Evaluation of new thinning agents versus hand thinning in ‘Fuji’ apple

C. Fernandes
ECOFRUTAS Lda
Rua dos Arelhais,29
Sobral do Parelhão
Bombarral, Portugal
C.M. Oliveira and M. Mota
CEER – Centro de Engenharia dos Biossistemas
Instituto Superior de Agronomia
Technical University of Lisbon
Lisboa, Portugal
Keywords: ammonium thiosulphate, 6-benzyladenine, crop load, Malus domestica, metamitron

Abstract
The chemical agents ammonium thiosulphate (ATS) (1%), 6-benzyladenine (BA) (100 ppm) and metamitron (low, medium, and high concentration) were evaluated as thinning agents on 3 years old ‘Fuji’ grafted onto M9 apples trees. The products were used alone, at single or double application or in combination. There were no differences between treatments regarding yield, mean fruit weight and number of commercial fruits (70 – 85mm) but there were differences concerning the greater and smaller fruit size classes. The fruit set was reduced by all the treatments, but when considering metramitron action on different crop loads an over thinning effect was observed at high concentration in trees with low crop load. Weather conditions during the application period of ATS+BA might influence the thinning results. Hand thinning, leaving one fruit per cluster spaced of 15 cm is a good option but labour costs make it impractical.

INTRODUCTION
‘Fuji’ fruit meets the main qualities that are required in an apple: fruit size (75-80mm), high quality taste and good storage capacity. Flowering is abundant in one year old wood and in spurs and these flowers have an excessive fruit set, displaying four or more fruits per flower cluster, which results in excessive production. The high production of flowers necessary to obtain a good and regular yield makes thinning a compulsory practice in this particularly hard to thin cultivar. Manual thinning is, nowadays, a very time consuming and costly operation for large areas, but due to its great selectivity can be used complementary to chemical thinning. Moreover, the option for manual thinning can lead to excessive thinning because the fruit has a small stem attached making it difficult to thin. On the other hand, under mild winter conditions flowering tend to be irregular and long-lasting, fruit set is irregular and the use of chemical thinning after flowering is safer than during the flowering period, which makes the active substance selection crucial for a successful thinning. There are several chemicals that have been used, with different modes of action and a wide range of application timing from full bloom to fruit set, ranging from 4-5 mm to 18-20 mm in diameter, depending on the active ingredient, indole acetic acid (IAA), ethephon, naphthaleneacetamide (NAD), 1-naphthyl-N-methylcarbamates (carbaryl), naphthalene acetic acid (NAA), and NAA amide, the 6-benzyladenine purine and gibberellins (GA4+7). More recently, studies have been carried out by Dorigoni and Lezzer (2007), Clever (2007) 1 Janoudi and Flore (2005) and Maas (2007), among others, seeking alternatives to carbaryl, since its withdrawal from the market opened a gap in terms of efficiency and ease. The purpose of this study was to test the effect of ammonium thiosulphate (ATS), metamitron and 6-benzyladenine (BA) in three years old Fuji apple trees. Trees with low and high initial fruit set were thinned with metamitron at different concentration, in order to evaluate the effect of crop load in its effectiveness.

MATERIAL AND METHODS
A field experiment was carried out in 2010 in a commercial orchard near Alcobaça, Portugal, using three years old ‘Fuji’ Kiku® 8 Frubax grafted on M9. The trees were spaced of 4.0 m x 1.5 m and trained as central axis. The treatments are listed in table 1. Metramitron was tested in trees that had very different initial fruit set at 1st of May and two crop levels were considered: high and low crop level corresponding to 200 fruitlets and 113 fruitlets per 100 clusters, respectively or corresponding to 21.4 and 9.1 fruits per cm2 branch sectional area, respectively. The experimental design was set in a three randomized blocks with eight trees per block. Regarding timing of application, the products were applied accordingly to manufacturer’s recommendations. In two labelled branches per tree branch diameter was recorded. Number of flower clusters, number of fruits set per cluster, fruit number (at 1, 16 of May, 22 of June and at harvest) was recorded. Fruit growth was measured on two labelled representative fruits on each tree, for each treatment and replication (8 trees x 3 block x 2 fruits) starting when the fruits were approx. 40 mm in diameter (22 June), and was carried out fortnightly until harvest. At harvest, for each tree, weight and fruit diameter was recorded, a random sample of
100 fruits per treatment was analysed for fruit firmness, soluble solids contents (SSC) and titratable acidity (TA). SSC (%) was measured using an Atago hand-held refractometer, on juice collected during the measurements of flesh firmness. Firmness was measured using a penetrometer (Effegi model 327) with an 11 mm probe. For starch index determination, apples were cut in half at the equator, immersed for 2 min in 3% iodine–potassium iodide and rinsed with fresh water. Starch pattern index was measured using a Center Technique Interprofessionnel des Fruits et Legumes (CTIFL) scale. TA was determined in a 10 ml fruit juice and titrated against 0.1 N NaOH solution after adding 2-3 drops of phenolphthalein as indicator. The number of hours under 7.2º C i.e. accumulated chilling hours from 1st October to 15th February was 451 h. Bud break was on the 22nd March, full bloom was on 12 April, fruit set started on the 3rd May and harvest on the 1st October. Standard orchard management practices such as pest and disease management control and nutrition applications were conducted according to commercial standards. Data was analyzed by one way ANOVA, and the means compared by Tukey test (α= 0.05) using Statistix 9.

RESULTS AND DISCUSSION
Twelve hours after the application of ATS flower petals and leaves exhibited significant damage but the symptoms were not severe as a consequence of flowering delay and initial development of small leaves due to the lack of chilling. ATS has been reported to 2 cause leaf burning in apple when applied at concentration of 1.5% or more (Fallahi and Willemsen, 2002). Concerning metramitron, during the first 10 days after the metramitron application any adverse symptoms were observed but after 15 days, the older leaves showed severe symptoms of toxicity, these symptoms are characterized by large necrotic spots affecting between 30-40% of leaf. By the end of the trial metamitron had no detrimental effect on growth of the fruit or the tree. According to Anderson and Crocker (2009), ‘Fuji’ has 575 winter chill hours required for normal bloom and foliation and in the year of the trial this requirement was not fulfilled resulting in both temporal and spatial heterogeneity of budbreak and flowering. In the beginning of the trial the number of flowers was not statistically significant between treatments (Table 2), but the number of fruits per branch sectional area varied according to the treatments’ timing. At 22 June, after June drop, there were differences between the trees with high crop load treated with metramitron at medium concentration and all the other treatments, the treatment seemed to be not effective since the number of fruits was not statistically different from not thinned tress. This trend was the same at harvest, for metramitron at the concentration of medium concentration there was an effect of crop load, but at high concentration the action of metamitron was independent of the initial load of fruit. The ATS+ BA and metramitron at high concentration in trees with initial low crop load treatments resulted in lowest fruits number per branch sectional area. As the trees were only three year old the effect of the thinning treatments was not as clear-cut as if the trees were full bearing. Most thinning treatments reduced yield (Table 3) and as in a previous trial for eight year old ‘Fuji (Dorigone and Lezzer, 2007) no thinned trees had the highest yield and the smaller fruit weight. Fifteen tonnes ha-1 in the third year is considered a standard yield for this cultivar, however the treatment with metamitron at high concentration
resulted in lower figures (11.1 and 11.3 tonnes per hectare), suggesting that this concentration might be too high for trees with this age. When comparing low, meium and
high concentrations of metramitron there was dose response result, as previously reported. Concerning quality (Table 4), commercial requirements for ‘Fuji’ are firmess 6 – 7 kg cm-2, Brix > 16 and TA < 4 g/l. Quality parameters for all the treatments were within these ranges, except for the fruits treated with ATS+BA that were firmer. This result might be due to the BA application as reported by Greene (1993) BA increases cell division and firmness. The main differences in quality parameters between treatments may also be due to irregular fruit ripening due to the prolonged flowering and fruit set a characteristic of lack of chilling. Seasonal fruit growth patterns showed the effect of increased fruit size when fruit thinning was conducted early in the season mainly with ATS + BA treatment (Fig. 1). As expected, the lowest crop load gave the largest fruits, mainly with metramitron at high concentration and in the non thinned trees treatments the fruits were the smallest. ATS and BA are two active substances that can be used alone or in combination. This
mixture is ideal for replacing the naphthalene acetic acid (NAA) in combination with BA, since NAA causes fruit pygmies in 'Fuji'. However, the ATS has a strong temperature dependence observed in between two days before and after application. Metramitron shows very promising results, as it might be more flexible to use and its concentration may be adapted to different crop loads. Hand thinning was carried out as to leave only one fruit per cluster and a spacing of 15 cm between them. This model does not correspond to the usually practiced by fruit growers in the region and although it proved to be a good strategy, it should 3 be seen as a complementary method of chemical thinning given high cost, skilled labour and lengthy operation.

Literature cited
Anderson P.C. and Crocker T.E. (2009) Low Chill Apple Cultivars for North Florida and North Central Florida – HS764, a publication of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date: December 2000. Revised August 2009. Web site at http://edis.ifas.ufl.edu. Clever, M. 2007. A comparison of different thinning products applied to the apple variety’ Elstar Elshof’ in the Lower Elbe region. Erwerbs-Obstbau 49 (3) 107-109. Dorigoni, A. and Lezzer, P. 2007 Chemical thinning of apple with new compounds. Erwerbs- Obstbau 49 (3): 93-96. Fallahi, E. and Willemsen, K.M., 2002. Blossom thinning of pome and stone fruit. HortScience 37, 474–477 Greene D.W. 1993. A review of the use of benzyladenine (BA) as a chemical thinner for apples. Acta Hortic. 329: 231–236 Janoudi A. and Flore J.A. 2005. Application of ammonium thiosulfate for blossom thinning
in apples. Scientia Horticulturae 104 (2005) 161–168. Maas, F. 2007. Thinning strategies for ‘Elstar’ apples – Experiences with ammonium thiossulphate, calcium hydroxide and benzyladenine. Erwerbs-Obstbau 49(3): 101-105.

Tables
Table 1. Treatments and application dates.

Table 2 – Effects of ATS, ATS + BA, Metramitron at different crop levels and concentration and hand thinning on the numbers of flower clusters (No. flower buds) and number of fruits per branch sectional area (Fruits/BSA cm2) during the growing season in ‘Fuji’ apple trees.

SEM – Standard Error of the Mean. Means followed by the same letter are not significantly different with Tukey’s multiple-range t-test at P≤0.05

Table 3. Effects of ATS, ATS + BA, Metramitron at different crop levels and concentration and hand thinning on yield per tree, calculated yield per hectare, mean fruit weight, and percentage of fruit with diameter >70 mm.

SEM – Standard Error of the Mean. Means followed by the same letter are not significantly different with Tukey’s multiple-range t-test at P≤0.05.

Table 4. Effects of ATS, ATS + BA, Metramitron at different crop levels and concentration and hand thinning on fruit quality characteristics: firmness, SSC, starch index and TA.

SEM – Standard Error of the Mean. Means followed by the same letter are not
significantly different with Tukey’s multiple-range t-test at P≤0.05.