250 Auxins in Agriculture 



yet did not interfere with the prevention of fruit abscission. The 

 abihty of maleic hydrazide to counteract auxin effects in growth have 

 been discussed in chapter VII. The firmness of apple fruits was ac- 

 tually increased by maleic hydrazide and the early ripening effect 

 resulting from the auxin treatment was essentially erased. 



The effects of auxins in altering ripening and storage of fruits 

 and vegetables is discussed in more detail in chapter XV. 



Excessively high concentrations of auxins applied for pre-harvest 

 drop control can result in excessive prevention of development of 

 abscission tendencies. As a consequence harvesting the fruit can cause 

 extensive breakage to the flowering spurs. This type of damage is 

 not excessive if auxin concentrations of less than 20 mg./l. are used 

 (Thompson, 1951). Another abnormal result which sometimes appears 

 is excessive enlarging and radial cracking of the fruits. 



PRE-HARVEST DROP CONTROL IN OTHER FRUITS 



Oranges 



Although the fruit drop problem with oranges is not as great as 

 with apples, some orange groves are particularly prone to drop their 

 fruit early and, as in the case of apples, the advent of warm weather 

 before harvest will result in losses due to excessive drop. In contrast 

 to the effect on apples, naphthaleneacetic acid has no effect on drop 

 of oranges but 2,4-D has been found to furnish good control (Stewart 

 et al, 1951). Of several derivatives tested, the isopropyl ester of 2,4-D 

 has given the best results (Erickson, 1951). The duration of the effect 

 is remarkably long, for these workers have found that sprays applied 

 as early as October could effectively control drop until harvest in May. 

 Whereas the first experiments were carried out by applying the auxins 

 shortly before harvest time, it was soon discovered that applications 

 made at or near the time of full bloom would not only control drop 

 at harvest time but would also cause an increase in the yield of large- 

 sized fruits. For this reason the 2,4-D is commonly applied either at 

 full bloom or shortly before. The concentration used is 8 to 10 mg./l. 

 in an ordinary aqueous spray, generally combined with a fertilizer 

 spray of zinc or manganese (Stewart et al, 1951). In some orchards 

 it is applied by helicopter spraying a concentrated solution of 2400 

 mg./l. 



The auxin spray applied to oranges at the time of flowering stim- 

 ulates the rate of growth of the individual fruits. Gains in fruit sizes 

 as great as 13 per cent have been reported (Stewart et al, 1952). One 



