When you use 2,4-D to reduce preharvest 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 mature grapefruit and Valencia orange trees, 2,4-D can be applied to reduce preharvest drop of mature fruit or as a dual-purpose spray (to reduce mature fruit drop and to improve fruit size for the next year's crop). 2,4-D can also be added to pesticide oil sprays to reduce leaf and fruit drop caused by the oil.
When making later season applications, be sure to review the effect of this treatment on young setting fruit in the section INCREASING FRUIT SIZE WITH 2,4-DICHLOROPHENOXYACETIC ACID (2,4-D).
Precautions When Using 2,4-D
Observe all precautions below and comments given with each plant growth regulator (PGR) use.
- Avoid 2,4-D spray drift to susceptible plants, which include cotton, grapes, roses, beans, peas, alfalfa, lettuce, ornamentals, and all broadleaf species.
- Applying 2,4-D shortly before or during a flush of growth may damage vegetative and reproductive growth. This may result in lower fruit production, especially if the spring flush is affected.
The effectiveness of 2,4-D for reducing 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.
| Variety | Amount to use | REI‡ | Application timing | |
|---|---|---|---|---|
| (g a.e. = grams acid equivalent) | (hours) | |||
| Always read the label of the product being used. | ||||
| TO REDUCE PREHARVEST FRUIT DROP | ||||
| A. | NAVEL ORANGES* | 30 g a.e./acre in water spray | 12 | Sept.–Nov. |
| . . . or . . . | ||||
| 15 g a.e./acre in water spray | Dec.–Jan. | |||
| . . . or . . . | ||||
| 45 g a.e./acre in hydrated lime spray | Sept.–Nov. | |||
| COMMENTS: The 30 g/acre rate applied in water is more effective than the 45 g/acre rate applied in hydrated lime (whitewash). This is important during long harvest seasons. The preferred October through November application is a 30 g/acre water spray 3 days before or 3 days after whitewash. October through December sprays are commonly used and are generally effective. However, September and October may be too early to provide good fruit drop reduction when conditions favor fruit drop (e.g., warm winter, protracted harvest). On the other hand, January sprays may be somewhat risky, especially when environmental factors favor an earlier-than-usual spring flush of growth. For example, consider adding 6 to 18 g/acre in a water spray containing gibberellic acid (GA3) followed by up to 30 g/acre in a November water spray. The 2,4-D in the GA3 spray may reduce the GA3-induced leaf and fruit drop and will provide some help with mid- to late-season mature fruit drop. However, the 2,4-D applied with the GA3 early in the season will not provide adequate mid- to late-season reduction of mature fruit drop in most years. | ||||
| B. | VALENCIA ORANGES* | 15 g a.e./acre in water spray | 12 | See comments below |
| COMMENTS: If the desired response is to reduce drop of mature fruit and minimize effects on size or quality of next year's fruit, apply when fruit of the following crop is at least 0.5 inch (13 mm) in diameter. This treatment also reduces fruit stem dieback. | ||||
| C. | GRAPEFRUIT* | |||
| (summer fruit drop) | 30 g a.e./acre in water spray | 12 | See comments below | |
| (winter fruit drop) | 45 g a.e./acre in hydrated lime spray | 12 | Oct.–Nov. | |
| . . . or . . . | ||||
| 30 g a.e./acre in water spray | Oct.–Jan. | |||
| COMMENTS: If the desired response is to reduce drop of mature fruit and minimize effects on size or quality of next year's fruit, apply when fruit of the following crop is at least 0.75 inch (19 mm) in diameter. The 30 g/acre rate applied in water is more effective than the 45 g/acre rate applied in hydrated lime (whitewash). This is important during long harvest seasons. The preferred winter fruit-drop control application is a 30 g/acre water spray applied 3 days before or 3 days after whitewash. January sprays may be somewhat risky, especially when environmental factors favor an earlier-than-usual spring flush of growth. Grapefruit may require special spray application techniques to achieve coverage of interior fruit. These sprays also reduce fruit stem dieback. | ||||
| D. | LEMONS* | 15 g a.e./acre in water spray | 12 | Oct.–Dec. |
| COMMENTS: Apply a single treatment. Applications to coastal lemons are risky because of their everbearing nature. | ||||
| E. | TANGELOS and OTHER CITRUS HYBRIDS* | 30 g a.e./acre in water spray | 12 | Sept.–Nov. |
| COMMENTS: Only apply once per season. | ||||
| TO PREVENT FRUIT DROP CAUSED BY AN OIL SPRAY | ||||
| A. | ALL* | 2.2 ml 2,4-D isopropyl ester formulation/gal oil | 12 | See comments |
| COMMENTS: Used in pesticide oil sprays to counteract leaf drop and fruit drop caused by the oil. Gives some degree of preharvest fruit drop control. However, because of the long time span between most pesticide oil sprays and the onset of mature fruit drop, the 2,4-D/oil spray may not give adequate mature fruit drop control, especially if the per acre 2,4-D dosage is less than 24 grams acid equivalent (60 ml of a formulation containing 3.34 or 3.36 lb of 2,4-D/gal). Under these conditions, an aqueous 2,4-D preharvest spray may be needed. If the 2,4-D/oil spray occurs within 3 to 4 months of the fruit drop period, and if the per acre dosage was at least 24 grams, no additional 2,4-D should be applied. If the fruit drop period will occur 4 to 6 months after the 2,4-D/oil spray was applied, apply an aqueous spray at about 50% of recommended rates for preharvest fruit drop control. If the fruit drop period will occur more than 6 months after the 2,4-D/oil spray, apply full rates recommended for preharvest fruit drop control. | ||||
| ‡ | 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. |
| * | Note: (1) Do not apply to trees less than 6 years old. (2) Do not apply within 7 days of harvest (PHI= 7 days). (3) 2,4-D 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. (4) Use an isopropyl ester formulation of 2,4-D containing 3.34 or 3.36 lb of acid equivalent/gallon. These formulations provide about 1,520 g acid equivalent/gal, 12 g acid equivalent/fl oz, 402 g acid equivalent/liter, or 0.402 g acid equivalent/ml. An example for 30 g acid equivalent/acre: (30g)/(0.402 g/ml) = 75 ml. Thus, 30 g would be contained in 75 ml or 2.5 fl oz of formulation. Apply this to one acre in volumes up to 500 gal. (Added to 500 gal it would be a 16 ppm solution). If spray volumes exceed 500 gal/acre, follow restrictions specified on the label. |