Thinning the apple crop early by mechanical or chemical means is beneficial to reduce fruit set, crop load, and the high labour costs associated with hand thinning after natural fruit abscission. In addition, thinning early to improve fruit size and return bloom, particularly of biennial bearing cultivars such as Honeycrisp, is important.
There is increasing grower interest in thinning multiple times during the spring, starting with judicial pruning, followed by blossom thinning, and finally fruitlet thinning. Many growers are already doing this with the overall goal of early thinning to enhance flower bud initiation for the flowing season’s crop, and to reach a target crop load with minimal hand thinning. Managing the crop load of apples remains a significant challenge to producers, in part because of the unpredictability of fruit set and fruitlet abscission.
I have conducted blossom thinning experiments over the past decade and decided to revisit these efforts with a study in spring of 2021.
In 2020, a 4-yr-old block of ‘Brookfield Gala’/M.9 T337 rootstock located at the University of Guelph Horticultural Experiment Station, Simcoe, ON was used for this study. Trees were spaced 0.90 m x 3.5 m (3175 trees ha-1) and trained to a spindle solid hedgerow orchard system. Treatments consisted of:
- Untreated control
- Hand thinned control (flower clusters were singled, fruit space ~10 cm apart)
- 75 mg/L 6-BA (Maxcel, Valent Bioscience. Guelph, ON) combined with 750 mg/L Carbaryl (Sevin XLR; Tessenderlo Kerley Inc., Phoenix, Arizona) applied at 12 mm king fruitlet diameter
- 2% ammonium thiosulphate (ATS; 12-0-0-26S Norwich Fertilizer, Norwich, ON) at bloom
- 2% ATS at bloom follow by 75 mg/L 6-BA combined with 750 mg/L Carbaryl applied at 12 mm king fruitlet diameter
- 2.5% lime sulphur (LS; Oligo-S, Axter Agroscience, Mont-St-Hilaire, QC) combined with 2% (v/v) mineral oil (Purespray Green Spray Oil 13E, Intelligro, Mississauga, ON) at bloom
- 2% (v/v) ammonium thiosulphate combined with mineral oil followed 75 mg/L 6-BA combined with 750 mg/L Carbaryl applied at 12 mm king fruitlet diameter.
All spray treatments included 0.05% Regulaid® non-ionic spray adjuvant. The experimental design consisted of a randomized complete block with 5 replications and 7 treatments. All sprays were applied using a commercial air blast sprayer at 1379 kPa, 1217 L ha-1, which equated to tree row volume (TRV) pesticide dilute. To minimize treatment interference caused by spray drift, experimental units were separated by at least one guard tree.
Blossom thinners will be applied at approximately 30% full bloom when most of the king flowers were open but prior to lateral flowers opening. A propriety computer model, known as the pollen tube growth model, was used to time when to apply the blossom spray (Peck et al, 2016; Peck and Olmstead, 2018). Using cultivar specific pollen tube length, pollen growth rate, and air temperature, the model predicts the time required for the flowers to be fertilized after the pollen reaches the stigma. In addition, the predicted time to apply blossom thinners is based on the desired number of flowers to be fertilized per tree, which is approximate 25% higher than the desired number of fruit per tree.
Fruitlet thinners were timed by periodically measuring the longitudinal diameter of 50 king and 50 lateral fruits (5 fruits of each across 10 trees) using digital calipers. The date of full bloom was 24-May 2020. Only the hand-thinned control trees were thinned on 8 July 2020 by removing all but one fruit per cluster and spacing fruit ~10 cm apart.
Tree trunk circumference at 30 cm above the graft union was measured at the beginning and end of each growing season, from which trunk cross-sectional area (TCSA) was calculated. Four scaffold branches—two on the east and two on the west side of the tree—were selected prior to bloom to determine fruit set. On 13-May 2020, the number of flower clusters per branch were counted on each marked limb. The number of fruit set per limb were counted again after natural abscission (‘June drop’) on 24-June 2020. These data were averaged and used to calculate percent fruit set (number of fruit set divided by number of flowers).
Leaf fruit phytotoxicity was assessed 14 June 2020 by rating the incidence and severity (0=none, 1= very slight, 2= slight, 3= moderate, 4= high, 5= very high), the incidence and severity of leaf necrosis (0=none, 1= very slight, 2= slight, 3= moderate, 4= high, 5= very high).
Fruit were harvested on 14-Sept 2020. During harvest, the total number and weight of fruit was recorded. The number of unmarketable fruit (undersize, poor colour, premature fruit drop) were also counted and weighed. Mean fruit size was estimated by dividing total mass of marketable fruit by the number of fruit in the sample. A random sample of 20 kg fruit per tree was taken from each experimental unit and in cold storage (~2oC) for subsequent grading on a commercial colour sorting and sizing grading line in November 2020.
There was a significant treatment effect on fruit set (P<0.0001) (Figure 1). The untreated control and ATS blossom thinning treatments had the highest fruit set, whilst the LS blossom thinning followed by the CB and 6-BA fruitlet thinning treatment had the lowest fruit set. The post-bloom fruitlet treatment, LS blossom treatment and ATS blossom thinning followed by CB and 6-BA fruitlet thinning treatment resulted in moderate reduced fruit set. Fruitlet thinning alone reduced fruit set by 54% compared to the untreated control. ATS alone was not effective in reducing fruit set, while trees treated with LS and oil had 37% lower fruit set compared with the untreated control. The addition of the fruitlet thinner following the ATS and LS blossom thinning treatments decreased fruit set, but the response was sufficiently variable that the effect was not statistically significant.
In 2020, based on the relatively low crop load of 4.2 fruit TCSA-1 for the untreated control trees, in retrospect all trees required little additional blossom or fruitlet thinning apart from singling fruiting clusters. The light fruit set can be attributed to a spring frost in early May. Trees during this period were in the tight cluster stage, were subject to air temperatures that fell below -2˚C. On 8-May, 9-May, 12-May, and 13-May minimum temperatures fell to -1.8˚C, -3.4˚C, -1.2˚C and -1.9˚C, respectively. Notwithstanding, total yield, marketable yield, total number of fruit per tree and crop load were highest from the untreated control trees, followed by the ATS blossom thinning treatment (Figure 2).
Trees receiving LS alone, or LS or ATS followed by CB and 6-BA, or CB and 6-BA alone had the lowest total yield, marketable yield, total number of fruit per tree (Figure 2), and crop load. Generally, treatments that resulted in the greatest degree of fruit thinning also produced the lowest yield, number of fruit and crop load. The percent of marketable fruit (P=0.693) and adjusted mean fruit weight (P=0.3217) (Figure 3) was unaffected by the thinning treatments. However, it is notable that fruit weight from trees treated with LS, without CB and 6-BA, was numerically (but not statistically; P=.0697) lower than any of the other treatments, even though they were thinned that greatest.
Leaf curling and necrosis was observed in all treatments, including the untreated control, but severity was low, ranging from very slight to slight (data not shown). The severity of leaf curling, however, was similar among all treatments. Leaf necrosis was more prevalent in treatments that were bloom thinned compared to the untreated control. Bloom thinning increased the incidence of necrosis compared to fruitlet thinning (P=0.05). Trees blossom thinned with ATS, with or without fruitlet thinning, exhibited significantly greater severity of leaf necrosis sever compared to untreated trees. On average, leaf necrosis was also more severe in trees that were bloom thinned compared to trees that were fruitlet thinned (P=0.001). Some leaf phytotoxicity is expected when using caustic blossom thinners, buy level overall was low and likely not of any significant commercial importance.
- Lime sulphur plus oil effectively reduced fruit set and number of fruit per tree.
- 2% ATS did not reduce fruit set when applied alone compared with the untreated and hand-thinned control treatments. Perhaps higher rates are required to obtain adequate reductions if fruit set on Gala, but since the data are preliminary, this remains inconclusive.
- When the blossom thinner ATS and LS were combined with the fruitlet thinners (carbaryl and 6-BA) applied at 12 mm, fruit set and number of fruit per tree were reduced further
- Leaf phytotoxicity was slight on trees treated with ATS and lime suphur plus oil. There was no difference in the amount of leaf necrosis on trees treated with 2% ATS and those treated with 2.5% lime sulphur plus 2% oil.
- In 2020, based on the relatively low crop load of 4.2 fruit TCSA-1 for the untreated control trees, in retrospect all trees required little additional blossom or fruitlet thinning apart from singling fruiting clusters. The light fruit set can be attributed to a spring frost in early May.
- The preliminary results indicate when early spring frost damage occurs, caution should be exercised when using blossom thinners as over-thinning can occur. Fruitlet thinners can be used during the 6-15 mm window if thinning is deemed necessary.
- One observation of potential concern is the reduced fruit weight in trees treated with LS. While the difference was not significant when compared with the other treatments (P=0.32), we would expect fruit weight to increase with decreasing crop load.
- These data are preliminary, and therefore must be interpreted with caution. Annual variations in thinning response can occur, and cultivars may also respond differently to the blossom thinner compounds and rates used in this study.
This blossom thinning research will continue for another two years (with Erika DeBrouwer), and we will have more to report in the future on the benefits of blossom thinning with ATS and LS and using the pollen tube growth model.
This research is part of a project supported by the University of Guelph Ontario Agri-food Innovation Alliance program with industry support by the Ontario Apple Growers, Adama Canada, Valent BioSciences, and the Norfolk Fruit Growers’ Association (NFGA). Special thanks to Dr. Greg Peck of Cornell University for assistances with the pollen tube growth model and Hayden Dooney (NFGA) for assistance with grading fruit.
Peck, G.M., D. Olmstead. 2018. Implementing the Pollen Tube Growth Model on NEWA. Fruit Quarterly 26:11-15.
Peck, G.M., L. D. Combs, C. DeLong and K.S. Yoder. 2016. Precision apple flower thinning using organically approved chemicals. Acta. Hortic. 1137: 47-52.