Plant growth regulators are registered for use on California citrus to
- increase fruit set.
- increase fruit size, either directly or indirectly by thinning (removing) fruit to increase the size of the remaining fruit.
- prevent fruit senescence.
- control preharvest fruit drop.
- control leaf and fruit drop following an oil spray.
- control suckering.
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. If temperatures are projected to reach approximately 85ºF or higher during the day, applications should be made just after daybreak or after the temperature drops in the evening. Uptake is improved with these application times due to higher relative humidity. In addition, the nightly increase in relative humidity in the field may cause renewed uptake of the compound from the spray residue. Make winter applications during the warmest part of the day, not to exceed 85ºF.
Consider factors such 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. All plant growth regulator applications are more successful on healthy, well-watered trees with adequate nutrition. Note: plant growth regulators are potent compounds; be careful when applying them.
Considerations When Using Plant Growth Regulators
Older recommendations and product labels specified plant growth regulator 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 a plant growth regulator will have a greater potential for efficacy 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. However, some can cause rind blemishes on citrus fruit. Find a suitable surfactant for citrus either through direct experimentation or by contacting an experienced citrus pest control operator.
Research has shown that a suitable organosilicone adjuvant such as Silwet L-77 can increase the efficacy of plant growth regulators applied to citrus trees. Two cautions are in order when using adjuvants:
- increased efficacy also means increased risks of negative effects, such as excessive leaf drop.
- twig dieback 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, on an active ingredient basis.)