Plant growth regulators (PGRs) are compounds that modify natural hormone activity to influence plant growth and development. They regulate key physiological processes such as cell division, elongation, flowering, fruit set, and senescence.
In citrus, PGRs are used to manage developmental processes such as vegetative growth, flowering, fruit retention, maturation, and to reduce preharvest fruit drop.
The major hormone groups include auxins, gibberellins, cytokinins, abscisic acid, and ethylene, along with other regulators such as jasmonates, salicylic acid, strigolactones, melatonin, and brassinosteroids, which also affect growth and stress responses.
In citrus, as in other plants, a number of processes both developmental and in response to stresses involve the combined action of many of these phytohormones. In general, a single hormone can regulate various processes, and at the same time, multiple hormones could impact a single process as well. Knowledge of the role of every PGR group and understanding their interactions will help to adopt better grove management practices. Last but not least, when applied as treatments, PGR action and effectiveness depend on the physiological status of the plant or the target organ (fruits, leaves, roots). Usually, PGRs have a narrow window of efficacy. Knowing when the best time of application is for each one maximizes the desired effects.
Plant growth regulators (PGRs) directly affect plant metabolism, so their effectiveness can vary widely depending on citrus variety, tree health, and stress level. Growers should first become familiar with PGR behavior by conducting small-scale field trials before applying them to large acreage. Most PGRs perform best when used with an adjuvant such as a surfactant, sticker, or spreader. Since PGRs are regulated as pesticides, label directions must always be followed, the label is the law.
When applying PGRs, consider the following factors:
- Concentration of active ingredient
- Spray volume and coverage
- Method and timing of application
- Compatibility with other tank-mix chemicals
- Type of adjuvant used
- Weather conditions (humidity, temperature, sunlight, wind)
- Tree health and canopy density
CAUTION: Growth regulators may cause serious problems if misused. Excessive rates, improper timing, and fluctuating environmental conditions can result in phytotoxicity, crop loss, or erratic results. Under certain environmental conditions, 2,4-D may drift onto susceptible crops in surrounding areas. Observe wind speed restrictions and follow all label directions and precautions.
- Do not use in spray solutions with pH above 8.
- November–December applications of GA may reduce return bloom in spring. If low flowering has been an issue, avoid spraying during this period.
Use of PGRs for Huanglongbing (HLB)-affected Trees
HLB-affected citrus trees often suffer from severe canopy loss and preharvest fruit drop. Among available PGRs, 2,4-D and gibberellic acid (GA₃) have shown potential to reduce fruit loss when applied correctly.
- Single sprays of 2,4-D or GA₃ alone are not effective for HLB-affected trees.
- A combined application of 2,4-D (3.2 oz/acre) and GA₃ (10 oz/acre) in November–
- December significantly reduces fruit drop in ‘Valencia’ sweet orange.
- Multiple GA₃ applications in fall (September–January, 20 g a.i./acre) with a surfactant can improve yield, canopy density, and fruit retention.
- Avoid GA₃ sprays after January, as late applications can reduce flowering and lower next season’s yield.
Commercial grove management practices are reviewed and updated annually in the Florida Citrus Production Guide: Plant Growth Regulators
