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 harvesting of citrus. Concentrations of 1, 2, and 4 mg·L metsulfuron-methyl were applied to ‘Hamlin’ orange [Citrus sinensis (L.) Osbeck] trees at 470, 1900, and 4700 L·ha (0.5 to 19 g·ha a.i.). Effective fruit loosening was achieved with all applications >1.9 g·ha (4 mg·L at all volumes, 2 mg·L at 1900 and 4700 L·ha, and 1 mg·L at 4700 L·ha). Heavy defoliation and twig dieback were observed on trees receiving 2 and 4 mg·L 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-2yl) amino] carbonyl] amino] sulfonyl] benzoate (metsulfuron-methyl). Mechanized harvesting of citrus fruit has been researched in Florida since the 1960s with limited success (Whitney and Harrell, 1989). Recently, trunk shakers have been developed and are being evaluated on a commercial scale for oranges intended for juice (Brown, 1998). Since mechanical harvesters seldom remove all of the fruit on a tree, chemicals that increase the harvesting efficiency have been evaluated (Wilson, 1978). Chemical looseners, such as glyoxal dioxime (5-chloro-3-methyl4-nitro-1H-pyrazole) and cycloheximide, have been tested but none has been developed because of phytotoxicity, failure to gain registration, or lack of efficiency (Kender, 1998). Wilcox and Taylor (1997) reported that metsulfuron-methyl (“WTX-901”) effectively loosened citrus fruits with no detrimental side effects. In studies on 11 citrus varieties, however, we observed variable loosening effects of metsulfuron-methyl and some defoliation (Kender et al., 1998). Differences in environmental and physiological factors, fruit responThis study was conducted to determine to what degree the concentration of metsulfuronmethyl and spray volume affect fruit loosening and phytotoxicity in ‘Hamlin’ oranges. Materials and Methods A field trial was conducted in Jan. 1998 to evaluate rates and volume of application of metsulfuron-methyl on ‘Hamlin’ orange trees. Seventy-two trees, spaced at 7.3 × 7.3 m (185 trees/ha), were selected in a grove of the Florida Dept. of Agriculture and Consumer Services, Division of Plant Industry, in Dundee, Fla. A group of 36 of these trees were 10 years old, averaged 3.7 m in height, and yielded an average of 196 kg/tree. The other 36 trees were 23 years old, averaged 7 m in height, and yielded 660 kg/tree. Brix-acid ratio of fruit from the two groups of trees were 15.7 and 18.0, respectively. Metsulfuron-methyl (Ally, DuPont Co., Wilmington, Del.) was applied at concentrations of 0, 1, 2, and 4 mg·L at three volumes (470, 1900, and 4700 L·ha) on 6 Jan. 1998 using a commercial Power Takeoff airblast sprayer (Rear’s Manufacturing Co., Eugene, Ore.). Kinetic, a 25% organosilicone–75% nonionic-oil adjuvant (Setre Chemical Co., Memphis, Tenn.), at 0.125% was included with all treatments, as well as Buffer Xtra Strength (Setre Chemical Co.) at 0.027% to adjust solution pH to 6.5. All 12 treatment combinations were replicated six times (three siveness, cultivar, and application method, including dosage, may account for the difference in response. Fig. 1. Fruit detachment force of ‘Hamlin’ oranges following applications of metsulfuron-methyl at three concentrations and spray volumes on 6 Jan. 1998, Dundee, Fla. Vertical lines indicate the standard errors