Injurious effects of metsulfuron-methyl when used to stimulate abscission of 'Hamlin' oranges

A field experiment was conducted to determine effects of concentration and spray volume of metsulfuron-methyl as an abscission aid for mechanical harv esting of citrus, Concentrations of 1,2, and 4 mg.L-1 metsulfuron-methyl we re applied to 'Hamlin' orange [Citrus sinensis (L,) Osbeck] trees at 470, 1 900, and 4700 L.ha(-1) (0.5 to 19 g.ha(-1) a.i.), Effective fruit loosening was achieved with all applications >1.9 g.ha(-1) (4 mg.L-1 at all volumes, 2 mg.L-1 at 1900 and 4700 L.ha(-1), and 1 mg.L-1 at 4700 L.ha(-1)). Heavy defoliation and twig dieback were observed on trees receiving 2 and 4 mg.L- 1 at all volumes. Defoliation and dieback became more severe and flower dev elopment and fruit set were inhibited as fruit loosening increased. The use of metsulfuron-methyl 1 as an abscission agent for 'Hamlin' oranges is not recommended until conditions for its safe application can be determined. C hemical names used: methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) am ino] carbonyl] amino] sulfonyl] benzoate (metsulfuron-methyl).