Copper sprays are protectant fungicides that must be applied evenly to the plant or fruit surface before the disease develops to prevent infection. Copper is not a systemic chemical and cannot be carried internally through the plant to kill the pathogen; it only protects where it lands.

Copper is a metal, but when it's dissolved and sprayed on a tree, it breaks down fungal diseases, such as peach leaf curl, fire blight, powdery mildew, shot hole and brown rot, says Shonnard's Nursery, as well as pests that burrow into a tree's cracks and crevices.

Copper fungicides are also effective for fungal diseases. Copper ions kill by denaturing proteins and enzymes in cells of pathogens they contact that have not yet infected the plant. They have no post-infection activity and are non-selective, which is why phytotoxicity occurs when they get inside plants.