210 PLANT GROWTH SUBSTANCES 



days' time. Tangerines, on the other hand, adhere tenaciously, and it 

 is only with difficulty that mature fruit of any stage can be pulled intact 

 from the tree. 



The response of citrus fruits to sprays of naphthaleneacetamide is 

 quite in contrast to the response of apples. Gardner (15) found that 

 10 ppm. of this compound had no effect on Pineapple oranges but that 

 100 ppm. reduced drop markedly if applied early in the harvest season 

 (November) and was still effective 12 weeks later when the fruit was 

 harvested in February. When applied in January even this high concen- 

 tration was without effect. Most citrus growers would prefer to take 

 the chance that they will harvest their crop ahead of heavy droppage 

 rather than invest in the cost of such a concentrated spray, and for this 

 reason, in part, the naphthalene compounds are not used for citrus. 



In California, Stewart and Klotz (45) reported appreciable reduction 

 in the preharvest drop of Valencia and Washington Navel oranges from 

 2,4-D sprays applied in early summer. Marsh grapefruit, in experiments 

 by Stewart and Parker (46), also responded but apparently not as 

 satisfactorily as oranges. At the highest concentration (225 ppm.) some 

 of the young grapefruit on the trees at time of spraying (not the mature 

 crop for which the sprays were applied) developed quite abnormally, 

 having cylindrical shapes, thick rinds, many prominent rudimentary 

 seeds, and even navels. The oranges responded similarly at high con- 

 centrations (45). From numerous reports by Stewart and his coworkers 

 it appears that low concentrations, 5 ppm. for example, are nearly as 

 effective as 25 ppm., at which concentration and above occur increasing 

 injury to the tree, abnormal fruits, and abscission of young fruits of the 

 new crop. This reduction in the quantity of fruit might well account 

 for the slightly larger fruit sizes of oranges reported by Stewart (44). 



In Florida, Gardner (unpublished results), using the sodium salt of 

 2,4-D on several citrus varieties in October at 10 ppm,, did not secure 

 as outstanding control of drop as that reported in California. The 

 Pineapple oranges picked 12 weeks after spraying dropped half as much 

 fruit during that period as the controls. The sprays on Valencias and on 

 Marsh grapefruit were somewhat less effective and, in the case of grape- 

 fruit, the effect did not persist as long as with oranges. "Seedling 

 oranges," which approach a varietal status in Florida because of the 

 high incidence of nucellar seedlings, failed completely to respond to the 

 2,4-D. It appears that here again is an instance of differential varietal 



