FRUIT THINNING WITH CHEMICAL SPRAYS 15 



a sparse bloom that no thinning would be needed. A thorough knowl- 

 edge of the fruit-setting characteristics of a variety under a par- 

 ticular set of conditions is the best guide in determining the amount 

 of bloom below which spray thinning is inadvisable. 



Thoroughness of application is essential for effectiveness. Failure 

 to thin sufficiently is often a result of applying the spray material too 

 lightly. At Wenatchee, Wash., 15 to 20 percent more thinning was 

 obtained when the amount of spray material applied was increased 

 from 20 to 32 gallons per tree on 20-year-old Golden Delicious, Wine 

 sap, and Delicious. 



In many orchards, particularly the older and more crowded ones, 

 the lower limbs are likety to be weak and blossoms fail to set as 

 heavily as on the more vigorous and well-exposed branches. Depend- 

 ing upon weather and pollination, these lower weak limbs should be 

 sprayed lightly or not at all. 



Experience has shown that dinitro sprays are usually more effective 

 in reducing fruit set on young trees. Trees under 10 to 12 years of age 

 are likely to be more vigorous and frequently fail to set fruit in 

 proportion to the amount of bloom they produce. Only by trial under 

 one's own conditions can it be determined whether these sprays can 

 be safely used on such trees. There seems to be less risk involved on 

 young trees of the Golden Delicious, Duchess, Wealthy, and other 

 heavy-setting varieties than with Winesap and Delicious of com- 

 parable age. 



Thinning sprays are best applied with manually operated "guns" 

 or "brooms" that deliver the spray material under high pressure from 

 either portable or stationary spray equipment. Portable spray equip- 

 ment that applies the spray through fixed outlets is not too well 

 adapted for use with thinning sprays, except under conditions of 

 uniformly good vigor and heavy bloom. Frequently the use of such 

 equipment results in overspraying the low or weaker wood, which 

 in many cases should be sprayed lightly if at all. It is also difficult 

 to avoid spraying trees or parts of trees that do not have a sufficient 

 amount of bloom. 



EFFECTIVENESS OF GROWTH-REGULATING CHEMICALS 

 IN THINNING APPLES 



Effectiveness of Different Chemicals 



Most of the work and experience with hormone or growth-regu- 

 lating chemicals for thinning apples has involved the use of prepara- 

 tions containing naphthaleneacetic acid or the sodium salt of naph- 

 thaleneacetic acid as the active ingredient. These are the same 

 preparations that have come into general use as harvest sprays for 

 the control of fruit drop. 



In attempting to increase the set of the Starking apple, Burkholder 

 and McCown (6) sprayed halves of Starking apple trees during the 

 bloom with naphthaleneacetic acid at concentrations of 10 parts per 

 million (p. p. m.), or 0.001 percent, and 50 p. p. m., or 0.005 percent. 

 The number of blossoming spurs setting fruit on the sprayed parts 

 of the trees as compared with the unsprayed parts was reduced 15 



