Plant Bioregulator Strategies to Alleviate Biennial Bearing, Enhance Precocity, and Control Vegetative Growth of 'Northern Spy' Apple Trees
Biennial bearing, low precocity, and vigorous vegetative growth are major production constraints of ‘Northern Spy’ apple trees. Experiments were conducted in bearing and non-bearing ‘Northern Spy’/M.9 orchards to determine whether plant bioregulator applications of ethephon (ETH), napthaleneacetic acid (NAA), prohexadione-calcium (P-Ca), and/or benzyladenine (BA) could be used to overcome these production constraints of ‘Northern Spy’. Ethephon application at 150, 300, or 450 mg∙L-1 in the ‘on’ year increased return bloom, fruit yield, and alleviated biennial bearing in the ‘off’ year in a positive linear relationship to concentration when trees were moderately biennial in cropping. Applications applied 22 June were more effective than 27 July or 31 Aug. applications. However, single or multiple (2, 3, or 4) application(s) of 150 mg∙L-1 ETH or 5 mg∙L-1 NAA were ineffective when trees were not biennial. Two ETH applications at 1500 mg∙L-1 to non-bearing trees significantly increased flowering and fruit yield the year following treatment. The combination of P-Ca with ETH had an additive effect on shoot growth and improved growth control compared to P-Ca alone. Two BA applications at 500 mg∙L-1 had no effect on lateral branching of young trees.