Plant growth regulators to increase fruit set in two different 'Williams' pear orchards in Uruguay: with and without pollinizer

Pear production in Uruguay covers 628 ha being 'Williams' the most planted cultivar. Orchards in Uruguay are managed without the use of pollinizers (parthenocarpy). Pear production in recent years has not been stable, there are many factors that could be influencing, including climatic conditions, low fruit set, among others. The aim of this study was to evaluate the use of plant growth regulators to increase fruit set in 'Williams' pears in orchards with and without pollinizers. The study was performed during the 2021/2022 growing seasons using two different 'Williams' pears orchards, the first one five-year-old orchard grafted on 'Adams' with 'Packham's Triumph' as pollinizer, and the second one, a three-year-old orchard grafted on ' OHxF40' without pollinizer. Treatments consisted of an untreated control, and different aminoethoxyvinylglycine (AVG) and 6-benzyladenine + gibberellins 4+7 (6-BA + GA4+7) rates sprayed at full bloom and 7 days after full bloom. Productive and quality parameters were assessed. Fruit set was affected differently in the two orchards (with and without pollinizer), where in the orchard with pollinizers, AVG and 6-BA + GA4+ 7 increased fruit set, number of fruit and yield of 'Williams' pears. A negative effect of 6BA + GA4+ 7 sprays compared to untreated trees and AVG-treated trees was fruit misshapenness, where the combination of these active ingredients showed a higher level of misshapen fruit. In the orchard without pollinizers, the best results were observed with 6-BA + GA4+ 7 sprays, where AVG did not show significant differences between treated trees and control trees. Collectively, our results showed both plant growth regulators may represent an efficient tool to increase fruit set and yields of 'Williams' pears, however, AVG showed increase in fruit set only in the orchard with pollinizer and 6-BA + GA4+ 7 in both orchards. 6-BA + GA4+ 7 treated trees had an increase in the misshapen fruit level when compared to untreated control trees and AVG-treated trees and could reduce the fruit commercial value.