Efficacy of chemical bloom thinning agents to European plums

Abstract

During the period 1998–2000, thinning trials were conducted using bloom thinners on mature European plum trees at Ullensvang and at fruit growers’ sites in western Norway. In 1998, unsprayed control and hand-thinned ‘Victoria’ trees were compared with trees treated at full bloom with a single application of 1% Armothin® or 1.5% ammonium thiosulphate (ATS). The same programme was conducted in the following two years with the addition of a single full bloom treatment with 250 ppm ethephon and a post-bloom application one month after full bloom with the mixture 10 ppm 1-napththalene acetic acid (NAA) and 75 ppm ethephon. High volume sprays were conducted the first two years and low volume the last year. Thinning trials testing 1–1.5% ATS and 5–7% lime sulphur in comparison with unthinned and hand-thinned trees to the cultivars ‘Opal’ and ‘Victoria’ were conducted in 2000, at growers’ sites. Generally, thinning treatments reduced crop load and enhanced fruit quality (fruit size, soluble solid content, fruit firmness and ground and surface colour), but the results varied from year to year. Fruit set was reduced to about half of control values and the percentage of class 1 fruits was doubled compared to the control trees. High volume sprays to running off were more effective than low volume sprays. The cultivar ‘Opal’ was more sensitive to ATS than ‘Victoria’ and a low dosage is recommended. All thinning compounds caused some minor leaf injury but no fruit damage. The flower thinners were efficient at rather low temperature. No differences in the amount of gummosis (internal disorder of the fruits) were observed as a result of treatments on ‘Victoria’ plums. Return bloom was improved by thinning. In conclusion, a single dilute application at full bloom of 1% Armothin®, 1.5% ATS or 250 ppm ethephon or one application of the mixture 10 ppm NAA-75 ppm ethephon four weeks after bloom reduced fruit set and crop load and increased fruit quality and return bloom of the cultivar ‘Victoria’. Similar results were achieved with one dilute spray with 1% ATS or 5% lime sulphur at full bloom to the cultivar ‘Opal’.

Keywords:

• Armothin®
• ATS
• ethephon
• fruit quality
• lime sulfur
• Prunus domstica L
• return bloom
• yield
• yield efficiency

Introduction

Many fruit cultivars produce few flowers and even fewer fruits in the two or three years following planting of young trees. Thereafter they frequently initiate and form too many flowers and set too many fruits to obtain regular and marketable crops from year to year. If consistent cropping is to be sustained and fruit quality maximized, the fruit set of the tree must be balanced with the tree size and leaf area (Måge, 1994; Webster & Spencer, 2000). In addition, adjustment of the crop load protects branches from breakage due to the heavy weights of the fruit. The abundance of flowers produced is often far in excess of the tree's requirement in order to set and retain an optimum number of fruits. For example, a fruit set of only 5% of flowers on the plum cultivar ‘Victoria’ is required to provide a full crop (Webster & Holland, 1993) and for apple a fruit set of 10% of flowers is sufficient (Meland, 1998). At this crop load level, most of the fruits are of the size and quality preferred by the fresh fruit market. Fruits compete with each other and with other vegetative parts of the tree such as shoots and roots for water, nutrients and assimilates. The total leaf area of the tree, the amount of available light (light interception) and the ambient temperature are important for total carbon production and influence the optimal crop load (Palmer, 1992). Management factors such as the choice of rootstock, tree spacing and tree pruning also have strong impact on the crop load. The within-tree competition can be altered by other management techniques such as manipulation of the vegetative growth (shoot pruning/ training) and root growth (root pruning/restriction).

The self-fertile European plum cultivars ‘Opal’ and ‘Victoria’ (Prunus domestica L.) blossom abundantly in most years and too many flowers can be set. If these excessive fruitlets remain on the trees until harvest, the crop consists of small, unmarketable fruits of low fruit quality. Branches may break under the heavy crop load and flowering may be reduced in the following season. To achieve annual yields of high quality fruits, the commercial practice for many years has been to blossom-thin these cultivars with a chemical agent and make further crop load adjustments by hand post-bloom. This management technique saves labour costs and improves the value of the crop. Hand thinning of flowers or fruitlets alone is regarded too time-consuming.

Chemical thinning of blossoms permits reduction of the potential overset at the earliest possible stage. In Scandinavia, fruit thinning with the chemicals ethephon at the early bloom stage or lime sulphur at full bloom has been recommended (Kvåle, 1978, 1985). However, these chemicals occasionally produce inconsistent results on a commercial scale. Recently, limitations on the use of chemicals in orchards have been introduced. Traditionally used thinning products may not be acceptable for sustainable production or may be withdrawn by manufacturers because of the cost of registration. Therefore, more reliable alternative methods for thinning plums using environmentally safe agents are needed. If chemical thinning of blossoms is to be effective, physical removal of blossoms has to be stimulated (Meland, 2004). Sprays of highly phytotoxic chemicals at blossom time achieve this effect by killing the flowers, but may also damage other parts of the tree (Webster & Holland, 1993). High volume sprays with ammonium thiosulphate (ATS) at blossom time have thinned peaches (Byers et al., 1986) and plums (Webster & Holland, 1993). The concentrations used are in the range of 1.0–1.5%. Armothin®, a poly-fatty acid amine, is used as a bloom thinner in peach at concentrations between 1.5 and 2.0% (Baroni et al., 1998). Fruit thinning following bloom permits a more exact evaluation of the fruit set before any application of a thinning agent. Jakob (1998) found that the mixture ethephon-NAAm applied 30–40 days after bloom produced a significant thinning effect. Using ethephon alone post-bloom was too unpredictable and could cause overthinning.

The experiments described in this paper examined the responses of ‘Opal’ and ‘Victoria’ plums to various chemical thinning agents applied during the blossom period or one month later over three seasons.

Materials and methods

Field trials with ‘Victoria’ trees grafted on St. Julien A rootstock were conducted in the experimental orchard at Ullensvang, Lofthus (60°N) in 1998, 1999 and 2000. The trees were trained as free spindles, spaced 2.5×4 m and kept at about 2.5 m height by pruning. Tree age was three years at beginning of the experiment. The soil was a loamy sand with approximately 4% organic matter. Soil management combined frequently mown grass in the alleyways with 1 m wide herbicide strips along the tree rows. The trees were irrigated with trickle irrigation when water deficits occurred. All trees received the same quantity of fertilizers based on soil and leaf analysis.

All trials employed randomized complete block designs with single-tree treatments and six to eight replicates of trees of uniform bloom. In 1998 unsprayed control and hand-thinned trees were compared with trees sprayed either with 1% Armothin® or 1.5% ATS at full bloom (11 May). The maximum air temperature on the day of bloom application was 10.4°C. In 1999 unsprayed control and hand-thinned trees were compared with trees sprayed either with 1.0% Armothin® at full bloom (13 May), 1.5% ATS at full bloom, 250 ppm ethephon at full bloom, or a mixture of 10 ppm NAA and 75 ppm ethephon applied 27 days after full bloom (9 June). The maximum air temperature on the day of the full-bloom application was 11.8°C and post-bloom application 16.9°C. Hand thinning was conducted on 17 June to a thinning distance of about 5 cm between fruits. In 2000 the same treatments applied in 1999 were repeated. The full bloom treatments were applied on 9 May when the maximum air temperature was 21.3°C and the post-bloom treatment on 6 June when the maximum temperature was 23.5°C. Due to the heavy crop load on these trees, yield per tree was estimated and sub-samples were collected for fruit weight and quality testing. In 2000, field trials were conducted at growers’ sites also in cooperation with The Regional Fruit Advisory Service (Nordfjord Fors⊘ksring and Indre Sogn Fors⊘ksring). In both regions unsprayed control trees were compared with trees sprayed with 1.0% ATS to the cultivar ‘Opal’ and 1.5% to ‘Victoria’ and 5–7% lime sulphur applied at full bloom to the same cultivars. Single trees were treated and the experimental design was a randomized block with three replicates. In Sogn the trees were sprayed with the two thinning agents the day after full bloom (15 May) and in Nordfjord at full bloom (12 May) at a maximum temperature of 17°C. The hand thinning of the trees in Sogn was conducted on 30 June and in Nordfjord on 28 June.

The experimental procedure was similar in all trials. The treatments were applied to whole trees as dilute sprays with a handgun and a small knapsack sprayer the two first years and as low-volume sprays the third year. The Regional Fruit Advisory Service sprayed the trees to run-off stage with a hand sprayer and the plum flowers turned yellow when treated with lime sulphur. No surfactants or any additives were included in the sprays. Trunk circumference was recorded 25 cm above the graft union. The trees were judged for possible leaf damage one month after application. Fruit set was calculated by counting the number of flower clusters per tree prior to the application and fruit number at harvest. Total yields were taken from each tree at harvest and graded according to current standards (The Standardization Organization in Norway, 1999). The number of fruits per tree was recorded and mean fruit weights estimated. A sample of 10 randomly selected fruits from each experimental tree was used to measure fruit-soluble solids content and fruit firmness, to judge ground and surface colour, and to estimate the amount of internal gummosis, a physiological disorder of ‘Victoria’ plums. Return bloom was recorded as total number of flower clusters per tree in 1999 and by visually rating bloom in 2000 on a scale of 1 (no bloom) to 9 (very heavy flowering).

Results

Yield and fruit quality

No significant differences in flower clusters per tree or tree size were found at the beginning of the experiment in 1998 (Table I). All thinning treatments reduced crop load. ATS was the strongest thinner, reducing fruit set to less than half and yield to about 40% of control trees. Armothin® was less aggressive, with a total yield similar to the hand-thinned trees. All treatments produced high percentages of grade 1 fruits. No differences in return bloom the next year were observed. There was abundant flowering on all trees including also the unthinned trees.

Table I. The effects of different thinning agents on fruit number, fruit set, yield efficiency and total yield of ‘Victoria’ plums in 1998 and on return bloom the following year.

Treatment	No. of flower clusters per tree	Final fruits per 100 clusters	Trunk cross-sectional area, cm^2	Yield, kg per cm^2 TCSA^1)	Yield, kg per tree	Yield, % > 36 mm fruit size	Return bloom. No. of flower clusters per tree
Untreated	232	118	23.8	0.50	11.1	89	705
Hand thinned	232	59	24.4	0.26	6.2	98	686
Armothin®,1%	212	85	24.7	0.32	7.9	96	602
ATS, 1.5%^2)	223	50	24.5	0.18	4.2	97	709
LSD, p = 0.05%	NS	30	NS	0.35	4.4	NS	NS
^1)TCSA: trunk cross-sectional area.
^2)ATS: trade name Ammonium Thiosulphate Solution. Liquid formulation, 62% ATS stabilized with water, ammonium sulphate and ammonium sulphite.

Fruit weight from the thinned trees was slightly increased, but not significantly (Table II). Soluble solids appeared to be increased by ATS in 1998, but there were no other effects of thinning treatments on fruit quality factors. Both Armothin® and ATS produced minor damage on the leaves. Gummosis increased slightly but not significantly on the thinned trees. Untreated trees showed significant gummosis symptoms in 1998.

Table II. The effects of different thinning agents on fruit size, fruit quality and leaf damage of ‘Victoria’ plums in 1998.

Treatment	Fruit weight, g	Soluble solids,%	Fruit^1) firmness	Ground^2) colour	Surface^3) colour	Gum-mosis,%	Leaf^4) damage
Untreated	44	15.0	62.3	4.4	2.4	42	1.0
Hand thinned	48	16.2	65.8	4.4	2.8	56	1.0
Armothin®, 1%	47	15.7	62.5	4.8	3.0	51	1.6
ATS, 1.5%	47	17.0	64.8	4.6	2.8	58	1.2
LSD, p = 0.05%	NS	1.8	NS	NS	NS	NS	0.6
^1)Fruit firmness measured with Durofel, Copa-Technology SA.
^2)Ground colour scores 1–9, where 1 = dark green and 9 = bright yellow.
^3)Surface colour scores 1–9, where 1 = no red colour and 9 = red colour covering the surface completely.
^4)Leaf damage scores 1–9, where 1 = no damage and 9 = very severe damage.

No significant differences in flowering or tree size were found at the start of the trial and the trees were uniform in 1999, the second year of application. All thinning treatments reduced fruit set and yield and increased the percentage of marketable fruit (Table III). The chemicals produced greater thinning effects than hand thinning. All chemical thinning treatments at full bloom improved return bloom the following year.

Table III. The effects of different thinning agents on fruit number, fruit set, yield efficiency and yield of ‘Victoria’ plums in 1999 and on return bloom the following year.

Treatment	No. of flower clusters per tree	Final fruits per 100 clusters	Trunk cross-sectional area, cm^2	Yield, kg per cm^2 TCSA^1)	Yield, kg per tree	Yield, % > 36 mm fruit size	Return bloom ^2)
Untreated	628	130	30.1	0.76	22.7	26	3.7
Hand thinned	621	83	34.6	0.53	18.3	64	4.3
Armothin®, 1%	630	62	36.4	0.42	14.9	75	5.3
ATS, 1.5%	648	65	34.4	0.44	15.5	72	6.2
NAA^3), 10 ppm + ethephon^4), 75 ppm	580	63	33.5	0.38	12.6	68	5.3
Ethephon, 250 ppm	651	58	33.1	0.41	13.5	67	4.7
LSD, p = 0.05%	NS	10	NS	0.12	4.5	14.2	1.2
^1) TCSA: trunk cross-sectional area.
^2) Return bloom scores 1–9 where 0 = no flowering and 9 = very abundant flowering.
^3)NAA: trade name: Pomoxon, 0.15% NAA.
^4)Ethephon: trade name: Cerone, 48% ethephon.

The different thinning agents increased fruit weight, soluble solids content and ground or surface colour but decreased flesh firmness (Table IV). The thinners appeared to advance fruit maturity. Both ATS at bloom and the post-bloom NAA-ethephon mixture produced minor damage on leaves visible in the middle of June. Incidence of gummosis was generally low during the 1999 season and no differences between the treatments were found.

Table IV. The effects of different thinning agents on fruit size, fruit quality and leaf damage of ‘Victoria’ plums, 1999.

Treatment	Fruit weight, g	Soluble solids, %	Fruit firmnessDurofel	Ground^1) colour	Surface^2) colour	Gummosis, %	Leaf^3) damage
Untreated	29	12.4	64.2	3.2	3.3	2	1
Hand thinned	37	12.9	61.7	4.0	3.7	2	1
Armothin®, 1%	40	14.7	56.2	4.2	4.0	3	1
ATS, 1.5%	40	15.3	54.3	4.5	4.0	2	1.3
NAA, 10 ppm + ethephon, 75 ppm	36	14.5	56.0	4.7	4.3	5	1.2
Ethephon, 250 ppm	38	14.5	58.5	4.3	3.7	5	1.3
LSD, P = 0.05%	5.8	1.4	5.5	0.8	NS	NS	NS
^1)Ground colour scores 1–9, where 1 = dark green and 9 = bright yellow.
^2)Surface colour scores 1–9, where 1 = no red colour and 9 = red colour covering the surface completely.
^3)Leaf damage scores 1–9, where 1 = no damage and 9 = very severe damage.

All thinning treatments except Armothin® and ethephon reduced fruit set significantly in the third year, 2000 (Table V). The hand-thinned trees had the smallest fruit set, 24% of the control trees. The most effective thinners, ATS and the post-bloom mixture NAA/ethephon reduced the fruit set to only 65% and 59% of the untreated trees, respectively. Estimated yield was significantly reduced by hand thinning. The fruit weight of the hand-thinned trees was significantly improved, with 39% larger fruits compared to the unthinned trees. The other thinning treatments did not increase the fruit weight owing to their heavy crop load. Soluble solids content and fruit firmness were not influenced by treatment. However, ground colour was slightly increased on the hand-thinned trees.

Table V. The effects of different thinning agents on fruit set, yield, fruit size and fruit quality of ‘Victoria’ plums in 2000.

Treatment	Final fruits per 100 clusters	Yield^1)	Fruit weight, g	Soluble solids,%	Fruit firmness Durofel	Ground^2) colour	Surface^3) colour
Untreated	93	8.7	23	14.7	51	7.0	6.8
Hand thinned	22	7.0	32	15.7	52	7.8	7.8
Armothin ®, 1%	83	8.3	25	15.4	50	6.5	6.8
ATS, 1.5%	60	8.3	26	13.3	51	6.5	7.0
NAA, 10 ppm + ethephon, 75 ppm	55	8.0	23	15.0	50	7.1	7.2
Ethephon, 250 ppm	77	8.7	24	15.2	53	6.3	7.2
LSD, p = 0.05%	18	0.7	3.2	NS	NS	0.3	NS
^1)Yield scores 1–9 where 1 = no yield and 9 = heavy yield.
^2)Ground colour scores 1–9, where 1 = dark green and 9 = bright yellow.
^3)Surface colour scores 1–9, where 1 = no red colour and 9 = red colour covering the surface completely.

Results from the trials in the two fruit production regions are listed in Tables VI–VIII. In Nordfjord the thinning treatments reduced the crop load of the cultivar ‘Opal’, but not significantly. ATS was the most effective thinner. The total amount of marketable yield was not enhanced because many fruits became cracked due to rain during ripening time. However, the contents of soluble solids in the fruits from the thinned trees increased significantly. In Sogn region all thinning treatments reduced the crop load significantly. Lime sulphur thinned the trees to almost the same level as the hand-thinned trees. The percentage of grade 1 (>36 mm fruit diameter) fruit was doubled compared with the unthinned trees. Fruit weight and the content of soluble solids increased significantly and return bloom the following year was slightly enhanced.

Table VI. The effects of different thinning agents on yield efficiency, total yield, fruit size and fruit quality of ‘Opal’ plums in 2000 in the fruit production region Nordfjord, western Norway.

Treatment	Flower clusters per tree^1)	Trunk cross-sectional area, cm^2	Yield, kg per cm^2 TCSA^2)	Yield, kg per tree	Yield, % > 36 mm fruit size	Fruit weight, g	Soluble solids,%
Untreated	7	104	0.10	10.3	93	35	15.4
Hand thinned	5.8	110	0.07	7.5	90	36	16.0
1.0% ATS	4.3	100	0.04	3.7	66	35	16.2
7% lime sulphur	5.3	105	0.07	6.8	84	35	16.2
LSD, p = 0.05%	NS	NS	NS	NS	7	NS	0.5
^1) Flower clusters scores 1–9 where 0 = no flowering and 9 = very abundant flowering.
^2) TCSA: trunk cross-sectional area.

Table VII. The effects of different thinning agents on yield efficiency, total yield, fruit size, fruit quality and return bloom of ‘Opal’ plums in 2000 in the fruit production region Sogn, western Norway.

Treatment	Flower clusters per tree^1)	Yield, kg per tree	Yield, % > 36 mm fruit size	Fruit weight, g	Soluble solids, %	Return bloom ^2)
Untreated	7.3	42.5	41	28	12.8	7.3
Hand thinned	7.7	16.6	93	41	16.8	9.0
1.0% ATS	8.0	28.2	88	38	16.2	8.3
5% lime sulphur	7.0	20.9	92	41	16.7	8.3
LSD, p = 0.05%	NS	11.6	22	5	2.0	NS
^1) Flower clusters scores 1–9 where 0 = no flowering and 9 = very abundant flowering.
^2) Return bloom scores 1–9 where 0 = no flowering and 9 = very abundant flowering.

Table VIII. The effects of different thinning agents on yield efficiency, total yield, fruit size, fruit quality and return bloom of ‘Victoria’ plums in 2000 in the fruit fruit production region Sogn, western Norway.

Treatment	Flower clusters per tree^1)	Yield, kg per tree	Yield, % > 36 mm fruit size	Fruit weight, g	Soluble solids, %	Return bloom ^2)
Untreated	6.3	11.3	76	45	16.9	8.0
Hand thinned	6.3	8.7	99	58	18.8	8.3
1.0% ATS	8.0	8.1	97	51	17.5	8.3
7% lime sulphur	8.3	8.9	95	52	17.1	8.6
LSD, p = 0.05%	1.6	NS	6	6	1.1	NS
^1) Flower clusters scores 1–9 where 0 = no flowering and 9 = very abundant flowering.
^2) Return bloom scores 1–9 where 0 = no flowering and 9 = very abundant flowering.

Similar results were obtained by thinning the cultivar ‘Victoria’ chemically with ATS and lime sulphur. Both thinning agents thinned to the same crop level as the hand-thinned trees. The pack-out percentage, fruit weight and the soluble solid contents increased significantly compared to unthinned trees. However, the amount of return bloom was not much influenced by the thinning programme and the trees blossomed abundantly the following year.

Discussion

General thinning effect

From the results in 1998 and 1999 it is evident that all the thinning agents tested are effective thinners of the plum cultivar ‘Victoria’ when applied as dilute sprays to run-off during or after blossoming. In 2000, the thinning treatments had much less effect on crop load. Both ATS and the mixture NAA/ethephon reduced the fruit set to some extent but not to the desired level. The treatments in 2000 were applied using a dilute, but low volume spray to the point of just wetting the trees. This may explain why the thinning effects were less than expected. Also, the temperature was high during the period of application. The experiments at the growers’ sites testing ATS and lime sulphur reduced significantly the crop load and promoted fruit quality.

The temperatures at the blossom application where rather low both in 1998 and 1999 −10.4°C and 11.8°C, respectively. At the regional trial in Sogn in 2000 the application temperature was 17°C. As previously mentioned, all the thinning agents reduced the crop load significantly, even with the rather low temperature. These results validate the opportunity for fruit growers to flower-thin plums in regions having cool weather during blossom time.

Armothin®

The thinning agent Armothin® is mainly used as a blossom thinner in peaches. Baroni et al. (1998) found that sprays applied at the end of bloom (80–100% open flowers) at a concentration ranging between 1.5 and 2% gave the best cropping in two peach cultivars. The thinning action of the chemical is concentration-dependent and negatively related to the percentage of open flowers. In the experiments reported here, 1% Armothin® was tested as a bloom thinner under rather cool temperature conditions. Armothin® treatment reduced fruit set from 50–70% of the control trees and increased the percentage of no. 1 fruit, fruit size and soluble solids content. Optimum crop density, good fruit size and fruit quality varies from season to season and is influenced by several factors. Under Norwegian growing conditions, a fruit set around 50 fruits per 100 flower clusters produces satisfactory fruit size.

ATS

The fertilizer ammonium thiosulphate (ATS) has been extensively tested as a flower thinner for many fruit crops in the last decade (Wertheim, 1998). ATS is thought to thin by the desiccation of flowers and damage to the base of the flower peduncle. Its efficacy is therefore influenced greatly by the proportions of flowers at vulnerable stages in relation to the time of spraying (Byers & Lyons, 1983; Byers et al., 1986). In experiments conducted in 1998 and 1999 the final fruit set was reduced to about half of the unthinned trees when 1.5% ATS was applied at full bloom to the cultivar ‘Victoria’. The yields were significantly reduced and the pack-out percentage increased. Webster and Holland, (1993) reported that two or three sprays at intervals during the blossom period were much more effective than a single spray. Blossoms sprayed at or soon after anthesis were more sensitive to ATS than those sprayed earlier. The stigma is the most sensitive tissue part of the flower; disruption of pollination and fertilization appears to be the main effect of blossom thinning treatments. Multiple applications in these trials would probably have increased the thinning effect because of the low temperatures and extended flowering period in 1998 and 1999. Some minor leaf damage was recorded when treating the trees with ATS. These leaf symptoms disappeared during the season and seemed to have no effect on the growth of the trees.

Similar results were achieved when testing a lower dosage of ATS (1.0%) to the cultivar ‘Opal’. The crop load became reduced and the fruit quality enhanced to the same levels as when thinning ‘Victoria’ with 1.5% ATS. Probably the cultivar ‘Opal’ is more sensitive for blossom thinning than ‘Victoria’ and needs a lower dosage. Similar results were reported from Holland where 1.5% ATS in combination with a wetting agent proved to be toxic to ‘Opal’ plums (Wertheim, 1998).

Ethephon

In Europe, ethephon is used as a flower thinner for biennial-bearing apple trees. Ethephon has been shown to be efficient as a thinning agent for apples at the early bloom stage. The thinning effect is dependent on temperature, concentration and stage of development (Jonkers, 1979; Wertheim, 2000).

In the experiment conducted in 1999, 250 ppm ethephon applied at bloom reduced the fruit set and the crop load to about half and increased fruit weight, soluble solids content and ground colour relative to untreated trees. Return bloom became enhanced. Kvåle (1978) reported similar results in ‘Opal’ plums where the ethephon treatment also had a strong effect on return bloom.

NAA

The thinner naphthalenacetic acid (NAA) is widely used in the apple industry around the world. Thinning results with NAA are often variable and are seriously influenced by spray volume, weather around application time and additives. Both concentration and timing also have a major impact on the thinning results in apples (Jones et al., 1992).

Thinning after bloom, when the actual fruit set of the trees can be judged, can be an advantage. The mixture NAA and ethephon applied four weeks after flowering, thinned plum trees to the same level as the blossom thinners. The fruit quality and the fruit weight were improved to the same extent as hand-thinned trees. Late chemical thinning may be an advantage if inappropriate weather conditions are experienced during bloom. However, in the UK, fruitlet thinning with high dosages of NAA (60 ppm) to the cultivar ‘Victoria’ retarded vegetative shoot growth and reduced fruit bud numbers in the season following spaying (Webster, 1980).

Lime sulphur

The thinning agent lime sulphur has a caustic effect on the flowers and is mainly used at full bloom on plums (Kvåle & Ystaas, 1969). Many trials have given inconsistent results from year to year and it has been suggested that this may be attributable to the different sensitivity of flowers at different stages of their development (Byers & Lyons, 1986). However, in these trials at growers’ sites there was a consistent thinning effect when applying at full bloom a dilute spray until the flowers turned yellow. Over the years Norwegian plum growers have considered this thinning agent as reliable.

Gummosis

The plum cultivar ‘Victoria’ is susceptible to the physiological disorder gummosis of the fruit, reducing the value of the crop. The incidence of this disorder varies from year to year (Webster, 1980). Gummosis was severe in 1999 but not in the two other years of trials. During these experiments no chemical thinning treatment produced either a positive or negative effect on the incidence of gummosis.

Return bloom

The thinning treatments improved return bloom in the experiment conducted in 1999 (Table III) but not in the year earlier (Table I). The trees were young in 1998, only three years old when the experiment started, and had strong vegetative growth. All trees produced a heavy return bloom in 1999, the year after the first experiment. Apparently the crop loads in 1998 were not heavy enough, even in unthinned trees, to depress flowering the next year. Kvåle (1978) found increased return bloom the year after flower thinning with ethephon concentrations from 180–250 ppm to the cultivars ‘Opal’ and ‘Victoria’ was used.

Conclusion

A single dilute application at full bloom of 1% Armothin®, 1.5% ATS or 250 ppm ethephon or one application of the mixture 10 ppm NAA-75 ppm ethephon four weeks after bloom reduced fruit set and crop load and increased the fruit quality and return bloom to the cultivar ‘Victoria’. Similar results resulted from one dilute spray with 1% ATS or 5% lime sulphur at full bloom to the cultivar ‘Opal’.

Acknowledgements

Thanks are expressed to the staff at Ullensvang for technical support and to The Regional Advisory Service in Nordfjord and Sogn for carrying out field trials.