For 2 years, 'Redchief Delicious' apple (Malus ×domestica Borkh.) trees were treated with combinations of NAA and oxamyl and 'Smoothee Golden Delicious' trees were treated with combinations of NAA and carbaryl. Oxamyl, at concentrations of 250 to 750 mg·L-1, but not NAA at concentrations of 1 to 6 mg·L-1, reduced fruit set, yield and crop value. NAA did not consistently affect average fruit weight or the percentage of small fruits on 'Redchief Delicious' trees. Carbaryl reduced fruit set and yield on 'Smoothee Golden Delicious' trees one of the two years. Fruit set and yield were negatively related to NAA concentration both years. In one of the two years the combination of NAA plus carbaryl was more effective than NAA alone. Treatments that provided adequate thinning tended to reduce crop value because the increase in fruit size did not compensate for the reduced yields. Chemical names used: 1-naphthyl-N-methylcarbamate (carbaryl); methyl N′,N′-demethyl-N [(methylcarbomoyl)oxy]-1-thiooxamimidate (oxamyl); 1-naphthaleneacetic acid (NAA).
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