R/R, and R/LS), each in a mixture with JMS Stylet-Oil, to chemically thin apple flowers in an organically managed ‘Honeycrisp’/‘MM.111’ orchard. There was also a nontreated control, a “grower standard” control (LS at 11 mm fruitlet diameter), and a hand-thinned control. The treatments were evaluated for their ability to reduce crop load, as well as to control powdery mildew [Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon], cedar apple rust (Gymnosporangium juniperi-virginiana Schwein.), and quince rust (Gymnosporangium clavipes Cooke & Peck). All treatments reduced crop load compared with the nontreated control, and after the first application of LS or R, the number of fertilized king blooms was reduced and fertilization was prevented in all side blooms. All bloom thinning treatments had more fruit peel russet than the control and russet was more severe when LS was one of the applications. Bloom thinning applications of LS and R did not reduce powdery mildew leaf infection compared with the nontreated control. Cedar apple rust incidence was reduced by all bloom thinning treatments, though some lesions were detected in all treatments. There were minimal quince rust infections in any of the treatments, including the nontreated control. These results suggest that when LS and/or Regalia® are mixed with JMS Stylet-Oil and applied as bloom thinners, they can reduce crop load, and, as a secondary benefit, they can also decrease cedar apple rust incidence from infections that occur during bloom.