Copper compounds

Copper is a foliar fungicide and bactericide with protective action. Copper compounds can be highly toxic to many fruit crops and must be used with extreme care. Read the label of the product you intend to apply to determine the type of copper, the percent active ingredient, and any cautions that accompany its use. 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. The blue copper material made the grapes very unappetizing. In 1882 a French scientist observed that this antitheft treatment was effective in reducing a disease called downy mildew on grapes. 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 soluble in water. This high degree of solubility is the fundamental cause of 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 (Basicop), basic copper chloride, copper oxide, and copper hydroxide (Champ, Kocide, Nu-Cop).

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 tissue, 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 high enough 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 provides low to moderate control of many fungal and bacterial diseases. Fixed coppers, plus lime, are safer than Bordeaux. They may be used for fire blight of apples and pears, leaf curl control on stone fruits and pre- and postharvest leaf spot control on tart cherries. Copper compounds should not be applied in a postharvest spray without adding lime. If copper is applied without lime, orchards will show toxicity symptoms such as chlorosis (yellowing), leaf drop on tart cherries, and necrosis (browning) on sweet cherries. If a copper material is applied without lime and yellowing and leaf drop occur, an application of lime within 2 to 3 weeks after the copper application may prevent further yellowing and leaf drop. Do not use immediately before or after using ferbam. These compounds are useful in plant nutrition as they supply copper to the plant.