The Effect of Plant Growth Regulators on Apple Graft Union Flexural Strength and Flexibility

Abstract

The apple rootstock 'Geneva® 41' ('G.41') forms weak graft unions with some scions. Exogenous plant growth regulators (PGR) can influence vascular differentiation and wood formation, and thus may improve graft union strength. A series of commercial and experimental PGR formulations were applied to trees on 'G.41' rootstock over two seasons in May and June, and graft union strength and flexibility were measured. Treatments included abscisic acid (S-ABA), 1-napthaleneacetic acid (NAA), prohexadione-calcium (PCa), and benzyl adenine (BA) as dilute sprays; and a concentrated formulation of BA applied in a latex paint solution to the graft union. BA in latex paint significantly increased the flexural strength per scion cross-sectional area and the flexibility of the union. Foliar applications of PCa also increased graft union flexural strength and flexibility, but temporarily limited scion extension growth. Applying PGRs in the nursery to more brittle rootstock-scion combinations may be an option for improving graft union strength and preventing tree losses. However, more efficient methods of application are needed for this approach to be commercially viable.