The 2012 ASHS Annual Conference

In apple (Malus X domestica Borkh.) production, crop thinning during bloom produces the largest fruit, the greatest return bloom in the following year, and reduces biennial bearing. The application timing for this spray has been subjective, and, in the past, was usually based upon the percent of full bloom open (e.g., an application at 20 and 80% full bloom). While this approach became a standard practice in some growing regions, more precise application timing can be achieved through modeling the fertilization of the desired percent of king bloom needed to achieve a full crop at the desired fruit size. When this target is achieved, a bloom thinner can be applied so that later blooming flowers are prevented from setting fruit. By measuring pollen tube growth rates under controlled atmospheric conditions using growth chambers, we have developed a model that calculates the time required to fertilize the king bloom after pollination. We have found that cultivar differences and temperatures affect pollen tube growth and the time required for fertilization, and can offer explanations for inconsistent bloom thinning results. Additionally, we believe that this model has the potential to improve crop load management when using liquid lime sulfur and fish oil for bloom thinning in organic apple orchards. There are a limited number of fruit thinning chemicals approved for use under USDA organic standards, and many organic apple growers rely on a combination of liquid lime sulfur with either fish or stylet oil applied during the bloom period. We are also evaluating these materials for their disease control potential when used for flower thinning. The pollen tube growth model is now being tested for flower thinning on a limited basis in commercial orchards with the goal to assemble specific modeling data for each commercially important apple cultivar.