Fruit weight, cropload, and return bloom of 'Empire' apple following thinning with 6-benzyladenine and NAA at several phenological stages

Thinning with BA reportedly increases size of 'Empire' fruit more than does thinning with NAA because of enhancement of cell division by BA. This stud y was conducted to determine the phenological stage at which BA application provides maximum fruit weight relative to degree of cropload reduction. In all years, treatments were applied at a range of timings: petal fall (PF), 5-, 10-, or 15-mm king fruitlet diameter (KFD). For each thinner, the same concentration was used throughout the study. In 1994, only Accel((R)) at 7 5 mg .L-1 was evaluated. In 1995, NAA (7.5 mg .L-1) + carbaryl (600 mg .L-1 ), Accel((R)), and a BA-onIy formulation were compared, but BA alone was ap plied only at PF, 10- and 15-mm KFD. In 1996, Accel((R)) and NAA were compa red with and without carbaryl at all timings. Most treatments reduced cropl oad and enhanced fruit weight. When data for all 3 years were combined, Acc el((R)) or BA increased cropload-adjusted fruit weight (CAFW) in 8 of 10 tr eatments made at 10- or 15-mm KFD, PF treatments never increased CAFW, and only one of four applications at 5-mm KFD increased CAFW. In contrast, NAA + carbaryl increased CAFW in four of four treatments applied at PF or 5-mm. KFD, but in only one of four treatments at 10- or 15-mm KFD. Accel((R)) wa s less effective than NAA in reducing fruit clusters to a single fruit per spur in most comparisons, either with or without carbaryl. Return bloom var ied greatly across years, but was always influenced by application time and types of thinners. In 1994 and 1996, return bloom was closely related to c ropload the previous year. Although return bloom was very low for most trea tments in 1995, 10- and 15-mm KFD applications of NAA + carbaryl increased it three-fold in comparison with other treatments (NAA + carbaryl at PF or 5 mm or BA at 10-mm KFD) that had similar effects on cropload. Chemical nam es used: 6-benzyladenine (BA); naphthaleneacetic acid (NAA).