Bloom or postbloom sprays of 2-chloroethylphosphonic acid (Ethrel) thinned peaches. Applications at full bloom did not produce consistent thinning in ‘Cardinal’. Postbloom sprays applied approximately at endosperm cytokinesis produced consistent thinning of 3 cultivars. Degree of thinning was related to time of Ethrel application. Most thinning was obtained when Ethrel was applied near the end of Stage I or during Stage II. Ethrel sprays within a month after full bloom caused significant fruit size reduction that persisted 2 months or more after treatment. Ethrel sprays within 50 days of harvest accelerated maturity of several cultivars.

The cytokinin 6-benzyladenine (BA) and gibberellins A4 + A7 (GA4+7), but not GA3, increased the fruit length-diam (L/D) ratio of ‘Delicious’ apples. Prominence of the points near the fruit calyx was increased more by BA than by GA4+7, but increased most when the 2 were combined. Sprays during bloom were more effective than either postbloom or prebloom sprays. A bloom spray combining GA4+7 and BA at 100 ppm each decreased fruit set of open pollinated flowers in one year, but not in another. Concentration of 50 ppm or less altered fruit shape without any significant effect on the number of seed, fruit set, firmness, or fruit size.

Applications of potassium gibberellate (KGA) were made to ‘Redskin’ peach trees at various times after bud differentiation. KGA applied before leaf fall caused flower bud mortality and retarded bud development, depending on timing and concentration. Response peaks for each of these effects occurred at different stages in the development of flower buds. The KGA-induced retardation of floral development reduced the damage caused by frost during the bloom period. In contrast to the delay in bloom obtained when KGA was applied before leaf fall, flower bud development was accelerated when KGA was applied after a substantial part of the chilling requirement had been satisfied. The cold hardiness of the dormant flower buds was not greatly affected by KGA.

Ethephon ((2-chloroethyl)phosphonic acid) at 50 to 200 ppm hastened the maturation of peach fruit of the cvs. ‘Cardinal’, ‘Ranger’, ‘Redhaven’, ‘Blake’, and ‘Richhaven’ over a range of application times. Stimulation of maturation was not attributed solely to reduced fruit load, since an influence on maturation was found even when ethephon-treated trees bore no fewer fruits than hand-thinned controls. When treated and nontreated ‘Richhaven’ fruit were harvested at a comparable maturity based on firmness, ground and flesh color a and a/b values were increased by ethephon, as were soluble solids. No differences were found in b values for ground or flesh color or in total titratable acidity. Fruit treated with ethephon exhibited more uniformity in firmness and color than untreated fruit at shipping maturity, a characteristic which has potential value in facilitating once-over mechanical harvesting operations.

Several peach varieties were sprayed with 3-chlorophenoxy-α-propionamide (3-CPA) at different stages of fruit development. The timing of the application was critical, and varieties differed greatly in their thinning response. The ‘Ranger’ variety was thinned with ease, but attempts to thin ‘Cardinal’ with 3-CPA were unsuccessful. Fruit thinning apparently increased the cold hardiness of the flowers during the following bloom period. Several spray additives were found to increase the thinning effectiveness of 3-CPA.

Gibberellic acid (GA3) was applied at 75 ppm to mature ‘Redskin‘ peach trees at various times during the summer and fall. Flowering was reduced by GA3, the degree of reduction depending on time of application. Two response peaks were evident: the first occurred with application in early summer, at the time of flower initiation; the second occurred in late summer and resulted from mortality of developing flower buds. Application of GA3 shortly before leaf fall caused the most retardation of flower development the following spring. The nature and degree of flowering response was, therefore, related to time of GA3 application.

Application of succinic acid,2,2-dimethylhydrazide (SADH) to peaches at the onset of pit hardening advanced maturity and caused more rapid softening during ripening on the tree. Flesh color at a given firmness was improved by SADH over a wide maturity range. The color difference was maintained after processing; canned halves from SADH trees had a higher USDA color grade than control fruit of comparable firmness. Uniformity of firmness and flesh color among fruit within trees varied with crop maturity, but uniformity was not substantially affected by SADH. However, enhancement of flesh color by SADH improved the quality of fruit obtained by once-over harvest and tended to concentrate the grade distribution of canned halves in USDA color grades A and B.

Postbloom application of gibberellin A3 (GA3) to open pollinated peach trees prevented seed development in some fruit, and also resulted in the persistence and maturation of both seeded and seedless fruit on the same trees. GA3 temporarily promoted fruit growth and delayed abscission of both seeded and seedless fruit, but the final crop load was not altered. Both seeded and seedless fruit abscised from treated trees during physiological drop. The persisting seedless fruit, therefore, competed successfully with seeded fruit. Seedlessness was associated with smaller fruit size at maturity.