Return bloom of 8th leaf Honeycrisp/M.26 apple trees at FREC, May 1, 2015. Row on the left received no return bloom sprays. Row on the right received 3 “sides” of RB sprays - one spray of NAA and 2 sprays of ethephon - June/July, 2014.
The first step in an effective return bloom program is an effective chemical thinning program. Chemical thinning removes a portion of the fruit crop, reduces competition, and can increase return bloom. When NAA or ethephon are used in a chemical thinning program, these chemistries can directly promote return bloom, in addition to the beneficial effect from the actual thinning. For some annual bearing, or mildly alternating varieties, a good chemical thinning job can promote adequate return bloom for next year. For alternate bearing varieties, additional action is needed. Here is a concise review of how to improve return bloom.
What to Use
NAA or ethephon
Both are effective for promoting RB when used in a multi-spray program such as that described below. Ethephon is the stronger promoter of the two.
When to Use
Spray every 7 to 10 days, starting after June Drop, and when fruits are larger than 30 mm in diameter. Return bloom sprays should be started after the current year's crop becomes unresponsive to chemical thinning, but before the crop becomes sensitive to the ripening effects of these plant growth regulators. Return bloom sprays later in July, particularly ethephon, can promote premature ripening and accelerate preharvest fruit drop. This is a greater concern for early ripening varieties such as Honeycrisp and McIntosh types. In most seasons, the effective window is early June to early July. For later ripening varieties, such as Delicious, York and Fuji, the sprays can go on until mid-July.
Low rates of either NAA or ethephon are effective. Two oz per acre of NAA or 8 oz of ethephon per acre per spray are adequate. There is little benefit from higher rates, and use of lower rates minimizes the risk of unwanted side effects. Increasing the number of low rate return bloom sprays is more effective than one or two heavy shots. For consistent results, 3 to 4 is recommended.
Can I Add to a Tank Mix?
Return bloom promoters can be applied as stand-alone sprays or tank-mixed with other crop protectants. Return bloom can be applied as part of regular cover sprays, either complete or alternate-row-middle. As with all chemical applications, good coverage is necessary.
While additives may increase uptake and activity of return bloom promoters, be careful with adding these to your tank mix. As Dr. Dave Rosenberger once said to me "Captan on the surface of a leaf is a wonderful thing. Captan under the surface of a leaf is terrible." If applying return bloom sprays as part of a tank mix, it is probably better to skip the adjuvants.
Some years ago, Dr. Ross Byers at Virginia Tech recommended using an ammonium form of nitrogen in combination with return bloom sprays to increase their activity. I have not been able to demonstrate a benefit from this practice, but some growers continue to add foliar N to return bloom sprays.
Do Return Bloom Sprays Work?
See the figure with this article. The photograph shows rows of Honeycrisp trees. Row 2 received no return bloom sprays. The adjacent rows were treated once with NAA and 2 times with ethephon from one side. The results speak for themselves.
What Can Go Wrong?
Return bloom spray applied too early
Return bloom sprays applied too early increase the potential for unwanted additional thinning of the current season's crop. This risk is greater with ethephon than with NAA, and higher rates (>12 oz ethephon, or > 3 oz NAA per acre) can increase the risk of unwanted thinning. This is particularly the case with varieties that are sensitive to late thinning, such as Macoun, Golden Delicious, and Rome.
Return bloom sprays applied too late
Return bloom sprays applied too late increase the potential risk of premature ripening of early maturity varieties such as Honeycrisp. This risk is also made greater from use of higher rates, and is more acute when using ethephon. Avoid return bloom sprays when daytime highs are in excess of 85° F for the same reason.
Many factors influence return bloom. Excessively over-cropped trees, weak trees, excessively vigorous trees, unpruned/heavily shaded trees, water stress and/or heat stress, can all contribute to poor results.
What about Post-harvest Return Bloom Sprays?
Researchers at Michigan State University discovered that stop-drop sprays of NAA could, in some cases, increase return bloom. This has led some consultants to recommend post-harvest return bloom sprays. In 2014, a post-harvest spray of ethephon had no effect on return bloom of Honeycrisp/M.26 trees at the Penn State Fruit Research and Extension Center (FREC). A 2014 stop-drop NAA spray did not increase return bloom of York Imperial/Bud. 9 trees.
Several horticultural techniques (ringing, scoring, root pruning, pinching) have been documented to promote return bloom. With all of these techniques, the period shortly after bloom is the most effective timing, and this appears to be the best timing for return bloom sprays as well. Post-harvest return bloom may prove to be useful in certain circumstances, but even if this timing proves to be worthwhile, it is unlikely to replace the post-thinning timing.
For more information on return bloom sprays, see the Penn State 2018-19 Tree Fruit Production Guide.