I have heard from several growers that bloom has been much lighter than expected this year, in particular in Honeycrisp, even when crop loads last year were moderate to light. This has many growers scratching their heads as for why bloom levels are lower than expected. When trying to find a possible explanation we need to think of when flower initiation (i.e., the formation of floral structures within the buds) happens in apple trees. Research studies in multiple apple cultivars have reported flower initiation takes place 45 to 90 days after full bloom (DAFB). Cultivars prone to biennial bearing, such as Honeycrisp, have flower initials present at 45 DAFB, while those less prone to biennial bearing go through flower initiation later in the season. If we think about last growing season, there was a very dried period during this 45 to 90 DAFB period that could have stressed trees enough to affect flower initiation, resulting in the light bloom we experienced this year. There might be other factors that could have contributed to the low bloom levels, but when it is as widespread across the state it is very unlikely that it is not production management issue (i.e., excessive crop load, fertilization). There has also been some discussion about the warm weather we experienced in March affecting bloom levels. This is also unlikely, since flowers have already been formed by then, and if the warm spell would have affected the flowers buds, we would have seen flowers anyway.
Thinking ahead for next year, if you had a light bloom in your trees and you crop load is low, you will probably have a heavy bloom next year and you might not want to do any plant growth regulators (PGRs) applications to encourage return bloom. However, if you are not experiencing light bloom this year and you are considering PGR applications to secure adequate bloom levels for next spring, here are some recommendations.
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 temperatures are over daytime highs are in excess of 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 thinning program, as trees with excessive crop load will not respond to return bloom sprays.This article was posted in Apples and tagged Amaya Atucha, Apple return bloom, Apples, Honeycrisp, light bloom.