Effects of the ethylene inhibitor aminoethoxyvinylglycine (AVG) on fruit abscission and yield on pruned and unpruned macadamia trees

Abstract

Macadamia, a subtropical evergreen tree, flowers profusely but sheds more than 98% of flowers and fruitlets within 10 weeks of anthesis. This premature fruit abscission is accentuated when trees are hedged in spring, as is commonly practiced in Australian orchards. The aim of this study was to assess the effect of the ethylene inhibitor aminoethoxyvinylglycine (AVG) on fruit abscission in macadamia and if it could be used to mitigate the effect of pruning on abscission. Racemes on '849' trees that had been planted in 2002 were sprayed either pre or post anthesis with AVG at concentrations of 0, 30, 60 or 90 ppm in 2007, and 0, 200, 400 or 600 ppm in 2008. Whole trees that had been tip pruned at anthesis to simulate hedging, were sprayed twice (pre and post anthesis) with AVG at concentrations of 60 ppm in 2007 (10-year-old 'A4' trees) and 400 ppm in 2008 (6-year-old '849' trees). Post anthesis applications to racemes increased set only slightly and the effects of low concentrations in 2007 were similar to those of high concentrations in 2008. Pre anthesis application had a similar effect to post anthesis application in 2007 but decreased initial fruit set relative to post anthesis application and the control in 2008. The increase in fruit abscission associated with pruning was not prevented by AVG application; and whole tree sprays did not increase yield on either pruned or unpruned trees. Overall the response of fruit set in macadamia to AVG in this study was poor even at high rates and with two applications. This may indicate an alternative to ethylene as the primary regulator of abscission of young macadamia fruit.