15 



GROWTH REGUUTORS IN ORCHARD HAMAGEHENT 



Duane W. Greene 



Department of Plant and Soil Sciences 



University of Massachusetts 



Growth regulators are an important component of apple production. They 

 are used more intensively on apples than on any other horticultural crop, and 

 they can regulate important physiological processes, resulting in higher and 

 more consistent yields of high-quality fruit. Growth regulators include 

 hormones found naturally in the plant and similar synthetic compounds. 



Chemical Thinning 



The oldest, yet probably the most important, use of plant growth 

 regulators remains in the area of chemical thinning. Apple trees frequently 

 produce too many flowers, and if all flowers set fruit which develop to 

 maturity, the fruit will be too small and flower bud formation for the crop the 

 following year will be either reduced or eliminated. Chemical thinners can be 

 applied from shortly after bloom until about ^ weeks afterwards. Weather 

 conditions determine the exact length of time during which fruit can be thinned 

 chemically. 



Carbaryl is the mildest and safest thinning agent, but it should not be 

 used at bloom time because it is especially toxic to honey bees. Some growers 

 also are reluctant to use this compound because of its possible adverse effect 

 on predator mites. 



Naphthaleneacetic acid (NAA) is a more potent thinner and is used on the 

 more difficult to thin cultivars and on those cultivars that have a 

 particularly heavy bloom. Naphthaleneacetamide (NAAm or NAD) is milder than 

 NAA and is used when NAA could cause injury or leaf dwarfing. NAD is not 

 recommended for use on Delicious because of the possibility of producing many 

 small, seedless fruit called "pygmies." Promalin^M, applied with a surfactant, 

 has some thinning ability when used to elongate Delicious. Spray combinations 

 of carbaryl and NAA or NAD are becoming increasingly popular — especially where 

 growers want to get increased thinning, yet reduce the possibility of 

 overthinning or foliar damage seen with the higher rates of "AA or mad. 



Preharvest Drop 



Daminozide (Alar-85^^) is the most important compound used to control 

 preharvest drop. It is most effective when applied near harvest, but label 

 restrictions specify that it cannot be applied within 50 days of harvest. 

 Since daminozide is less effective when application is made earlier in the 

 season, it should be applied as close to harvest as the label will allow. 

 Daminozide is under review by the EPA and its future registration status is in 

 question. 



NAA also retards preharvest drop. It becomes effective within one day of 

 application, and it can control drop for 7 to 10 days. If needed a second 

 application may be made. NAA has the negative effects of advancing ripening 

 and causing fruit softening. 



