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Copper Compounds
Copper compounds are widely sold as fungicides for orchard and garden use. Copper is a foliar fungicide with protective action. These compounds can be highly phytotoxic to many fruit crops and must be used with extreme care. Check the label for type of copper and any cautions that accompany its use. Please remember to also check the label for rates and application times for best disease control. Many formulations of copper are available to the home gardener (see Table 2.4).
Copper was first used in the mid-1800s in grape vineyards in France to discourage theft of the grapes. Copper sulfate and lime were mixed in a slurry and spread over the grape vines. In 1882 a French scientist observed that this antitheft treatment was effective in reducing a disease called downy mildew. This observation was made near the town of Bordeaux, so the mixture of copper sulfate and lime became known as Bordeaux mixture.
Copper sulfate is readily soluble in water. This high degree of solubility is the fundamental cause of the toxicity problems, which copper sulfate can cause to all fruit crops. Fixed coppers have been developed that are relatively insoluble and therefore less toxic to plants; however, fixed coppers can also result in phytotoxicity under certain conditions. Fixed coppers include basic copper sulfate, basic copper chloride, copper oxides, and copper hydroxide.
The fungicidal activity of copper is based on its ability to destroy proteins in plants. This is true for all plants, fungi, and fruit plants. When lime is combined with copper compounds, it reacts with the copper, making it more stable. Thus, copper compounds in the presence of lime would generally produce lower, more uniform concentrations of free copper, which in turn would be less apt to injure plant tissues than if no lime were used. Because copper has the ability to kill all types of plant tissues, the use of copper fungicides carries with it the risk of causing injury to fruit plants. Ideally, copper on the leaf or fruit surface should be in high enough concentration to kill the fungus or bacteria but low enough not to cause injury to the plant. Factors that can promote injury include failure to use enough lime; cold, wet weather conditions that apparently increase copper's solubility, allowing more into the plant and resulting in toxicity; and application of excessive rates of copper. Even when no injury is evident on the plant, subtle effects of the copper on the plant may be occurring. In addition, to reduce growth and yields, it has been shown that the use of copper fungicides can reduce the maturity of the fruit as well as that of the shoots. Copper fungicides can have subtle, chronic negative impacts on fruit plants.
Copper will provide low to moderate control of many of the diseases. Bordeaux may be used on pears during bloom for fire blight control when temperatures are above 70°F and drying conditions are rapid. Fixed coppers, plus lime, are safer than Bordeaux. They may be used for leaf curl control on stone fruits and pre- and postharvest leaf spot control on tart cherries. These compounds are useful in plant nutrition since they supply copper to the plant. Strawberries are very sensitive to copper. Never apply copper to strawberries because severe phytotoxicity will result under almost any conditions.
Do not apply any of the copper compounds without adding lime. Lime should be used at a rate one to two times that of the copper. If a copper material is applied without lime and yellowing and leaf drop occur, an application of lime within 2 to 3 weeks of the copper application may prevent further yellowing and leaf drop. Again, check the label of the product you intend to use to see if lime has already been added in the formulation or if it is advised to add lime and at what rates. Do not use copper in cool wet weather. Do not use immediately before or after using ferbam. Most insecticides are not compatible with lime.