- Author: Ben Faber
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.
- Author: Ben Faber
Avocados in some parts of coastal California have been blooming. Some of them got hit by the cold weather in the first part of February. In the coldest areas there was a little bit of new leaf damage, but this has been minimal.
Some browning of some flowers and stems (pedicels - the little stalks the connect the flowers to the larger raceme/panicle) may have occurred, but I haven't heard of major flower damage.
It's early days for flowering, though, and most ‘Hass' trees are not very far along, but seem like they area about to burst. A recent visit on a 40 acre farm in Saticoy had trees in a whole range of stages, some with no flowers pushing, some with panicles just starting to open individual flowers and many trees on their north sides' completely quiet. Many are still just pushing into the cauliflower stage,
which is the ideal time is for applying Pro-Gibb to improve fruit set in healthy orchards.
Application time is when 50% of the trees in the block have 50% of their bloom in the cauliflower stage. This is a judgment call when there can be such huge variation in bloom across and orchard. It's going to be a best estimate call for when to do the application. As usual with a new technology/practice don't apply to the whole orchard so that you can see whether the application is warranted.
For a more detailed discussion of gibb application, read Carol Lovatt's article:
- Author: Ben Faber
At a recent grower meeting at South Coast Research and Extension Center in Irvine, Carol Lovatt gave a talk on the use of gibberellic acid to increase avocado fruit set and yield. The registration for the use only occurred this spring, barely in time for growers to use it, so not many applied it. Then we had this heat wave in July and a lot of fruit, whether it had been treated or not, fell off. A show of hands was asked of the growers present, who had applied it this spring. Of the 100 people or so in attendance, only five raised their hands. Of course, this is not a scientific survey, but most would try it again this coming spring, even though they might not have seen results this year.
The following are guidelines that were provided on how to use it, if you should so choose this coming spring:
Use of ProGibb LV PlusR Plant Growth Regulator on Avocado to Increase Fruit Size and Yield
Professor of Plant Physiology, Emerita, and Professor in the Graduate Division, Department of Botany and Plant Sciences, University of California, Riverside firstname.lastname@example.org
ProGibb LV PlusR. On March 27, 2018, gibberellic acid (GA3) was approved for use on avocado to increase fruit size and yield. The only material registered for this purpose is ProGibb LV PlusR, a low volatile organic compound (LVOC) formulation, manufactured by Valent BioSciences, Corporation (Libertyville, IL). Only this product may be used; the older formulation sold under the name ProGibbR and other generic GA3 products cannot be used. Note: (i)the restricted entry interval is only 4 hours; (ii) the preharvest interval is 0 days; and (iii) ProGibb LV PlusÒ can be used in certified organic orchards.
Application Time. ProGibb LV PlusR is applied as a foliar spray at the cauliflower stage of avocado inflorescence development. The applications should be made when 50% of the trees in the block have 50% of their bloom at the cauliflower stage. This means that 25% of the bloom will be at an earlier stage of inflorescence development and 25% will be approaching bloom (open flowers). If you are unable to make the application at this time, being slightly late in applying the treatment affords better efficacy than being too early. Note: applications made at full bloom are typically not effective.
ProGibb LV PlusR Dose and Dilution Rate. The sprays should be applied like a pesticide spray to give full canopy coverage, especially of the developing inflorescences, but not sprayed to run-off. For ground application, use 12.5 fluid ounces of ProGibb LV PlusR (25 grams active ingredient [g ai]) in 100 gallons of water/acre. For aerial (helicopter) application, use 12.5 fluid ounces (25 g ai) in 75 gallons of water /acre. The maximum allowable dose is 25 g GA3 (active ingredient)/acre. Note: the results of our research documented that lower and higher doses are less effective.
Spray Solution pH. The final pH of the spray solution in our research was between pH 5.5 to 6.0. ProGibb LV PlusR is stable at pH 4.0 to 8.5. The pH of the water used should be adjusted accordingly. Note: prolonged exposure of GA3 to a pH > 8.5 should be avoided to prevent breakdown of the material.
Additional Information on Spray Volume. In our research, for ground applications, we used the same amount of GA3 (25 g ai/acre) but a spray volume of 200 to 250 gallons of water/acre, depending on tree size, to achieve good coverage without causing the material to run-off the tree and with minimum spray volume left in the tank after application. Use of spray volumes greater than label rate of 100 gallons of water/acre for ground application is the decision of the Agricultural Commissioner for each county. Consult with your County Agricultural Commissioner, if you wish to apply ProGibb LV PlusR (25 g ai) in more than 100 gallons of water/acre as a ground spray. For the aerial (helicopter) application, the greatest efficacy was achieved with ProGibb LV PlusR (12.5 fluid ounces, 25 g ai) in 75 gallons of water/ acre.
Wetting Agent. In our research, we used the organosilicone surfactant Silwett L-77R or Widespread MaxR (Loveland Industries, Greely, CO) at a final concentration of 0.05%. Similar pure organosilicone type surfactants are acceptable.
Photo: Cauliflower stage inflorescence. Source: Salazar-García et al., 1998.