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

Abstract

Additional index words. apple scab, cedar apple rust, Japanese knotweed, liquid lime sulfur, powdery mildew, quince rust Abstract. Although demand for organic produce continues to increase in the midAtlantic, few apple (Malus 3domestica 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 Ò (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 Ò 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.There is a need to conduct research specifically for organic apple (Malus ·domestica) production in the mid-Atlantic region (including the states of Delaware,

Evaluation of Blossom Thinning Spray Timing Strategies in Apple

“…This study demonstrated that use of the 'Gala' PTGM reduced fruit set, yield, fruit number, and estimated yield per acre at both sites in most cases (Tables 2-4). This is in agreement with other research studies [18,25] which demonstrate that the PTGM is a reliable method for timing blossom thinning sprays. Additionally, lime sulfur sprays applied at 20% open bloom and 48 h later performed near equally well as the PTGM treatment in reducing fruit set, crop density, kg fruit/tree, fruit no/tree, and estimated yield per acre in most situations.…”

“…Lime sulfur effects on fruit russeting has varied between different studies and is likely dependent on multiple factors. Stopar [30] found that increasing concentrations of lime sulfur did not increase the occurrence of fruit russet, whereas Peck et al [25] found that model-guided lime sulfur blossom thinning sprays resulted in a significant increase of fruit russet incidence. The adjuvant used in lime sulfur sprays can also have significant impacts on fruit russet incidence [29].…”

Effects of Bloom Thinning with Lime Sulfur on Fruit Set, Yield, and Fruit Quality Attributes of ‘RubyS’ Apples

“…The reduction of fruits per tree (crop load) in bloom-thinned trees is due to the lime sulfur-induced reduction in the number of fruits within each fruiting cluster. Similar results were also reported in previous studies (Yoder et al, 2013;Peck et al, 2017). Additionally, a recent study reported significantly larger fruit weight and size in bloom-thinned trees at harvest, along with a lower yield and number fruits per tree, compared with untreated trees (Allen et al, 2021).…”

“…The application timing of thinning in apple trees is important to achieve the optimum result (Jones et al, 1992;Dennis, 2000;Guak et al, 2004), and the earlier thinning time was more effective than the later thinning time (Bayers and Carbaugh, 2002;Allen et al, 2021). In general, lime sulfur thins flowers by inhibiting pollen germination, pollen tube growth, and fertilization (Peck et al, 2017;Schupp and Kon, 2019). Inconsistent bloom thinning responses may be attributed to application timing since pollen tubes can reach the base of the style within 48 hours (Yoder et al, 2009).…”

“…Inconsistent bloom thinning responses may be attributed to application timing since pollen tubes can reach the base of the style within 48 hours (Yoder et al, 2009). Thus, delaying application by one or two days can result in failure of the bloom thinner treatment because the treatment would have little or no effect on fertilization and fruit set of earlier pollinated flowers (Yoder et al, 2009;Yoder et al, 2013;Peck et al, 2016;Peck et al, 2017). However, the growth rate of pollen tubes may vary by cultivar and temperature (Yoder et al, 2009;Yoder et al, 2013).…”