Fruit set of 'Jonagold': a small dose of gibberellins or Regalis
A treatment with a low dose of GA4+7 can improve fruit set. After bloom an initial increase in fruit set can be observed after a treatment with GA4+7, but often this is lost for the most part during June drop. To counter this heavy June drop even Regalis (Prohexadion-Ca) was tested to reduce the competition between fruits or between vegetative and generative growth. Regalis inhibits gibberellin formation, but acts also as an anti-ethylene agent when it is sprayed between 2 and 4 weeks after full bloom. Regalis can reduce June drop, but the timing of spraying is important and the effect is only short lasting (4 to 5 days). Therefore we recommend to spray 2 times at a rate of 0.5 kg ha-1 2 and 3 weeks after full bloom. The treatments must be done before the occurrence of stress. Stress can be caused by dryness, insufficient radiation (= 2 to 3 dark days), general poor weather conditions and other factors. On parcels in an off-year or after frost damage, it is also possible to spray twice with 0.5 or 1 kg ha-1 of Regalis at the beginning of bloom and 3 weeks after full bloom to reduce shoot growth, so there is less competition for nutrients. Between beginning of bloom and full bloom one must also spray 5 g GA4+7 ha-1, leaving at least 3 days between spraying this treatment and the one with Regalis.
Vercammen, J., Gomand, A., Siongers, V. and Bylemans, D. (2016). Fruit set of 'Jonagold': a small dose of gibberellins or Regalis. Acta Hortic. 1139, 431-438
GA4+7, prohexadion-Ca, anti-ethylene agent, June drop