Hydrogen cyanamide impacts on flowering, crop load, and fruit quality of red 'Fuji' apple (Malus domestica)

The extended flowering period of many apple (Malus domestica) cultivars often causes problems in the management of crop load. Hydrogen cyanamide (Dormex(R)) was examined over a 2-year period for its effect on flowering, crop load, fruit size, and quality of 'Fuji' apple. In Year 1, hydrogen cyanamide was applied at 5 application times (40, 30, 20, 10, or 0 days before estimated budburst (dBEB)) at 3% v/v. These treatments were compared with an untreated control. In Year 2, hydrogen cyanamide was applied at 1%, 2%, or 3% v/v at either 40 or 30 dBEB. The blossom thinner ethephon was applied to all hydrogen cyanamide treated trees at full bloom. An untreated control, a hand-thinned treatment, and a chemically thinned control were also included. Hydrogen cyanamide advanced flowering in both years when applied at 40 or 30 dBEB. In Year 1, full bloom was advanced by more than I week and the flowering period from pink bud to full bloom compressed by the 40 dBEB application, resulting in a very clearly defined full bloom date. Application of hydrogen cyanamide at budburst or 10 dBEB resulted in extended flower development and some bud scorching. Unseasonally warm spring weather conditions in the second year resulted in an abnormally short flowering period. It would appear that under these conditions hydrogen cyanamide can still be used to advance flowering but is not necessary to compress the flowering period. Hydrogen cyanamide reduced fruit set in the first year but not the second. Most treatments showed no detrimental effect on fruit quality. There was no evidence of any negative interactions between HCN and ethephon.