Common Tree Diseases in Wisconsin: A Green Bay Homeowner's Guide

Wisconsin's trees face a variety of diseases, and Green Bay's urban forest is no exception. Early detection is key to saving infected trees and preventing the spread to healthy ones. Here are the most common tree diseases affecting Green Bay homeowners.

Oak Wilt

Oak wilt is a fungal disease that affects all oak species but is most deadly in red oaks. It spreads through root grafts between nearby oaks and by sap-feeding beetles. Symptoms include leaf browning starting at the top of the canopy, often in late spring or early summer. Infected red oaks can die within weeks. Prevention includes avoiding pruning oaks during the growing season (April–July) when beetles are active.

Dutch Elm Disease

Dutch elm disease has killed millions of elm trees across the United States, and Green Bay has lost many of its majestic American elms. The fungus is spread by elm bark beetles. Symptoms include wilting and yellowing leaves on individual branches, progressing to the entire tree. There is no cure, but early treatment with fungicides can be effective for high-value trees.

Anthracnose

Anthracnose is a group of fungal diseases that affect many tree species, including sycamores, maples, oaks, and dogwoods. Symptoms include brown or black spots on leaves, leaf curling, and early leaf drop. While anthracnose rarely kills trees, repeated infections can weaken them over time. Good sanitation — raking and disposing of infected leaves — helps reduce recurrence.

Apple Scab

Apple scab affects crabapples and apple trees, causing olive-green to black spots on leaves and fruit. Infected trees may defoliate by midsummer. While not fatal, it significantly reduces the tree's ornamental value. Resistant crabapple varieties are available for new plantings.

Needle Cast Diseases

Evergreens like white pines and spruces in Green Bay can be affected by needle cast diseases, which cause needles to turn brown and drop prematurely. Good air circulation and avoiding overhead watering help prevent these diseases.

If you suspect your tree has a disease, contact Tree Service Green Bay for a professional assessment.