The first opportunity to visually assess apple tree crop potential is at bloom. The fragile and ephemeral nature of apple blossoms presents an early opportunity for adjusting crop load. Several chemicals have been evaluated as potential blossom thinners, with variable efficacy. One potential source of variability is the lack of precision in timing blossom thinner applications.
Application timing of apple blossom thinners is often based on arbitrary visual estimates of the percentage of open blossoms. Common timings include 20%, 60%, 80%, 100%, or all full bloom, and single or multiple applications can be applied. Inconsistent blossom-thinning responses may be attributed to application timing because pollen tubes can reach the base of the style within 48 h (Yoder et al., 2009). Increased fruit marking and russeting were attributed to blossom thinner applications that occurred at full bloom or later (Byers, 2003). The number of blossoms that are open can vary widely within a block or tree (Byers, 2003). An inherent challenge with blossom thinning is the short period of time that growers have to apply treatments over large acreage (Moran and Southwick, 2000).
Models have been developed to estimate the rate of pollen tube growth in apple styles. Child (1966) made one of the first attempts to evaluate apple pollen tube growth rates in vivo in the cider cultivar Michelin. Detached blossoms were subjected to constant temperatures (5 to 24 °C), and pollen tube growth rates were estimated using fluorescence microscopy. Using similar techniques, Williams (1970) developed an index that estimated pollen tube growth based on temperature; however, only relatively low temperatures were evaluated (7 to 15 °C) using detached blossoms. Jefferies and Brain (1984) measured pollen tube growth rate in detached flowers at a range of controlled incubation temperatures over 24 d. A relatively complex model was produced, and the authors indicated a few shortcomings, such as the overestimation of pollen tube growth at low temperatures, and that pollen tube growth under varying temperature regimes had not been evaluated.
Efforts to model pollen tube growth rate were revisited in 2003 using attached blossoms. Mature trees on ‘M.27’ were grown in root bags and were placed in growth chambers at a range of night and day temperatures (Yoder et al., 2009), and ‘Snowdrift’ crabapple pollen was applied to emasculated king blossoms. Using this system, pollen tube growth rates of several apple cultivars were determined and modeled (Yoder et al., 2013). Maternal cultivar, temperature, and style length are inputs in the model. Although pollen genotype can influence pollen tube growth rates in vivo (Jahed and Hirst, 2017), these relationships are complex and are dependent on maternal cultivar and temperature (DeLong et al., 2016). Pollen source is not currently an input in pollen tube growth models (PTGM) and may be difficult to incorporate.
The fungicide LS was one of the first chemical constituents with recognized activity to inhibit fruit set of apple (Bagenal et al., 1925) but has only recently been used for the purpose of crop load management. LS has multiple sites of action including inhibition of pollen tube growth and reduced net photosynthesis (Pn) (McArtney et al., 2006; Yoder et al., 2009). A 2% LS + 2% fish oil (FO) application prevented pollen tubes from reaching the base of the style when applied within 24 h of the pollination event, but later applications (48 h) did not influence the number of pollen tubes that reached the base of the style (Yoder et al., 2009). Because LS inhibited growth of pollen tubes that have partly traversed the style, Schmidt and Elfving (2007) suggested that LS thinning programs may have a longer application window when compared with other products. Photosynthetic inhibition following LS + FO treatments is not considered in the PTGM.
The fertilizer ATS was shown to desiccate floral tissues of peach (Byers and Lyons, 1985) and apple (Byers, 1997). Pollen tube growth was inhibited in vitro and in vivo following ATS application (Embree and Foster, 1999; Myra et al., 2006). Schroder [2001, as cited in Schröder and Bangerth (2006)] suggested the mode of action of ATS is a combination of damaged floral tissue and reduced photosynthesis due to leaf injury. In some experiments, ATS caused unacceptable leaf phytotoxicity (Byers, 1997; Embree and Foster, 1999), which resulted in reduced fruit growth (Wertheim, 2000). Conversely, Schmidt and Elfving (2007) suggested that ATS primarily influences blossoms that have recently opened and have not received pollen, and applications early in the flowering period (≈20% bloom) increased efficacy (Bound and Wilson, 2007). However, ATS was a potent inhibitor of pollen tube growth in vivo, and reduced pollen tube growth when applied 12 h before (Myra et al., 2006) or 24 h after pollination (Embree and Foster, 1999). Multiple applications of ATS at low rates were more effective than a single application (Bound and Wilson, 2007). Given the assumptions of the PTGM, model-based application timing may improve the consistency of ATS applications.
An aquatic herbicide, ET [7, oxybicylo (2,2,2)heptane-2-3 dicarboxylic acid] has been evaluated as an apple blossom thinner since 1993. The mode of action of ET was assumed to be as a desiccant (Williams et al., 1995) and it reduced the number of pollen tubes that reached the style base when applied 24 h after pollination (Embree and Foster, 1999). Two applications of ET during bloom reduced fruit set and improved fruit size when compared with a single application (Bound and Wilson, 2007; Greene, 2004; Williams et al., 1995), although this relationship was not consistent (Byers, 1997). Because the proposed mode of action of ET is similar to ATS, model-based application timing may be of benefit.
Naphthaleneacetamide is a hormonal thinner developed in the 1930s. NAD was developed as a milder analog of 1-napthaleneacetic acid thinner for use on summer ripening cultivars and has efficacy at bloom (Greene, 2002). Although NAD use has been limited in recent years, there is renewed interest in the use of NAD as a bloom thinner in the eastern United States (Greene et al., 2015). Greene et al. (2015) observed thinning activity at bloom and petal fall, with no observed dose response. Because activity of hormonal thinners is not linked to desiccation of stylar tissues, PTGM may not be suitable for timing NAD sprays.
The objective of these studies was to evaluate the efficacy of several blossom-thinning chemistries, using predictive models developed for ‘Golden Delicious’ and ‘Gala’ to determine application timing. Blossom-thinning treatments were evaluated as the sole method of ‘Golden Delicious’ crop load management, and as part of a ‘Gala’ crop load management program in a commercial orchard.
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