38 CIRCULAR 867, U. S. DEPARTMENT OF AGRICULTURE 



Apricots and Plums 



On apricots, results with chemical thinning experiments in central 

 Washington have been highly variable. The data in table 12 illustrate 

 this. Reduction in fruit set has varied from almost no thinning to 

 serious overthinning (Welch orchard, 1947). In general, 10 to 15 

 percent of the blossoms setting fruit is all that is required for a 

 satisfactory apricot crop. Under many conditions the Wenatchee 

 Moorpark variety (the most important commercial variety in Wash- 

 ington) will set fruit in excess of this amount, a condition that neces- 

 sitates thinning in order to obtain satisfactory size. However, in a 

 number of instances this variety fails to set excessively for no ap- 

 parent reason. In such cases, thinning sprays are likely to result 

 in overthinning. Tilton and Royal varieties are generally more con- 

 sistent in their fruit setting tendencies. Preliminary experiments 

 both in Washington and California indicate considerable promise in 

 thinning these varieties with dinitro sprays. 



On prunes, Lilleland (27), working in California, has reported 

 promising results with dinitro sprays in thinning various varieties. 

 In a number of tests he found that a spray of 2 pints of Elgetol per 

 100 gallons was generally the more satisfactory concentration. 

 Triethanolamine salt of dinitro-ortho-sec-butylphenol (DN No. 289) 

 used at 1 pint per 100 gallons (about one-half the toxicant concen- 

 tration of 2 pints of Elgetol) gave similar results. An outstanding 

 feature of his experiments was a marked benefit in increased fruit 

 size, but this was obtained at the expense of total yield. Lilleland's 

 work indicates that dinitro sprays have definite promise as a means 

 of thinning prunes. However, he cautions against the possible hazards 

 of overthinning and suggests that prune growers in California, 

 before using thinning sprays on an extensive scale, proceed on a trial 

 basis. 



Preliminary work with certain varieties of plums (Santa Rosa. 

 Beauty, Kelsey, and Wickson) indicate that dinitro sprays have con- 

 siderable promise for fruit thinning when applied during the blossom 

 ing period. More information is needed relative to concentration, 

 timing, and varietal response before specific recommendations are 

 possible. As far as is known, the effective concentration of dinitro 

 chemicals and other factors affecting results are essentially the same 

 as for other stone fruits. 



Timing the spray application with reference to blossom develop- 

 ment seems to be a more important factor in determining the amount 

 of thinning on stone fruits than is the case with apples and pears. It 

 has been previously pointed out that dinitro sprays applied to peache- 

 are usually less effective when applications are made rather late in the 

 bloom period as compared with sprays applied when about 90 per- 

 cent of the flowers are open but prior to any appreciable petal fall. It 

 may be noted from the data presented in table 12 that early petal fall 

 sprays on apricots were also generally less effective than when the 

 sprays were applied 2 days earlier (full-bloom stage). Lilleland ( 21 ) . 

 working with prunes, has emphasized also the importance of timing 

 in relation to results obtained. He obtained the greatest amount of 

 thinning by spraying when about 90 percent of the blossoms were 



