Common Tree Diseases
MINNESOTA
Minnesota's trees face a unique set of threats — from invasive beetles and fungal infections to bark diseases and scale insects. Left untreated, many of these issues can cause permanent damage or kill a tree entirely.
Use this guide below to identify what might be affecting your trees and reach out to our ISA-certified arborists if you need a professional eye on the problem.
Apple Scab
Leaf spots are circular, olive-hued, up to ½ inch in diameter with a velvety texture and fringed edges. With age, spots darken to brown or black and merge. Leaves covered in numerous spots become yellow and fall by mid-summer. Infected fruit displays olive-green patches that eventually become brown and corky.
Armillaria Root Rot
a destructive fungal disease that attacks the roots of trees and shrubs, weakening their ability to absorb water and nutrients. Often called “shoestring root rot” because of its dark, root-like fungal strands, it can cause yellowing leaves, branch dieback, slow growth, and eventually tree death. The fungus spreads underground through infected roots and thrives in stressed or weakened trees, making early diagnosis and preventative tree care especially important.
Ash Bark Beetle
a small wood-boring insect that attacks weakened or stressed ash trees by tunneling beneath the bark. These beetles disrupt the tree’s ability to move water and nutrients, which can lead to branch dieback, thinning leaves, bark splitting, and overall decline. Infestations are often a sign that a tree is already under stress from drought, disease, or environmental damage, making preventative tree health care and early intervention important for long-term survival.
Bark Inclusion
a structural tree defect that occurs when bark becomes trapped between two branches or stems as they grow together. Instead of forming a strong connection, the trapped bark creates a weak point that can increase the risk of splitting or branch failure during heavy wind, snow, or storms. Early pruning, cabling, and proper tree care can help reduce the risk and improve long-term tree stability.
Bronze Birch Borer
Metallic wood-boring beetles (Buprestid beetles) prevalent in Minnesota. Bronze birch borer (Agrilus anxius) targets birch; twolined chestnut borer (A. bilineatus) targets white and red oak. They feed in distressed, dying trees. Canopy dies back from the top down. Elevated strips visible on limbs and trunk. Maintaining tree health reduces appeal to borers. Insecticides can protect valued trees.
Cedar Apple Rust
a fungal disease that affects both cedar/juniper trees and apple or crabapple trees as part of its life cycle. It often appears as bright orange or yellow spots on leaves and can cause premature leaf drop, weakened tree health, and reduced fruit production. The fungus spreads through airborne spores, especially during wet spring weather, making preventative treatments and proper tree spacing important for controlling the disease.
Dutch Elm Disease
Dutch elm disease (DED) causes wilting and mortality in all elm species in Minnesota. Spread by elm bark beetles during feeding, and through root connections between trees. Fungicide injections can protect elms from bark beetle infection. If detected early, DED infections can be pruned out and fungicides can protect the tree. DED-resistant varieties are available.
Emerald Ash Borer
an invasive wood-boring beetle that attacks and kills ash trees by tunneling beneath the bark and disrupting the tree’s ability to move water and nutrients. Early signs of infestation include thinning leaves, bark splitting, increased woodpecker activity, and dieback in the upper canopy. Without treatment, infested ash trees often decline rapidly, making early detection and preventative care critical for protecting valuable trees.
Girdling Root
a root that grows around the base of a tree instead of outward into the soil. As the tree grows, the root can slowly constrict the trunk and restrict the flow of water and nutrients, leading to poor growth, thinning leaves, branch dieback, and decline over time. Early detection and corrective root care can help improve tree health and stability before serious damage occurs.
Oak Wilt
is a fast-spreading fungal disease that affects oak trees by blocking the movement of water within the tree. Infected trees may show wilting leaves, browning along the edges, rapid leaf drop, and canopy dieback, often progressing quickly once symptoms appear. The disease can spread through connected root systems and beetle activity, making early detection, proper pruning practices, and preventative treatments essential for protecting nearby oaks.
Spruce Gall Adelgid
a tiny sap-feeding insect that affects spruce trees by causing unusual cone- or pineapple-shaped growths called galls on new shoots. While light infestations are mostly cosmetic, heavy populations can stunt new growth, weaken branches, and reduce the overall appearance and health of the tree. Monitoring and early treatment can help prevent long-term stress and damage, especially on younger or already stressed spruce trees.
Eastern Tent Catepillar
The Eastern tent caterpillar is a common spring pest that builds distinctive silk tents in the branches of hardwood trees, especially cherry, apple, crabapple, and plum trees. While healthy trees can usually recover from occasional defoliation, repeated infestations can weaken trees and make them more susceptible to disease and other pests. Early identification and treatment can help protect your trees and prevent caterpillar populations from spreading.
