Use of GA3 to manipulate flowering and yield of 'Hass' avocado

Avocado trees (Persea americana Mill.) bearing a heavy crop produce a light "off" bloom the next spring. This results in a light crop and a subsequent intense "on" bloom the year after. The objective of the study was to quantify the effects of GA3 canopy sprays applied to 'Hass' avocado trees during the months preceding an "off" or "on" bloom on inflorescence and vegetative shoot number and yield. The experiment was initiated approximately seven months before an anticipated "off" bloom in an attempt to increase flowering intensity and yield. GA3 (25 or 100 mg(.)L-1) was applied to separate sets of trees in September (early stage of inflorescence initiation), November (early stage of inflorescence development), January (initial development of the perianth of terminal flowers), March (cauliflower stage of inflorescence development; only 25 mg(.)L1), or monthly from September through January (only 25 mg(.)L-1). Control trees did not receive any treatment. GA3 (100 mg(.)L-1) applied in September reduced inflorescence number in both years, but not yield. GA3 (25 or 100 mg(.)L-1) applied in November before the "on" bloom reduced inflorescence number with a concomitant increase in vegetative shoot number and 47% yield reduction compared to control trees. This treatment might provide avocado growers with a tool to break the alternate bearing cycle by reducing yield in an expected "on" crop year to achieve a higher yield the following year. GA3 (25 mg(.)L-1) applied in November or January stimulated early development of the vegetative shoot of indeterminate inflorescences. January and March applications did not affect the number of flowering or vegetative shoots produced either year. GA3 (25 mg(.)L-1) applied in March at the start of an "off" bloom increased 2-fold the production of commercially valuable fruit (213 to 269 g per fruit) compared to the control.