May 28, 2015
Promoting return bloom of apple trees

This is the time of the season when next year’s flower buds are initiating and beginning to form. Your return bloom program should begin after this year’s crop becomes unresponsive to chemical thinning but before the crop becomes sensitive to the ripening effects of NAA or ethephon.

The first step in an effective return bloom (RB) 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 RB, 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 RB for next year. For alternate bearing varieties, additional action is needed. Here is a concise review of how to improve RB for 2016.

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.

Spray every 7 to 10 days, starting after June Drop, and when fruits are larger than 30 mm in diameter. RB sprays should be started after this year’s crop becomes unresponsive to chemical thinning, but before the crop becomes sensitive to the ripening effects of these plant growth regulators. RB 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 ounces per acre of NAA or 8 ounces 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 RB sprays is more effective than one or two heavy shots. For consistent results, 3-4 is recommended.

RB promoters can be applied as stand-alone sprays or tank-mixed with other crop protectants. RB can be applied as part of regular cover sprays, either complete or alternate-row-middle. As with all chemical applications, good coverage is necessary.

—By James Schupp, Associate Professor of Pomology, Penn State University


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