The response of young ‘Braeburn’ and ‘Oregon Spur Delicious’ apple trees growing under an ultra-high density planting system to soil-applied paclobutrazol: I. Effect on reproductive and vegetative growth

Abstract

‘Braeburn’ and ‘Oregon Spur Delicious’ MM. 106 apple trees ( Malus domestica Borkh.) trees in an ultra-high density planting system were treated in autumn with paclobutrazol (PBZ) as a soil drench. One group of 3-year-old trees received 0 or 250 mg (a.i)/tree in 1989, and the other group of trees (4-years-old) received 0, 50 or 250 mg (a.i)/tree in 1990. Half of the trees in each treatment received PBZ at 0 or 250 mg (a.i)/tree in 1993 when trees were 7-year-old. PBZ applied at 250 mg increased fruit yield and fruit number in alternate years for up to four years. PBZ increased the biennial bearing index, however, the magnitude varied with cultivar. The additional 1993 application of PBZ did not increase yield or fruit number. Vegetative growth components were reduced the 2nd year after the PBZ application in both trials. The repeat application was not as beneficial as the initial 1989 or 1990 treatment of PBZ but did not cause yield losses in well managed trees. We conclude that soil-applied PBZ to the younger trees can control vegetative growth effectively but causes fruit bienniality. However, the repeat application did not seem to induce bienniality in the bigger (7-year-old) trees growing under an ultra-high-density apple-planting systems.