Abscisic Acid as a Defoliant for Deciduous Fruit Tree Nursery Stock

Abstract

Artificial defoliation of deciduous fruit tree nursery stock is often necessary so that plants can be dug early enough to escape inclement fall weather. In this research, we assessed the efficacy of abscisic acid (ABA) as a defoliant. ABA was applied as a foliar spray at one of three concentrations—500, 1000, or 2000 ppm a.i. Trees were sprayed either once or twice for a total of six chemical treatments, plus untreated controls. The defoliation and growth responses of eight cultivars were evaluated with the cooperation of commercial nurseries in Washington State. While all treatments caused significantly greater defoliation than was observed in untreated trees, ABA at 500 ppm applied once or twice, or 1000 ppm applied only once, was generally sufficiently effective only on `Bartlett', `Gibson Golden Delicious', and `Law Red Rome', but not on `Imperial Gala', `Scarlet Spur Delicious', `Granny Smith', `Braeburn', or `Red Fuji'. Single or double applications of 2000 ppm or double applications of 1000 ppm often produced faster defoliation than double applications of 500 ppm, but defoliation was not always superior after 4 weeks. No pre-digging field damage was noted, but some treatments appeared to reduce trunk diameter increase after replanting, with no consistent trends among cultivars, except with `Bartlett' pear, which was frequently negatively affected. ABA appears to be very promising as a nursery tree defoliant.