What Are the Dangers of Copper Spray on Fruit Trees?

Copper sprays are used against fruit tree disease.

Copper sprays are organic sprays used to control fungal and bacterial diseases on fruit trees. Copper, mixed with other minerals or ingredients, is sprayed on fruit trees, especially during the dormant season, to protect the tree from infection. When copper is not applied under the right conditions, the fruit trees can be damaged.

Uses

Copper sprays are used on apple and pear trees to combat fire blight, a contagious bacterial disease. Used on stone fruit trees such as peach, nectarine and apricot, copper sprays or Bordeaux sprays of copper and lime combat peach leaf curl. Leaf curl is a fungal disease that distorts and discolors the leaves, causing them to fall off the tree. Copper spray formulations are also used on other fruit tree diseases such as brown rot, canker and mildew.

Foliage

When copper dissolves in water, it releases copper ions that kill fungi and bacteria. These copper ions damage foliage and other plant tissue if the copper solution is too strong or if the foliage stays wet too long after the copper spray is applied. Cold, wet weather interferes with the drying time and exposes foliage to copper ions a longer time. To prevent foliage damage, copper sprays are applied before foliage develops and at specific dilutions, times and temperature conditions depending on the copper formulation. Water the trees well in dry spells to minimize foliage damage.

Russeting

Fruit russet is a brownish scar on the fruit skin. Caused by insects, disease or sprays, the russeting or roughening is a cosmetic flaw that affects the market value of fruit, especially apples and pears. Russeting often occurs when copper sprays are applied to fruit. To prevent russeting, copper sprays are applied before fruit develops or after fruit harvest. Like foliage damage, copper spray damage to fruit is minimized when fruit dries quickly after application. Many factors affect fruit russet, including weather and growing conditions, but copper sprays are a manageable variable.

Phytotoxic

Copper is a phytotoxic mineral, meaning it can kill all types of plant tissue. In diluted sprays it is effective against fungi and bacteria because of these effects, but applied incorrectly or in unfavorable conditions, the fruit tree foliage and fruit is also damaged. Essential to plant and animal life in small amounts, copper is toxic in large amounts. Copper is water-soluble, but binds to many soils and accumulates in most orchard soils over time. Used annually on fruit trees and as ground sprays, the copper washes into the soil and levels slowly increase. Applied excessively over many applications or decades, the copper deposits in soil reach toxic levels and fruit trees decline or die. Applying copper sprays only when needed and as directed reduces the phytotoxic effects on fruit trees and the soil.

References

• Colorado State University: Apples and Pears
• West Virginia University: Disease Management Guidelines for Organic Apple Production in Ohio
• Washington State University: Nutrient Sprays
• University of Georgia: Homeowner Fungicide Guide
• University of California: The California Backyard Orchard