Some reservations in thinning 'Fuji' apples with naphthalene acetic acid (NAA) and ethephon

A thinning trial was conducted on the apple cultivar 'Fuji' on MM106 rootstocks at the Grove Research Station, Tasmania, in the 1989-90 season using naphthalene acetic acid (NAA) at 7.5 ppm or ethephon at 400 ppm as thinning sprays. Both chemicals were sprayed alone at full bloom (FB) or 10 days post full bloom (PFB) or at both times and in combinations with NAA before ethephon or ethephon before NAA. Only combinations which included ethephon thinned consistently enough to produce fruit in excess of 200 g/apple. NAA, particularly when sprayed at PFB, produced some very small fruit (pygmy fruit). Ethephon, when sprayed at FB, caused excessive russeting. It is concluded that ethephon at FB is currently the only satisfactory thinning spray for 'Fuji'. The susceptibility of the cultivar to russet needs further study as do methods of improving the efficiency of NAA.