Notes on Applying Gibberellic Acid (GA3) to Navel Orange and other Citrus

in the San Joaquin Valley of California

Craig Kallsen, Citrus and Pistachio Farm Advisor, Kern County

Typically, the price of navel oranges drops during the peak of the navel harvest season. When the peak harvest is over, prices often increase for navels that are harvested later. There is no mystery here. The price curve is merely following the law of supply and demand. When supplies are plentiful for most commodities, prices fall. Products containing gibberellic acid (GA3) are registered and available to citrus growers. For many decades growers have been extending the harvest season of navel oranges by application of plant growth regulators (PGRs) such as gibberellic acid to retard navel orange rind maturity in combination with the isopropyl ester of 2,4-D to prevent pre-harvest fruit drop. Citrus fruits, generally, store on the tree much better than in refrigerated facilities. Growers also have the option of replanting mid-season maturing orange orchards with late-maturing navels (and have been doing so). Generally, the late navels do not require application of PGRs such as GA3. However, for those that do not have the luxury of having late-maturing navels in the orchard, PGRs provide an opportunity to take advantage of higher prices that may come with a later harvest. The following “notes” may help the grower in successfully timing and applying PGRs to navel oranges. Always read and follow label directions of any chemical product carefully before using.

Note 1: Dr. Coggins, a former professor at the University of California in Riverside, spent many years researching the use of foliar-applied GA3 to prolong storage of navel oranges on the tree (for more info see: https://www2.ipm.ucanr.edu/agriculture/citrus/Delaying-Fruit-Senescence-with-Gibberellic-Acid-GA3/ ). The late September to mid- October application window, was found to be best time to apply GA3 to navels in the San Joaquin Valley for reducing puff and crease, rind staining, and, generally, for maintaining a more juvenile rind longer. Applying the gibberellic acid two-weeks before the fruit begins to change color from green to orange (called “color break”) remains a handy rule-of-thumb. Color break in mid-season navels (like Washington, Frost Nucellar, Atwood and others) usually occurs about two weeks after color break in the early navels (like Beck and Fukumoto). Dr. Coggin's research showed the GA3 was significantly more effective when a nonionic silicon-based surfactant was included with the spray as an adjuvant. Note that the addition of an effective surfactant can increase the chance and/or severity of significant leaf drop. Always follow the surfactant's label carefully and make note of any cautionary statements regarding phytotoxicity.

Note 2: Treating with an auxin (an isopropyl ester of 2,4-D is registered for this purpose) in November or early December is necessary if fruit is treated with GA3. The auxin prevents fruit from dropping too early. There is no point in delaying the maturation of the rind with GA3 into April or May if the navel is going to drop from the tree in February.

Note 3: Uptake of GA3 by the peel is improved if the spray solution is acidic. A pH of the spray solution of about 4 to 5 is recommended and several acidifying agents and products are available to accomplish this. Zinc sulfate, applied at a rate of 1 lb. of zinc sulfate/100 gallons of spray solution, has been used as an acidifying agent with gibberellic acid, which, also, helps correct zinc deficiency. Obviously, many other acidifying and buffering agents are available. In general, tank mixing other pesticides or nutrient solutions with GA3 should be avoided.

Note 4: Growers have obtained good results with GA3 applications using the labeled rates of GA3 on a weight-of-product-per-acre basis using dilute or concentrated sprays. Whichever option is selected, good spray coverage of the fruit is essential, and all else being equal, better coverage is more likely with higher spray volumes. Most of the beneficial results of GA3 are obtained with about 25 grams (active ingredient) of gibberellic acid per acre.

Note 5: Not uncommonly, a navel grower in Kern County will report a significant drop of fruit and leaves as a result of a GA3 spray. Usually in these cases, GA3 was sprayed within a week or two of a narrow-range oil spray. There appears to be a connection here, but GA3 and oil have been sprayed a few days apart with no observed phytotoxic effects. However, erring on the side of caution suggests avoiding spraying petroleum oils and GA3 within a few weeks of each other. Make sure when applying either GA3 or oil that the trees are not under water stress and that GA3 or oil are not applied to trees that show phytotoxic affects from either a previous oil or other chemical spray. The addition of a spreader adjuvant may increase the risk of leaf drop with gibberellic acid. Monitor soil-water carefully in the fall before gibberellic acid or oil is applied. The temptation is to reduce irrigation too much in response to the first light rains of fall. Often these rains, especially in the southern San Joaquin Valley, will not meet the evapotranspiration requirements of citrus, especially on the hilltops where soils are thin, leaving the trees more susceptible to damage from chemical spray applications.

Note 6: Gibberellic acid works best on blocks of fruit that normally hold well on the tree. Past harvest records can play an important role here. A good strategy is to harvest blocks that are prone to early rind breakdown first and to treat only blocks where the fruit naturally holds longer with GA3. Applying GA3 to an orchard with poor fruit-holding qualities may extend the life of the fruit a few weeks, while applying it to fruit of a good-holding block may give the grower an additional six to eight weeks of tree storage.

Note 7: Sometime fruit does not grow as quickly as a grower would like, and a block that was scheduled for an early or mid-season harvest may be rescheduled for a late season harvest. Gibberellic acid applications can still delay harvest (although not for as long a period of time) if treated later than October. Do not apply GA3 to fruit that is in the process of changing color. A permanently two-tone fruit may result. If fruit is in the process of changing color, wait until the fruit has turned completely orange and then apply the gibberellic acid. Check the label for application timing. Gibberellic acid can negatively affect next year's crop if applied too late.

Note 8: Gibberellic acid and an isopropyl ester of 2,4-D can also be applied to some other citrus fruit with useful results. Read and follow the labels carefully before applying the commercially available PGRs. Label directions include crop registrations, uses, timings, rates, cautions and other necessary information that will vary with citrus variety. Puff and crease and rind staining of Minneola tangelo, lemons, and some mandarins may be reduced and fruit storage on the tree may be extended by the use of these growth regulators. The timing of application is similar to that of navels in most cases.

Note 9: Late harvested navel varieties have been readily available to citrus growers in California now for over four decades. Late maturing navels are not as likely to require the addition of gibberellic acid and 2,4-D to produce high-quality fruit late in the season. Growers wishing to compete in the late-navel market are encouraged to plant one of the many late navel varieties.