Response of evergreen perennial tree crops to gibberellic acid is crop load-dependent. I: GA3 increases the yield of commercially valuable ‘Nules’ Clementine Mandarin fruit only in the off-crop year of an alternate bearing orchard

Worldwide, gibberellic acid (GA3) is used routinely to increase fruit number and size of seedless mandarins. The efficacy of seven combinations of GA3 concentrations and application times to maximize total yield and yield of commercially valuable fruit (diameter 57.2–76.2 mm) of ‘Nules’ Clementine mandarin (Citrus reticulata Blanco) was determined in a commercial orchard. GA3 applied during the period of intense flower abscission failed to reduce the total number of abscised flowers in both the light off- and heavy on-bloom years. No GA3 treatment reduced fruit abscission when trees were setting the low yield off-crop. However, all trees receiving GA3 in the high yield on-crop year had fewer abscised fruit than untreated control trees (P = 0.0188) and GA3 applied 10 days after 75% petal fall and in July increased the number of fruit retained on tagged branches >20% compared to control trees (P = 0.0005). Maximum air temperature was not related to flower or fruit abscission. In the off-crop year (548 fruit per untreated control tree), it was necessary to apply 15 or 25 mg L−1 GA3 at 60% bloom, 90% bloom, 75% petal fall and 10 days after 75% petal fall to significantly increase the number of fruit per tree and yield of commercially valuable fruit (kilograms and number per tree) (P < 0.0001) above that of control trees, with no reduction in total kilograms per tree. In the following on-crop year, it was better not to apply GA3: no treatment increased total yield or fruit size and five of seven GA3 treatments tested reduced total yield as kilograms and number of fruit per tree (P = 0.0003). The results provide strong evidence that GA3 efficacy is crop load-dependent and dictate that crop load should be considered when using GA3 to increase fruit set or fruit size of mandarins.

► Gibberellic acid (GA3) is used worldwide to increase mandarin fruit set and size.
► Standard GA3 treatments proved problematic for mandarin growers in California.
► Seven combinations of GA3 concentrations and application times were tested.
► The results demonstrated that crop load influenced the effect of GA3 on yield.
► Multiple GA3 sprays were required to increase yield and fruit size of off-crop trees.
► For trees setting an on-crop year, it was better not to apply GA3.