Reducing fruit drop in fruit trees with the synthetic auxin TPA

Abstract

The project investigated if the synthetic auxin 3, 5, 6 - trichloro-2-pyridyloxyacetic acid (TPA) could be used to reduce fruit drop in mango, avocado or macadamia. Recent research has found that TPA can significantly reduce fruit drop and increase yields in a range of lychee varieties by inhibiting the development of the fruit abscission layer. Fruit drop is a problem in mango, avocado and macadamia reducing maximum potential yields. Application rates and timings for the trial work were adapted from the lychee research trials. Fruit at various stages of development were treated with a single foliar spray of TPA at 50 ppm and the level of fruit drop recorded. In avocado there was very little natural fruit drop in either the Hass or Sheppard variety in this season therefore the effect of TPA on fruit drop could not be determined, there were indications that the rate of TPA was too high with some phytotoxicity on the leaves. In mango fruit drop was greatly reduced with TPA applications however many of the fruit failed to develop properly, either dying or abscising before harvest in the young fruit or remaining until harvest but poorly formed and non-commercial in the older fruit. In macadamia fruit drop was also initially reduced with TPA applications but like mango not all the fruit developed properly especially in the younger fruit. Young fruit stopped growing and eventually abscised before harvest, in the older fruit, development was less effected and fruit was held until harvest, however most of this fruit was not of sufficient quality to warrant harvest. Some phytotoxic effects on young leaves were also observed. Lower rates and later application times of TPA to overcome some of the fruit development issues encountered in this research are suggested in future work.