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Sprays to Promote Return Bloom in Apple

Posted: June 1, 2012

Chemical thinning alone may not be sufficient to promote annual bearing for apple varieties which possess a genetic tendency to alternate bearing. Examples of these include York Imperial, Golden Delicious, Mutsu, Fuji, Macoun, Honeycrisp, and spur-type strains of Delicious.
Dr. Jim Schupp, Penn State Fruit Research and Extension Center Pomologist

Two materials are labeled for this use, NAA and ethephon. Several weekly or bi-weekly sprays of 5 PPM NAA, or 100-200 PPM ethephon may be used. Low rates of these chemicals are effective to promote flowering, and are recommended to avoid undesirable side effects, such as reduced fruit size and premature fruit ripening. Return bloom sprays should start when fruit diameter has exceeded 30 mm.  With the early spring and advanced crop development in 2012, it is now time to schedule sprays to promote return bloom in blocks of susceptible varieties with full crops.

Do they work? In a 2005 study on Fuji at FREC, NAA (Fruitone) or ethephon increased return bloom by 64%, and 168%, respectively. In a 2006 return bloom study on York Imperial at FREC, Fruitone increased return bloom by 48% compared to untreated controls. In the same trial we tried supplementing NAA with either N fertilizer or with 6BA, a cytokinin, but the return bloom with these supplements was no better than that of NAA alone.

Apply three-to-four sprays of 2 fluid ounces per acre of Fruitone L, or 8 fluid ounces per acre of ethephon (Ethrel, Ethephon 2SL), during the next month. Higher rates of growth regulator per acre are not desirable, as these higher rates can lead to premature ripening and possibly increase preharvest fruit drop. Instead, make more applications of the recommended lower rates to stimulate more return bloom. A buffer/ acidifier surfactant should be added to the tank if the spray water source is hard water from a well. These sprays can go on in an alternate row middle cover spray. Each half spray would count as a “complete” return bloom spray. To reduce the risk of advancing fruit ripening, these sprays should be applied before the second week of July, especially for early maturing varieties such as Honeycrisp.