Evergreen trees and shrubs are vulnerable to several common fungal diseases that are often not fatal to otherwise healthy plants. These infections can cause an evergreen to appear sick or dying and greatly weaken plants. Treatment of fungal diseases often depends on the extent of infection and the type of fungus.
Many types of fungi, such as those that cause sooty molds, blights and rusts, cannot be killed once they've infected an evergreen's limbs or foliage. In these cases, the only treatment option is to prune and destroy infected sections of the tree so spores don't spread to uninfected branches or a new host. Clean away foliage and twigs on the soil around the tree as well, because some types of evergreen fungus can spread from infected parts of the tree, even if they've fallen to the ground.
In terms of evergreen fungal infections, many fungicides work as a preventative measure, not as a method of killing existing infections. Apply fungicides according to label directions for application rates and method in early spring before mold spores spread. Typically, this can help prevent a previous fungal infection such as anthracnose from spreading.
Fungi can develop in evergreens heavily infested with homoptera insects such as aphids and white flies. Sooty molds feed on the honeydew secreted by these pests and, although not dangerous to a healthy tree, the fungi cause it to look unhealthy. Treatment of these fungi begins with ridding the tree of the insects because they give the mold a food source. Eradicate the infestation by applying contact or dormant oil insecticides labeled as safe for your particular tree. Also spray the plant with water to remove the sooty mold growing on the foliage and limbs.
Root rot is caused by a variety of fungi that attack the roots of an evergreen tree or shrub. Once the roots are severely infected, the only method of control is to remove the infected plant so it doesn't infect healthy plants.
Avoid Alternate Hosts
Species of gymnosporangium fungus that cause rusts, such as cedar-apple rust, require an alternate host to complete their life cycles. When these fungi attack evergreens, they rarely need treatment because their resulting galls are not harmful to healthy trees. Fungus spores can spread to nearby apple, pear, hawthorn and juneberry trees, however, causing damage. If the infection is already present in evergreen trees and shrubs, spray a fungicide on nearby uninfected plants and potential alternate hosts to prevent spores from spreading. Prune galls out of infected trees in late winter.