The window for chemical thinning is rapidly closing in many parts of the state. Carbaryl can be effective up to 20 mm fruit diameter. After this stage, options become limited and we’re in a “rescue thinning” approach. Below is some information from Penn State University on rescue thinning options:
Rescue Thinning Options – 20mm and Above
Ethrel® is a rescue thinner that thins fruit right up to 25 mm after which it becomes ineffective. Ideally, Ethrel® works well when applied at temperatures between 70-80 ˚F. 80 ˚F+ temperatures could cause excessive thinning. Ethrel® could be combined with 1 pint of Sevin® and two pints of a surfactant such as Regulaid® (per 100 gallons). Ethrel® use rates for different varieties has been established by researchers Autio and Cowgill (F-129R: Late-season “Rescue” Thinning with Ethephon). Use the following concentrations, in 100 gallons of dilute spray, by variety:
- McIntosh and Macoun, 200 to 300 parts per million (ppm) (0.7 to 1 pt)
- Spur-type Delicious and Fuji, 300 to 375 ppm (1.0 to 1.5 pt)
- Golden Delicious, Rome Beauty, and August varieties, 120 ppm (0.4 pt)
- Gala, Cameo, and GoldRush, 225 ppm (0.75 pt)
- Enterprise™, 150 ppm (0.5 pt)
- Jonagold, 150 to 225 ppm (0.5 to 0.75 pt)
From: Fruit Times, Penn. State University. Authors Shan Kumar, Lindsay Brown, Daniel Weber and Donald Seifrit.