By: Amaya Atucha, UW-Madison Department Horticulture
The formation of flower buds for next year’s crop is starting now and the use of plant growth regulators (PGRs) can be an effective tool to enhance return bloom in alternate bearing cultivars. However, an effective return bloom strategy must start with appropriate thinning program that will reduce competition and allow flower buds to form for next year.
There are two products that are effective at promoting return bloom: NAA and Ethephon. The applications of these products should start after the June drop, and once the fruits are over 30 mm in diameter and not sensitive to chemical thinners. If the return bloom sprays are done too early there is a risk of additional thinning of current year’s crop. On the other hand, late return bloom sprays (past mid-July) may result in premature fruit ripening and fruit drop for early ripening cultivars (for the same reason avoid return bloom sprays when daytime temperatures highs are over 85° F).
Application rates for NAA (sold under the name of Fruitone L®; PoMaxaTM; RefineTM) are 5 ppm (2 oz/100 gal) or 2.5 ppm + surfactant at 5, 7, 9 weeks after bloom. These sprays can be applied as stand-alone sprays or added to cover sprays during that period.
A single application of ethephon (sold under the name of Ethrel; Ethephon 2; Motivate TM) 5 to 6 weeks after bloom at rates of 16 to 24 fl. oz per acre for Red Delicious, Gala, and Rome; 24 to 48 fl. oz acre for Golden Delicious; and 48 to 72 oz/acre for Fuji and Cameo is recommended. If crop load is excessive apply a second time 14 to 21 days after the first application. Do not use ethephon for increasing return bloom on Honeycrisp.
PGRs can be a great tool to promote return bloom in cultivars with strong alternate bearing. However, the effectiveness of the sprays will depend on the implementation of an appropriate chemical thinning program, as trees will excessive crop load will not respond to return bloom sprays.This article was posted in Apples and tagged Apple return bloom, PGR.