Manipulation of avocado (Persea americana Mill.) trees for out of season fruit production

Abstract

The seasonality of avocado production leads to over-supplied markets during peak periods followed by shortages of fruit during other periods. The production of out of season avocado fruit was seen by Westfalia Estate as an opportunity to increase their profits. As the Estate is committed to supply Woolworths with fruit for 12 months of the year, this study was undertaken to investigate the production of out of season avocado fruit through chemical and physical manipulation of trees. Gibberellins (GA) are known to have many regulatory functions on various processes in plants, including flowering. Trials were conducted during 1999 in which the effect of gibberellic acid (G3) sprays on flowering of avocado was evaluated. A microscopic study revealed that the first signs of inflorescence development were already present in 'Hass' buds sampled in mid March. Secondary inflorescence axis meristems could be seen as small axillary meristems in the axils of the inner terminal bud bracts. GAJ sprays delayed the development of these secondary inflorescence axis meristems. In a more comprehensive study, the effect of single and multiple GAJ treatments (50 and 250 mg.L-1) on flowering of potted 'Fuerte', 'Hass' and 'Ryan' trees was evaluated. Depending on the timing of the application, single GA3 treatments did not affect (February), delayed (mid March and early April) or enhanced (late April and May) inflorescence development of avocado trees. However, none of these effects were significant. Multiple GA3 treatments applied over a four month period significantly inhibited inflorescence development. The effect on inflorescence development tended to increase with increasing number of sprays applied and with an increase in the concentration used. Timing of GA3 sprays proved to be crucial and determined what kind of effect was obtained. In order to inhibit flowering reliably, the first application must be applied before floral initiation and the last application during advanced stages of inflorescence development. In another trial, 'Fuerte' trees were de-flowered during the normal flowering period (August/September 1999) by the application of an Ethrel® spray. Three months after the de-flowering treatment, out of season flowering was induced by cincturing the trees. Out of season flowering and fruit set were observed in January/February 2000. Normal season fruit were harvested in April 2000 at a moisture content of 70%, whereas the out of season fruit were harvested in November 2000 at a moisture content of 62%. Although, the out of season yield (17.7 kg.tree-1) was significantly lower than the normal season yield (45.8 kg. tree-1), the extremely good prices obtained out of season makes this a highly profitable venture. Out of season fruit quality proved to be equal to or even better than the normal season fruit quality. The de-flowering treatment resulted in unnecessary stress being inflicted onto the trees and may be a limiting factor in the long term sustainability of out of season fruit production. In this regard, the use of G3 sprays to inhibit flowering during the normal season may eliminate the problems associated with the de-flowering treatment.