Effect of blossom thinners on fruit set and fruit size of peaches

Endothall (mono-N,N-dimethylalkylamine salt, 15.9%, endothallic acid 3.8%), ammonium thiosulfate (ATS, 55%), and Wilthin (1-aminomethanamide dilrydrogen tetraoxosulfate, 79%) were evaluated over 3 years for use as blossom thinners on mature 'Garnet Beauty' and 'Red Haven' Peaches (Prunus persica). Endothall and ATS were the most effective blossom thinners and reduced set in all 3 years. Wilthin reduced initial set in 2 of 3 years. Hand thinning was reduced by 50% to 80% on endothall and ATS treated trees. The reduction in crop load at bloom resulted in significant increases in fruit size at harvest. We suggest the use of ATS at a rate of 35 to 45 L(.)ha(-1) (3.5 to 5 gal/acre) and endothall at 1.8 L(.)ha(-1) (1.5 pt/acre), applied in 935 L(.)ha(-1) (100 gal/acre) on mature trees. The use of Wilthin at rates higher than 18.6 L(.)ha(-1) (8 qt/acre) may be required for adequate thinning, but phytotoxicity at higher rates was not tested. Other important components for successful blossom thinning include applying materials before the majority of flowers have been pollinated (slightly before full bloom) and to apply the spray when there is very little wind to assure good coverage and to prevent localized areas displaying increased phytotoxicity.