EVALUATION OF CHEMICAL AND THERMAL BLOSSOM THINNING STRATEGIES FOR APPLE

Open Access
- Author:
- Kon, Thomas Matthew
- Graduate Program:
- Horticulture
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 05, 2016
- Committee Members:
- James Rawlinson Schupp, Dissertation Advisor/Co-Advisor
James Rawlinson Schupp, Committee Chair/Co-Chair
Robert Michael Crassweller, Committee Member
Richard P Marini, Committee Member
Paul Heinz Heinemann, Outside Member - Keywords:
- crop load management
pollen tubes
Malus domestica
spur leaves
phytotoxicity - Abstract:
- Since 1989, approximately 150 compounds and multiple mechanical devices were evaluated as apple blossom thinners. Despite these efforts, blossom thinner adoption has been limited to a few apple producing regions or states. The purpose of this work was to: 1) compare the efficacy of promising chemical blossom thinners using a predictive model as a timing aid, and 2) evaluate the potential of short-duration thermal treatments as a blossom thinning strategy. Using a predictive model as a timing aid, the efficacy of several promising chemical blossom thinners was evaluated at Penn State’s Fruit Research and Extension Center in Biglerville, PA. Blossom thinner effects on pollen tube growth, fruit set, and yield responses were evaluated. Calcium polysulfide and ammonium thiosulfate inhibited pollen tube growth in vivo and reduced initial fruit set. Endothal was a potent thinner, but was ineffective in reducing pollen tube growth and caused excessive leaf injury. When used as the sole method of crop load management, none of the chemistries evaluated over-thinned or increased fruit injury. However, endothal caused excessive thinning when evaluated as part of a commercial crop load management program. In a series of experiments, short duration forced heated air treatments (thermal shock; TS) were evaluated as a potential blossom thinning strategy. TS treatments were applied to solitary blossoms and spur leaf tissue with a variable temperature heat gun. At effective temperatures, TS reduced stigmatic receptivity and pollen tube growth in vivo when applied up to 24 h after the pollination event. At the range of temperatures evaluated, a minimum of a 2 s application was required to influence pollen tube growth in vivo. While our data shows that TS was effective in reducing pollen tube growth in vivo, the onset of visible injury to leaf tissue occurred at similar temperatures. Environmental conditions appeared to influence heat gun performance and TS treatment efficacy.