238 Auxins in Agriculture 



after bloom. Peaches are more susceptible to late sprays than are 

 apples. Only poor and irregular thinning of peaches can be obtained 

 for the first three weeks after bloom, whereas good thinning can be 

 obtained from 3 to 5 weeks, as shown in figure 101, 



A very real advantage is the ability of auxin sprays to be used 

 effectively for several weeks after flowering. This permits the grower 

 to wait until there is less danger of frost and the degree of natural 

 set in the trees can be estimated before any thinning operations need 

 be started. Another advantage in late timing lies in the fact that the 

 foliage of fruit trees becomes more resistant to damage from auxin 

 sprays in the first few weeks after full bloom. This greater resistance 

 with maturity of the foliage is entirely consistent with the increased 

 resistance associated with age and increased differentiation discussed 

 in chapter V. 



FACTORS IN SUCCESS 



Physiological Factors 



Perhaps the most variable physiological factor affecting thinning 

 sprays is the vigor of the tree. It has been found that large, husky 

 spurs are more resistant to thinning sprays than are small weaker 

 spurs (Murneek, 1950). Vigorously growing plants or limbs are more 

 resistant to these sprays, whereas trees or limbs weakened by disease 

 such as fire blight or by insect damage are more susceptible to thin- 

 ning sprays. Much lower concentrations can be used effectively on such 

 weakened trees than on healthy ones. 



Again, more vigorous plants generally set a larger proportion of 

 fruit, and it has been observed in many instances that when the fruit- 

 set is great, relatively high concentrations of auxin must be used. More 

 vigorous trees are also more resistant to leaf injury by auxin sprays. 



The nitrogen level also influences abscission and thinning. Plants 

 which are relatively rich in nitrogen generally shed fewer fruits by 

 natural drop, and are more difficult to thin with auxins (Burkholder, 

 unpublished). 



Another physiological factor relating to the effectiveness of 

 thinning sprays is that of the age of the fruit at the time of spraying. 

 Jt has been pointed out that peaches become more susceptible three 

 weeks after full bloom, and it is thought that apples become more 

 resistant to thinning sprays after two weeks. After the fruit reaches 

 the age of four to six weeks the large amount of natural abscission 

 which is known as "June drop" is completed and after that even high 

 concentrations of auxins are ineffective in removing any fruits at all 



