Issue link: http://read.dmtmag.com/i/114539
5 TIPS for using copper 1 2 Solubility of fixed coppers increases under acidic conditions. Copper sprays will become more phytotoxic if they are applied in an acidic solution. Copper sprays generally cause more phytotoxicity when applied under slow drying conditions. "This concern is not relevant for delayed-dormant or green-tip applications," Rosenberger said. "However, if copper is used to control bacterial spot during summer, or if it is applied to nonbearing apple trees to control fireblight after leaves have emerged, then phytotoxicity can be minimized by applying the copper with relatively low volumes of water and under conditions where droplets dry quickly." Rosenberger recommends two sprays of copper on new trees after planting—to eliminate possibly strep-resistant fireblight strains from being introduced into an orchard. He recommends a copper spray at green tip for suppression of fireblight in any orchard that had active fireblight in either of the two previous seasons. Copper is labeled for fireblight during bloom, but it is not recommended. "It will almost always russet fruit under eastern U.S. conditions," he said. When buds are already showing green tissue, do not apply copper just prior to predicted frosts. Cells ruptured by frost crystals may resorb and be killed by the copper on the bud surfaces, he said. Adjuvants have highly variable and largely unpredictable effects on the efficacy of copper sprays. Copper products can be combined with oil in delayed-dormant or green-tip sprays on apples. A quart of oil per 100 gallons may enhance coverage of the wood in these early season sprays. Spray guides repeatedly warn against using oil and copper after the dormant season. "No other adjuvants are necessary or recommended when applying copper products on tree fruits," Rosenberger said. Bordeaux mixture is made by mixing copper sulfate and spray lime. With the fixed copper products, there is no evidence that adding spray lime will either reduce phytotoxicity or extend the residual activity of the copper. —R. Lehnert 3 4 PHOTO BY RICHARD LEHNERT 5 Cornell's David Rosenberger is a frequent speaker on fruit tree disease management. Here, he speaks to growers during the International Fruit Tree Association summer conference there in 2010. Guide advises using copper before bud swell to prevent peach leaf curl. Recommended rates are 8 to 16 pounds for copper hydroxide (Kocide) or copper oxychloride (COCS), which would translate to 2.5 to 5 pounds of elemental copper. Kocide 3000 is 46 percent copper hydroxide and has 0.3 pounds of elemental copper per pound. Kocide 2000 is 53 percent copper hydroxide and "Copper sprays control plant pathogens because copper ions denature proteins, thereby destroying enzymes that are critical for cell functioning." —David Rosenberger contains more copper. For cherry leaf spot, Kocide 2000 at 5 pounds per acre is recommended at first and third cover. No rate is specified for use of copper hydroxide, copper oxychloride, basic copper sulfate, or Bordeaux mixture on apples for suppression of fireblight, but the timing should be before growth starts. 12 March 15, 2013 GOOD FRUIT GROWER When used on stone fruits, Kocide 2000 is labeled at 6 to 12 pounds per acre with a maximum use rate of 51.4 pounds per season. When high rates are used year after year, copper can accumulate to high levels in orchard and vineyard soil. Coarse or fine? The finely ground coppers may be preferable for delayed dormant and dormant applications on apples and for fall and spring applications on cherries where the objective is to control bacterial canker, Rosenberger said. Most copper labels list a broad range of rates. In general, the upper end of labeled rates is suggested for applications that are made in spring or fall on cherries or for applications made at silver tip or green tip on apples. The lower labeled rates may be more appropriate for spring applications if spring weather has been delayed and one might therefore expect trees to advance rapidly from bud break to bloom, he said. Using excessive rates of copper, especially finely ground coppers that have good residual properties, could result in fruit russetting on some apple cultivars if copper ions are splash-dispersed to developing fruit tissue after flowers reach pink or bloom. Copper products such as Cueva and MasterCop contain very low concentrations of elemental copper, Rosenberger said. These products are better suited for applications during summer (e.g., for bacterial leaf spot on peaches or cherry leaf spot on tart cherries) when repeated applications with low rates of copper are desired to minimize phytotoxicity. • www.goodfruit.com