Injurious Effects of Metsulfuronmethyl 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 harvesting of citrus. Concentrations of 1, 2, and 4 mg·L –1 metsulfuron-methyl were applied to `Hamlin' orange [ Citrus sinensis (L.) Osbeck] trees at 470, 1900, 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 development and fruit set were inhibited as fruit loosening increased. The use of metsulfuron-methyl as an abscission agent for `Hamlin' oranges is not recommended until conditions for its safe application can be determined. Chemical names used: methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino] carbonyl] amino] sulfonyl] benzoate (metsulfuron-methyl).