7 Common Tree Diseases in Minnesota and How to Spot Them Early

Knowing What to Look For Can Save Your Trees

Minnesota's diverse tree population faces a wide range of diseases, from fast-moving fungal infections that can kill a tree in weeks to cosmetic issues that look alarming but rarely cause lasting damage. The key is knowing the difference — and catching problems early.

Here are seven of the most common tree diseases we see across Minnesota, along with what to look for and when to call in a professional.

1. Oak Wilt

Trees affected: Red oaks, pin oaks, white oaks, bur oaks

Oak wilt is the most serious tree disease in Minnesota. Caused by the fungus Bretziella fagacearum, it spreads through sap beetles and underground root connections between nearby oaks.

What to look for:

What to do: Do not prune oaks between April and July. If you see symptoms, contact a certified arborist immediately for a professional diagnosis. Early detection and prompt action are critical.

2. Dutch Elm Disease

Trees affected: American elm, red elm, rock elm

Dutch elm disease (DED) has been devastating Minnesota's elm population for decades. The fungus is spread by elm bark beetles and through root grafts between neighboring elms.

What to look for:

What to do: Avoid pruning elms between April and August. If you spot flagging branches, get a diagnosis quickly. Early-stage infections may be treatable, but advanced cases typically require removal to prevent spread.

3. Apple Scab

Trees affected: Crabapple trees, apple trees

Apple scab is one of the most common fungal diseases on ornamental crabapples in Minnesota. While it rarely kills trees, it can cause significant cosmetic damage and weaken trees over time if it recurs year after year.

What to look for:

What to do: Rake and remove fallen leaves in autumn to reduce fungal spores. Consider replacing highly susceptible varieties with scab-resistant cultivars. Fungicide sprays in spring can help for high-value trees.

4. Cedar-Apple Rust

Trees affected: Apple trees, crabapples, junipers, and eastern red cedars

This fungal disease requires two host plants to complete its life cycle — it alternates between juniper or cedar trees and apple or crabapple trees. It's common across Minnesota wherever these species grow near each other.

What to look for:

What to do: Separating host plants can reduce severity, though spores can travel long distances. Choosing resistant varieties is the most practical long-term solution. Fungicide applications in spring may help manage outbreaks.

5. Rhizosphaera Needlecast

Trees affected: Colorado blue spruce, white spruce, and some pines

Needlecast is extremely common on Colorado blue spruce in Minnesota. The fungus Rhizosphaera kalkhoffii infects needles and causes them to drop prematurely, leading to bare branches that work their way up the tree over several years.

What to look for:

What to do: Fungicide treatments in spring can help slow the spread but require multi-year commitment. Improving air circulation through proper spacing and pruning helps. In many cases, severely affected blue spruce may be better replaced with a more resistant species.

6. Anthracnose

Trees affected: Maples, oaks, ash, sycamores, and other hardwoods

Anthracnose is a group of related fungal diseases that thrive during cool, wet springs — which Minnesota has in abundance. It's one of the most common leaf diseases across the state.

What to look for:

What to do: Anthracnose is rarely fatal and usually doesn't require treatment. Raking up fallen leaves reduces spore buildup. Healthy trees typically recover on their own once warmer, drier weather arrives. Persistent cases over multiple years may warrant a tree health assessment.

7. Tar Spot

Trees affected: Silver maple, Norway maple, red maple, boxelder

Tar spot is one of the most visually dramatic leaf diseases in Minnesota, but it's also one of the least harmful. It's caused by fungi in the genus Rhytisma and is purely cosmetic.

What to look for:

What to do: Tar spot does not harm the tree. The most effective management is raking and disposing of fallen leaves in autumn, which reduces the amount of fungal spores the following spring. No chemical treatment is typically necessary.

When to Call an Arborist

Some of these diseases — particularly oak wilt and Dutch elm disease — require urgent professional attention. Others are cosmetic nuisances. Here's a general rule of thumb:

TreeTec provides expert tree disease diagnosis and tree health risk assessments throughout Minnesota. Whether you're dealing with a potential emergency or just want peace of mind, our team can evaluate your trees and recommend the right next steps.

Worried about a tree on your property? Contact TreeTec today to schedule a professional assessment.