2018 ASHS Annual Conference

The profitability of apple fruit producers is largely dependent upon consistent, high annual yield of quality fruit. In commercial apple production systems, this cannot be achieved without effective crop load management strategies that make a balance between vegetative and fruiting buds, reduce fruit-to-fruit competition, allow fruit enough room to grow, expose fruit to adequate sunlight and enhance return bloom. Winter pruning of dormant trees, chemical thinning of blossoms and fruitlets and hand thinning of fruit are the three common crop load management strategies. Chemical thinning can start at bloom stage and continue until fruits reach 25 mm in diameter. It is already well-established that crop thinning during bloom produces the largest fruit, the greatest return bloom and reduces biennial bearing. The application timing of bloom thinners has been subjective until a group of researchers at Virginia Tech has developed a model known as “the pollen tube growth model” (PTGM). This model predicts the time required for a pollen tube to reach the ovary and achieve fertilization to the fraction of king blooms needed for a crop, and the time for bloom thinners to be applied to prevent further set of later bloom. Over the last 13 years, models have been developed for six apple cultivars including Golden Delicious, Gala, Fuji, Cripps Pink, Honeycrisp and Granny Smith. In the present research, we describe the development of a PTGM for ‘Red Delicious’ and the validation of the model in commercial apple orchards in WA State. More than 100 WA Red Delicious beta-test blocks were bloom-thinned using the pollen tube growth model in 2016-17. From about 30 of those sites we received spray timing, yield data, and/or evaluated flower samples for evidence of fertilization. Microscopic evaluation of sampled flowers in the laboratory to determine the percentage of flowers that had been fertilized showed predictive effectiveness of Red Delicious model in the field. In addition, reported harvest data showed that the pollen tube growth model is helping growers to achieve their targeted crop loads.