18 CIRCULAR 867, U. S. DEPARTMENT OF AGRICULTURE 



areas where variable weather conditions may occur during or imme- 

 diately following the bloom period. For example, apple bloom oc- 

 curred relatively early in 1949 throughout fruit sections of the North- 

 east and excellent conditions for fruit setting prevailed for several 

 days. About petal fall a prolonged period of cool weather set in and 

 some frost damage occurred in localized areas. Two weeks after 

 bloom, with the advent of warm weather, a number of varieties that 

 had overset were successfully thinned with the hormone spray. 



Working with the variety Early Mcintosh, Southwick and Weeks 

 (4-0) reported that a calyx spray of the sodium salt of naphthalene - 

 acetic acid at 20 p. p. m. caused considerable permanent curling and 

 dwarfing of the spur leaves, while a concentration of 50 p. p. m. ap- 

 plied 3 weeks after the calyx formed did not produce any appre- 

 ciable extent of this type of injury. These investigators also observed 

 that calyx applications on Mcintosh were definitely more harmful to 

 foliage than those applied 2 to 4 weeks later. 



Although much of the emphasis has been placed on a late bloom 

 or calyx spray in order to obtain the benefits of early thinning, this 

 early application has in many cases caused rather serious foliage 

 dwarfing. This spray usually causes a temporary "flagged'* or wilted 

 appearance of the foliage for several days, with a gradual return to 

 normal. However, under some conditions and especially with some 

 varieties, a serious dwarfing and curling of the leaves may result 

 and persist throughout the season. This type of injury has occurred 

 most frequently on Duchess. Yellow Transparent, Early Mcintosh. 

 Delicious, and Winesap. Severe dwarfing of the foliage of these 

 varieties by naphthaleneacetic acid sprays has been observed to inter- 

 fere with proper sizing of the fruit and to prevent fruit -bud forma- 

 tion, even though adequate thinning was accomplished. While the 

 varieties Wealthy, Jonathan, Grimes, Baldwin, and Golden Delicious 

 appear to be less susceptible, moderate leaf dwarfing from calyx 

 sprays has been observed on these varieties in some seasons. 



Experience during the past several years indicates that leaf dwarf- 

 ing from naphthaleneacetic acid sprays can be largely or entirely 

 avoided by delaying the application until 2 to 3 weeks after the calyx 

 stage. The lack of injury at this stage is probably due to the com- 

 pletion by the spur leaves of certain growth processes such as cell 

 division, cell expansion, and cuticle formation. A prolonged period 

 of cloudy, rainy weather during the calyx and early postbloom period 

 has often been' associated with severe dwarfing. Such weather condi- 

 tions might tend to delay temporarily the development of the leaves 

 to a stage at which they are resistant to dwarfing. 



Varieties that have shown susceptibility to serious leaf dwarfing 

 from calyx sprays of the hormone should not receive this treatment 

 until approximately 2 weeks after petal fall. This includes some of 

 the heavier setting summer varieties such as Yellow Transparent. 

 Duchess, and Early Mcintosh. Even though some of the benefits 

 from the earliest possible thinning may be sacrificed by delaying the 

 spray, a normal healthy leaf surface as compared with dwarfed 

 foliage will more than compensate for the loss of 2 weeks in the time 

 of thinning. 



Apples are more subject to abscission from any cause prior to the 



