Submitted to: British Journal of Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 12, 1996
Publication Date: N/A
Interpretive Summary: More than $120 million of Florida grapefruit are shipped annually to Japan and the fruit must meet certain quarantine regulations imposed by Japan. This report is on the horticultural aspects of applying gibberellic acid (GA) as an alternative to malathion bait sprays for fly-free certification of grapefruit groves through the end of February. The entomological aspects of this research are reported in another report. The research shows that GA-treated grapefruit exhibit early-season levels of resistance through at least February. It will enable growers who cannot otherwise meet the current 1/2 mile buffer zone requirement after December 20 to continue to certify their fruit as being fly-free. Growers will be able to recover at least $1,000 per acre of extra value from GA-treated groves during the period from December 20 to February 28. Approximately 70,000 acres will be affected, and it should increase the export of Florida grapefruit.
Technical Abstract: 'Marsh' grapefruit (Citrus paradisi Macf.) peel senescence was reduced by gibberellic acid (GA) treatment as indicated by resistance to puncture and the retention of green color at different production sites in Florida. Applications of GA at 49 g ha-1 plus 0.05% Silwet L-77 were equivalent to 25 g ha-1 plus 0.1% Silwet L-77. GA at 49 g ha-1 plus 0.05% Silwet L-77 was more effective than the maximum label amount of GA (145 g ha-1) without surfactant. GA applications just prior to colorbreak were found to be most effective. GA treatments maintained peel oil at levels higher than untreated fruit, but did not affect fruit quality. Chlorophyll and carotene levels were higher in GA-treated fruit harvested in October and November than fruit harvested the remainder of the season. Generally, an additional day was required to degreen GA-treated fruit compared with nontreated fruit. Leaf drop was a function of grove site and generally was unaffected by GA treatment. The data suggest that GA efficacy is not dependent on application method, but on spray volume and GA dose.