Screening of eco-friendly thinning agents and adjusting mechanical thinning on ‘Gala’, ‘Golden Delicious’ and ‘Fuji’ apple trees

Abstract

Fruit thinning is the most important yet difficult practice that drives orchard profitability. High labor costs and difficulty to improve return bloom by hand thinning have left chemical thinning as the main method used by growers. However, unpredictability and safety/environment concerns regarding chemical thinning have set mechanical thinning as a sound alternative. Thirteen field experiments were performed during 2004-2016 in order to evaluate several agents for their use as new thinners, and adjust mechanical thinning on 'Gala', 'Golden Delicious' and 'Fuji'. Olive oil applied at bloom reduced crop load, but russetting was also increased. Therefore, while their use is not advisable for russetting prone cultivars such as 'Golden Delicious', it could be a good thinner for cultivars like 'Red Delicious'. Lime sulfur did not have a consistent thinning effect in our study when applied at bloom. Overall, no differences regarding economic value between hand, chemical, and mechanical blossom thinning were observed, suggesting mechanical thinning as a valid alternative approach. For 'Gala' strains, 6 km•h-1 and 250 rpm with 270 strings was the best configuration to provide an ideal crop load of ~6 fruit/cm 2 of TCSA and an average fruit size of 170 g. For 'Fuji', 5 km•h-1 and 320 rpm with 270 strings provided a crop load in accordance to the optimum range for this cultivar in our conditions. However, combination of mechanical thinning plus chemical treatments might be the ideal strategy for 'Fuji' strains when the initial number of flower clusters per tree is above 500. For 'Golden Delicious' strains, 6 km•h-1 and 230 rpm with 270 strings was the best configuration to provide an ideal crop load within the optimum range. Mechanical thinning timing was also examined at different phenological stages (E2, F1, F2, and G), with no significant differences regarding yield, fruit size or crop load between them. Two prediction models ('Gala' & 'Golden Delicious') were developed to adjust the right tractor and rotational speeds depending on the initial number of flower clusters. The method begins with first calculating the final fruit number needed per tree (crop load for each particular cultivar) in order to achieve the desired yield. Then, tractor and rotational speeds can be determined by the model once knowing the initial number of flower clusters per tree.

Effect of different application rates of metamitron as fruitlet chemical thinner on thinning efficacy and fluorescence inhibition in Gala and Fuji apple

“…Crop load management on apple trees remains a significant challenge to producers (Cline et al, 2018). Crop thinning is a vitally important but difficult agricultural practice that has a significant impact on orchard profitability (Lordan et al, 2018;Robinson et al, 2016). Apple flowers are initiated the year prior to bloom, and inadequate thinning can result in biennial bearing (Cline et al, 2018).…”

“…Hand thinning is costly in terms of labour and time-consuming. It requires waiting until the natural drop is complete, which often occurs late and may consequently affect fruit size and the return bloom (Lordan et al, 2018;McArtney et al, 1996). Mechanical thinning can be a valuable tool to initially reduce crop load prior to chemical or hand thinning (McClure and Cline, 2015).…”

“…Finally, chemical thinning is a commonly used practice because it acts early on the fruit and reduces production costs. However, its efficacy is variable as its use is dependent on climatic conditions and cultivar (Byers, 2003;Gonzalez et al, 2019b;Lordan et al, 2018;Robinson and Lakso, 2004). Currently, in Spain, chemical thinning can be carried out during flowering (naphthalene acetamide (NAD)) and after fruit set on young fruitlets at the 6-16 mm stages (using the hormones 6-benzyladenine (BA) and naphthyl acetic acid (NAA)).…”

Improving of Cherry Fruit Quality and Bearing Regularity by Chemical Thinning with Fertilizer

“…Mechanical thinning is an alternative to manual thinning, which is a laborious process [20,21] and which is increasingly difficult to apply from year to year due to the shrinking availability of workforce [22][23][24]. Mechanical thinning is typically applied to apples, pears and peaches [25][26][27][28][29]. However, the first devices were designed for the treatment of trees trained as narrow spindle, axis or fruit-bearing wall rather than for trees maintained in a broad spindle or vase shape [30].…”

Assessing and Minimizing the Development and Spread of Fire Blight Following Mechanical Thinning and Pruning in Apple Orchards

“…Chemical thinners include lime sulfur or plant hormones such as carbaryl (a carbamate insecticide), benzyl adenine (a synthetic cytokinin), or naphthaleneacetic acid (a synthetic auxin) alone or in combination (Cline et al 2019;Greene and Costa 2013;Petracek et al 2003;Sazo et al 2016). However, chemical thinning is problematic because of the often erratic results related to weather conditions, cultivar, phytotoxicity for bloom applications of caustic materials in humid climates, and restrictions on many of these products related to concerns about food safety and environmental protection (Cline et al 2019;Greene and Costa 2013;Lordan et al 2018a). Carbaryl was completely banned in Europe in 2008; in the United States, its use as an insecticide was recently removed from the label, and its use exemption for fruit thinning is likely to expire in the coming years (He and Schupp 2018).…”