FLOWER BUD THINNING AND WINTER SURVIVAL OF REDHAVEN AND CRESTHAVEN PEACH IN RESPONSE TO GA(3) SPRAYS

The timing and concentration of dilute foliar sprays of gibberellic ac id (GA(3)) were tested for their flower bud thinning effect during thr ee consecutive years on two common cultivars of peach [Prunus persica (L.) Batsch,] grown in commercial culture under midwestern conditions. There was a strong trend for June sprays to minimize the total flower bud density (buds/cm shoot) of unbranched shoots on mature 'Redhaven' and 'Cresthaven' trees. The GA(3) treatments applied between early Ju ly and late October did not reduce the total flower bud density. Incre asing the concentration from 25 or 50 mg.L-1 to 200 mg.L-1 tended to d ecrease total flower bud density, especially when applied May through July. During the same period the GA(3) treatments reduced total flower bud density of short shoots (<10 cm), but only about two-thirds as ef fectively as on long shoots. The 'Redhaven' live flower density on tre es treated in May, June or September was up to 2 times greater then th e control in March following exposure to extreme fluctuations in winte r temperatures to near critical lows in 1988 and 1989. Although 50 mg. L-1 GA(3) applied in June 1989 reduced total flower bud density by 70% , it resulted in a live flower density only 35% lower than the control . The treatment induced 2.3 times greater survival of the total flower buds existing after thinning when winter temperatures gradually decli ned to critical levels. The increased live flower density caused by th e GA(3 )sprays translated to a cropload 1.3 to 2.25 times greater than control. The length of neither fruiting quality shoots in the bearing canopy nor 1-year-old upright branched shoots in the top of trees (wa tersprouts) was appreciably affected by the GA(3) applications. GA3 tr eatments at 100 and 200 mg L-1 in late July and early August slightly delayed time of full bloom. Defoliation was delayed slightly by treatm ents applied in September and late July. Moderate doses of appropriate ly timed GA(3) sprays reduced flower bud densities without adverse eff ects on winter survival, yield, defoliation or bloom time. Our results support the use of GA(3) as a reliable peach thinning tool.