Phytotoxicity of Copper-Based Bactericides to Peach and Nectarine

Organometallic copper, consisting of a mixture of copper abietate, copper linoleate, and copper oleate (CuALO), is important for postbloom management of bacterial spot in New Jersey peach and nectarine orchards. Rotation of CuALO with oxytetracycline reduces cost and helps prevent (or delay) resistant organisms. However, because copper is also phytotoxic, higher rates and inorganic coppers have not been utilized. A study was conducted on 'Encore' peach and 'Redgold' nectarine to determine the quantitative relationship between copper concentration (metallic equivalent) and phytotoxicity. Different rates of CuALO and copper hydroxide (CuOH) were applied seven times postbloom. Foliar injury and defoliation increased with number of applications and copper concentration. Maximum 'Encore' defoliation was 10 to 17%, while 'Redgold' sustained 30 to 35% leaf loss. CuOH caused similar or less leaf injury and defoliation than CuALO. No injury was observed on fruit, even at three times the labeled metallic copper rate of CuALO. Fruit size and soluble solids were not influenced by copper. Estimates of foliar density and tree volume indicated that sufficient photosynthetic capacity existed to allow acceptable leaf damage and loss from copper applications. These results showed that inorganic coppers may be viable alternatives to organometallic copper for postbloom bacterial spot control; nectarine may be more sensitive to copper injury than peach; and higher rates of metallic copper, above the current commercial rate, can be applied to peach. In the latter case, greater amounts of copper may provide longer residual activity and improved bacterial spot control on highly susceptible cultivars.