Make applications while the fruit are still physiologically young but are approaching maturity. GA3 can have a negative effect on flowering and thus on production for the following year, especially if it is applied much later than specified on the current label or in these guidelines. It delays changes in rind color, an effect that can be considered either desirable or undesirable. For example, if you apply GA3 to navel orange trees while the fruit still have green rinds, delayed coloring will have a negative effect on your ability to harvest and market the fruit early in the season. In contrast, this effect is desirable for late-harvested fruit because it delays rind senescence, which results in fruit that are paler than the deeper-colored fruit from untreated trees.
GA3 applications amplify the re-greening of Valencia oranges. This is considered undesirable and can be minimized if you apply the compound no later than the date specified on the label or in these guidelines.
There is little need for delaying fruit senescence on young trees. This plus the possibility of excessive leaf drop argue against applying GA3 to young citrus trees. Recommendations are also included in these guidelines for using GA3 to delay lemon and lime fruit maturity and to delay aging and rind softening of tangerine (mandarin) hybrids.
GA3 seems to be compatible with urea, potassium foliar sprays, zinc and manganese micronutrient sprays, and neutral copper sprays, but the timing of its applications may not coincide with the best time for nutrient sprays.
When making later season applications, be sure to review the effect of this treatment on young, setting fruit in the section INCREASING FRUIT SET WITH GIBBERELIC ACID (GA3).
Precautions When Using GA3
Observe all precautions below and comments given with each plant growth regulator use:
- GA3 is slowly hydrolyzed by water and rapidly converted into an inactive isomer in highly alkaline solutions. Protect liquid and powder formulations from moisture and do not add GA3 to highly alkaline spray mixtures.
- Although GA3 seems to be stable in solutions up to pH 11 for short periods of time (2 hours), its activity is diminished rapidly at the high pH values found in Bordeaux and whitewash mixtures. Keep the pH of your GA3 solutions below pH 8, or lower according to manufacturer's label.
- Some observational evidence suggests that foliar petroleum oil sprays applied within two weeks of GA3 may result in significant leaf drop.
| Variety | Amount per acre | REI‡ | Application timing | |
|---|---|---|---|---|
| (g a.i. = grams active ingredient) | (hours) | |||
| Always read the label of the product being used. | ||||
| Note: Certain formulations emit high amounts of volatile organic compounds (VOCs); use low-VOC formulations of gibberellins whenever possible. Regulations affect the use of high-VOC gibberellic acid on citrus for the San Joaquin Valley between May 1 and October 31. The San Joaquin Valley includes all of San Joaquin, Stanislaus, Merced, Madera, Fresno, Kings, and Tulare counties and the valley portion of Kern County. Growers, PCAs, Pest Control Dealers, and interested parties can learn when high-VOC prohibitions are in effect at www.cdpr.ca.gov or by contacting the local County Agricultural Commissioner’s Office. | ||||
| A. | NAVEL ORANGES# | 10–40 g a.i. in water spray | 4 | See comments |
| COMMENTS: Used to reduce rind staining, water spot, and sticky rind (delayed aging and softening of rind). Apply between September and November to groves that will be harvested later than February 15 (San Joaquin Valley) or later than March 1 (Southern California). Preferred application time is 2 weeks before color break. If delayed coloring cannot be tolerated, apply after marketable color has developed. In both cases, there will be delayed rind aging but when it is applied after color has developed, considerable aging will have already occurred, resulting in less potential delay in aging. The effect of the later spray may be inadequate to provide the desired protection. Make application at least 3 days before or 3 days after whitewash (hydrated lime) and avoid December and January applications, as they may decrease subsequent production. Treatment with GA3 probably lowers intensity of puffy rind and fruit appears to be less susceptible to postharvest decay and mechanical injury. GA3 may result in a minor amount of leaf and fruit drop. Occasionally, leaf drop and fruit drop is excessive. When this happens, twig dieback can occur. Including 2,4-D in the GA3 spray may reduce these negative effects. | ||||
| B. | VALENCIA ORANGES# | 40–80 g a.i. | 4 | Aug.–Sept. |
| COMMENTS: Used to reduce rind creasing and delay aging and softening of rind. Apply at an early stage of fruit development. Application at a later stage may lead to more regreening than is acceptable. Some regreening or slower color development should be expected in the target crop and some increased regreening of mature fruit, if present, may occur. To improve coverage, use high spray volumes (500 gallons). | ||||
| C. | TANGERINE (MANDARIN) HYBRIDS# | 20–40 g a.i. | 4 | See comments |
| COMMENTS: Used to delay rind aging and softening and to reduce puffiness of rind. Apply about 2 weeks before color break, but only to groves where early harvest will not occur as this treatment delays coloring; satisfactory color should not be expected until late January. Later GA3 applications may produce undesirable results: applications made during coloring may result in unacceptable variations in rind color and applications made after coloring may cause preharvest rind staining to occur. | ||||
| D. | LEMONS# | 10–20 g a.i. | 4 | Oct.–Dec. |
| (nondesert areas) | ||||
| COMMENTS: Used to delay fruit maturity. Apply when target crop is 1/2 to 3/4 full size and still green. Reduces the number of small tree-ripe fruit and delays flowering, which shifts second-year crop toward summer. | ||||
| E. | LIMES# | 20 g a.i. | 4 | Sept.–Oct. |
| (nondesert areas) | ||||
| COMMENTS: Used to delay rind coloration. Apply when target crop is 1/2 to 3/4 full size and still green. Use amount given for large mature trees and less for smaller trees. | ||||
| F. | GRAPEFRUIT/PUMMELO# | 16–48 g a.i. | 4 | See comments |
| COMMENTS: Apply as 1 or 2 dilute sprays; either one application 2 weeks before color break (Aug.–Sept.) or one application after marketable color has developed (Oct.–Dec.), or at both application times. Do not apply in combination with an organo-silicone surfactant. | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. |
| # | Acceptable for use on organically grown produce. |