F^UIT THINNING WITH CHEMICAL SPRAYS 9 



humidity, little or no difference is obtained on normal healthy trees 

 with concentrations varying from 1 pint to 1 quart per 100 gallons. 

 Thus, during the latter type of bloom period set reduction may be 

 largely a result of the toxicant acting as a pollinicide or causing 

 injury to the stylar tissue within a short time after pollination ensues. 

 Supporting this premise MacDaniels and Hildebrand {28) have 

 shown that pollen germination is completely inhibited at concentra- 

 tions well below 0.1 percent (0.8 pint per 100 gallons) Elgetol. 



Aside from weather conditions, concentration may play a role in 

 results, depending on such factors as variety and tree condition. As 

 Verner and Franklin (J$) have pointed out, concentration may prove 

 to be of greater consequence as it approaches critical limits beyond 

 which overthinning results. With a given variety the critical limit 

 of concentration varies greatly, depending upon tree condition and 

 environmental factors. In order to obtain any appreciable indirect 

 effect of the thinning sprays during seasons of warm, dry weather, 

 it may be necessary under many conditions to use concentrations 

 higher than the conventional range. 



Under Northwest conditions Elgetol sprays are generally used at 

 higher concentrations with a greater margin of safety than under 

 most eastern conditions (5, 1$). The probable reason is due to the 

 uniformly greater vigor of the trees and usually more favorable 

 weather during the bloom period. 



Varietal Response 



In general, chemical thinning can be used with greater safety and 

 dependability with varieties that tend to set heavy crops. With a 

 moderately heavy bloom, under conditions prevailing in the North- 

 west, production of a commercial crop usually requires about 35 to 40 

 apples per 100 blossoming spurs. It is necessary to remove fruit in 

 excess of this amount if satisfactory fruit size and annual bearing 

 habits are to be obtained. 



In this fruit area the partially self-fruitful varieties, such as 

 Golden Delicious, Yellow Newtown, Jonathan, and Rome Beauty, 

 usually set from 70 to 100 fruits per 100 blossoming spurs when 

 carrying a heavy bloom. Thus one may expect a fruit set on these 

 varieties of at least double the number considered desirable for a 

 satisfactory crop. Therefore, under Northwest conditions the varie- 

 ties listed above can be chemically thinned with little or no likelihood 

 of overthinning. On the other hand, Delicious and Winesap in the 

 Northwest may or may not set heavily, depending upon the season 

 and provisions for pollination. If these self -unfruitful varieties are 

 adequately pollinized, fruit set is usually about 50 to 60 fruits per 100 

 blossoming spurs. Thinning sprays are safely used in the Northwest 

 on these varieties when weather conditions and cross-pollination are 

 favorable. 



In fruit sections other than the Northwest, varieties considered 

 best adapted for chemical thinning include Duchess, Early Mcintosh, 

 Wealthy, Baldwin, Yellow Transparent, Wagener, and Golden 

 Delicious. These varieties are partially self -fruitful (or at least they 

 set fruits with a few seeds) and tend to set heavily even under rather 



