Oxamyl is an effective apple fruit thinner when used alone or when combined with other thinners

In five experiments with 'Redchief Delicious' and one with 'Braeburn', oxamyl (Vydate 2L) was used alone or combined with other chemicals to thin apples. The thinning response to oxamyl depended on dose. In most cases, oxamyl at 600 mg · L^-1 and carbaryl at 900 mg · L^-1 thinned trees similarly, but the combination of oxamyl plus carbaryl was no more effective than either chemical alone. The combination of oxamyl plus NAA (2.5 to 5 mL · L^-1) was slightly more effective than either material alone. The thinning response to oxamyl and carbaryl was related to the concentration of superior oil added to the spray solution; for both chemicals, adding oil at 5 mg · L^-1 or Tween 20 at 1.25 mL · L^-1 gave equivalent thinning. Apples on trees sprayed with oxamyl plus oil had a dull finish. Adding Tween 20 at 1.25 mL · L^-1 improved the thinning activity of carbaryl (Sevin XLK-Plus) more than oxamyl. Similar thinning occurred whether oxamyl was applied when fruit diameter averaged 4 or 10 mm. On 'Braeburn' oxamyl, carbaryl, Accel, and NAA were mild thinners, but all combinations of oxamyl or carbaryl plus Accel or NAA overthinned the trees without improving fruit size. In general, oxamyl at 600 mg · L^-1 (2 pints of vydate 2L/100 gal.) and carbaryl thin apple trees similarly, and the efficacy of both chemicals is improved by adding a surfactant.