How to Manage Pests

UC Pest Management Guidelines

Citrus

Plant Growth Regulators: General Information

(Reviewed 9/08, updated 9/08)

In this Guideline:


The plant growth regulators 2,4-dichlorophenoxyacetic acid (2,4-D), gibberellic acid (GA3), and naphthaleneacetic acid (NAA) are registered for preharvest use on California citrus crops. 2,4-D is used mainly to delay and reduce unwanted fruit abscission (fruit drop), GA3 is used mainly to delay senescence (overripening), and NAA is used to promote abscission of excess fruit (thinning to increase the size of the remaining fruit) and to inhibit the growth of suckers on the trunk.

In order to be effective, plant growth regulators must be absorbed by plant tissue. Good spray coverage is essential and climatic conditions that favor absorption (warm and humid conditions) are therefore desirable. Consider such factors as tree size, canopy density, location of fruit, and type of spray equipment when deciding how much spray material will be required to achieve good coverage. Apply all spray materials uniformly to the fruiting canopy. Be advised that plant growth regulators are potent compounds and care is warranted in their use.

Both 2,4-D and GA3 seem to be compatible with urea, potassium foliar sprays, zinc and manganese micronutrient sprays, and neutral copper sprays, but the timing of growth regulator applications may not coincide with the best time for nutrient sprays.

2,4-dichlorophenoxyacetic acid (2,4-D)

2,4-D is used to control preharvest fruit drop, increase fruit size (oranges, grapefruit, mandarin, and mandarin hybrids), and to control leaf and fruit drop following an oil spray. When you use 2,4-D to reduce drop of mature fruit, apply the compound before (preferably shortly before) fruit drop becomes a problem, but far enough ahead of flowering to minimize undesirable effects that 2,4-D would otherwise have on the spring cycle of growth. For navel oranges, October through December sprays are common. October, however, may be too early to effectively reduce fruit drop if conditions favor it (e.g., warm winter, protracted harvest). January sprays may be somewhat risky, especially when environmental factors favor an earlier-than-usual spring flush of growth.

For mature grapefruit and 'Valencia' orange trees, 2,4-D can be applied to control drop of mature fruit or as a dual-purpose spray (to control mature fruit drop and to improve fruit size for the next year's crop). Fruit-sizing sprays require excellent coverage. In general, 'Valencia' orange is more responsive than grapefruit to fruit-sizing sprays. For mandarin and mandarin hybrids, 2,4-Dfruit sizing sprays are applied 21 to 35 days after 75% petal fall.

Recommendations are also included in these guidelines for the use of 2,4-D in pesticide oil sprays to counteract leaf and fruit drop caused by the oil.

Precautions When Using 2,4-D
  • Avoid 2,4-D spray drift to susceptible plants, which include cotton, grapes, roses, beans, peas, alfalfa, lettuce, ornamentals, and all broadleaf species.
  • If 2,4-D is applied shortly before or during a flush of growth, vegetative and reproductive growth may be damaged. This may result in lower fruit production, especially if the spring flush is affected.
  • The effectiveness of 2,4-D for controlling fruit drop is enhanced by oil and decreased by calcium hydroxide (calcium hydrate, hydrated lime). The magnitude of the oil enhancement and the magnitude of the reduction caused by calcium hydroxide are not sufficiently understood to permit any extrapolation of University of California recommendations or product label instructions. Do not vary from label rates.
  • Application of 2,4-D as a fruit-sizing spray can cause fruit dryness in mandarin and mandarin hybrids that tend to be less juicy, e.g., 'Nules' Clementine or in orchards prone to granulation.

Older recommendations and product labels specified 2,4-D dosages in terms of concentration (ppm or mg/liter). Because current spray volumes vary widely, University of California recommendations and current labels specify the amount of product per acre rather than ppm or mg/liter. If applied properly (i.e., if coverage is adequate and if the spray deposit does not dry rapidly), an application of a particular per-acre dosage of 2,4-D has the potential to be effective for controlling mature fruit drop and for delaying fruit senescence when applied at spray volumes of100 to 750 gallons per acre. Lower-volume applications (100 gal/acre) are less forgiving of imprecise spraying than are higher-volume applications (500 gal/acre). Success with 2,4-D applications to improve fruit size requires excellent coverage and wetting; low-volume applications are not known to be effective. In general, surfactants (wetting agents) help achieve good spray coverage. Many surfactant formulations are available in the marketplace. Some can cause rind blemishes on citrus fruit, so you need to find a suitable surfactant for citrus, whether through direct experimentation or by contacting an experienced citrus pest control operator.

Gibberellic acid (GA3)

The purpose of applying GA3 to citrus trees in California is to delay fruit senescence. 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 in color 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. GA3 application may result in leaf drop, which can be severe, especially when it is applied to navel orange trees that are under heat or water stress. When this happens, the tree may also suffer twig dieback. by including 2,4-D in the GA3 spray, you may be able to reduce this kind of damage. 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, to delay aging and rind softening of tangerine (mandarin) hybrids, and as a fruit-setting agent for clementine mandarin.

Precautions When Using GA3
  • 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. As a general rule, do not expose GA3 to solutions higher than pH 8. Values below pH 8 may provide greater stability for GA3 and better absorption by plant tissue.
  • Older recommendations and product labels specified GA3 dosages in terms of concentration (ppm or mg/liter). Because current spray volumes vary widely, University of California recommendations and current labels specify the amount of product per acre rather than ppm or mg/liter. If applied properly (i.e., if coverage is adequate and if the spray deposit does not dry rapidly), an application of a particular per-acre dosage of GA3 will have the potential for controlling mature fruit drop and for delaying fruit senescence when applied at spray volumes of 100 to 750 gallons per acre. Lower-volume applications (100 gal/acre) are less forgiving of imprecise spraying than are higher-volume applications (500 gal/acre).
  • In general, surfactants (wetting agents) help achieve good spray coverage. Many surfactant formulations are available in the marketplace. Some can cause rind blemishes on citrus fruit, so you need to find a suitable surfactant for citrus, whether through direct experimentation or by contacting an experienced citrus pest control operator. Research has shown that a suitable organ silicone adjuvant such as Silwet L-77 can increase the efficacy of GA3 applied to navel orange trees. Two cautions are in order: increased efficacy also means increased risks of negative effects, such as excessive leaf drop and twig die-back from GA3; and rind blemishes have been reported from relatively high adjuvant concentrations. (Silwet L-77 has a good rind blemish safety record at a concentration of 0.025%, v/v, active ingredient basis.)

Naphthaleneacetic acid (NAA)

NAA can be a very effective fruit-thinning agent for certain types of citrus. A different formulation is registered for sucker control. For fruit thinning, label rates are100 to 500 ppm. Within this concentration range, an application may result in inadequate to excessive thinning. In general, inadequate thinning occurs from the lowest label rate when maximum daytime temperatures on the day of application and several days thereafter are relatively low (~85°F [29°C]). Excessive thinning generally occurs from the highest label rate when maximum daytime temperatures on the day of application and several days thereafter are relatively high (~100°F [38°C]). In addition, excessive thinning can occur when NAA is applied to unhealthy or water-stressed trees. Heavy application of NAA, 1.15% liquid concentrate, to inhibit sucker growth on tree trunks, may result in tree damage. Trees should be nonbearing. Do not apply after September 1.

Precautions When Using NAA
  • Success with NAA as a fruit-thinning agent requires excellent coverage and wetting; low-volume applications are not known to be effective. In general, surfactants (wetting agents) help achieve good spray coverage. Many surfactant formulations are available in the marketplace. Some can cause rind blemishes on citrus fruit, so you need to find a suitable surfactant for citrus, whether through direct experimentation or by contacting an experienced citrus pest control operator.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441

Plant Growth Regulators

  • C. J. Lovatt, Botany and Plant Sciences, UC Riverside
Acknowledgment for contributions to Plant Growth Regulators:
  • C. W. Coggins, Jr., Botany and Plant Sciences, UC Riverside

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