Possible Methods to Increase Efficacy of Gibberellic Acid Applied to Navel Orange Trees

Reports from Israel and South Africa indicate the effectiveness of preharvest applications of gibberellic acid (GA₃) to citrus can be improved substantially by lowering the pH of the treatment solution or by the addition of surfactant. Silwet^® L-77 (Union Carbide) — a polyalkylene oxide modified dimethylpolysiloxane — has been reported to be especially effective. Since increased GA₃ efficacy on citrus is of both scientific and economic interest, we conducted a field experiment at two California locations (Riverside and Tulare Counties) to assess the impact of pH (~4.2 vs. ~7.8), L-77 (0.025% v/v), and combinations thereof on the biological effectiveness of preharvest applications of GA₃ (2.5, 5, and 10 ppm) in delaying Citrus sinensis (L.) Osbeck rind senescence. Treatments were applied in October (8 trees/treatment/location) at the rate of ~8000l/ha and fruit were evaluated 14 to 29 weeks later. Based on rind-color and rind firmness data, we observed the following trends: (1) low pH, by itself, did not lead to a significant increase in efficacy for any GA₃ rate, and (2) presence of L-77 increased GA₃ efficacy regardless of pH level. Clearly, of the two factors evaluated, inclusion of L-77 in the spray solution appeared most promising.