Investigating the efficacy of endothal as a chemical thinner of red 'Delicious' apple

The aquatic herbicide, endothal (7, oxabicyclo (2,2,1) heptane-2-3 dicarboxylic acid), was examined for its effectiveness as a blossom thinner of spur red 'Delicious' at the Grove Research Station in southern Tasmania. Endothal was applied as a single application at 80% bloom at a range of concentrations from 0.75 ml l -1 to 3.0 ml l -1 . These chemical treatments were compared with an unthinned control and a hand thinned treatment. Crop load was reduced linearly with increasing concentrations of endothal. The increase in fruit weight and size corresponded to the level of thinning achieved. Leaf damage was proportional to the concentration applied. Endothal had a flattening effect on the fruit. There was minimal effect on pip numbers. Fruit firmness and soluble solids were improved most endothal treatments. Return bloom was increased the higher concentrations of endothal which achieved greater thinning. Endothal shows distinct promise as a blossom thinner of red 'Delicious'. More work is needed to determine optimal concentrations, time of application and use in a programme with other thinning chemicals.