Enhancing abscission of mature macadamia nuts with ethephon

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Abstract
The effect of (2‐chloroethyl)phosphoric acid (ethephon) on the abscission of mature ‘Beaumont’ macadamia nuts (Macadamia integri‐folia × M. tetraphylla) was investigated in two experiments. In 1988, 6‐year‐old trees were treated with 200 or 400 mg/litre of ethephon, with and without pH neutralisation or a surfactant, 2 weeks after nut maturity. Nut abscission increased from 6% on untreated trees to a maximum of 87% on trees treated with a 400 mg/litre neutralised solution of ethephon which included a surfactant. Trees treated with 400 mg/litre ethephon sprays showed a large but nonsignificant reduction in yield in the following season. A second experiment in 1990 determined the effects of three ethephon rates (400, 500, and 600 mg/litre) applied on three separate dates (nut maturity, nut maturity +2 weeks, and nut maturity +4 weeks). In this study 70% of nuts abscissed after the final application compared to only 7% on untreated trees. However ethephon applied 2 or 4 weeks after the nuts matured caused significant flower damage and crop loss in the following season. Despite the enhancement of abscission rates, ethephon cannot be recommended for application to ‘Beaumont’ macadamia trees in New Zealand because of the loss of subsequent yields.