Efficacy of chemical bloom thinning agents to European plums

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’.