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Trees with Pest or Disease Damage
in Rochester, MN
Emerald ash borer arrived in the Rochester area and has killed thousands of ash trees over the past fifteen years. Ash trees that have not been treated are nearly all gone or going. Oak wilt is also spreading through connected root systems in neighborhoods where oaks grow close together, including parts of northwest Rochester. Trimming at the wrong time of year can spread some of these diseases, so timing matters.
Quick Answer
Several insects and diseases are actively killing trees in Rochester right now. Emerald ash borer has already destroyed most untreated ash trees in the area. Oak wilt and Dutch elm disease are present too. The visible damage often shows up after the tree is already in serious decline. Early pruning of affected branches can slow some diseases. Once the main trunk is involved, removal is usually the only option.
Telltale Signs
Warning Signs to Watch For
- Canopy is thinning from the top down over two or more seasons
- S-shaped galleries are visible under bark that has fallen off an ash tree
- Leaves are wilting and browning from the top of the canopy downward in summer
- Woodpeckers are working heavily on the upper trunk or main branches
- Bark is falling off in sections revealing discolored wood underneath
- Adjacent trees of the same species are also declining at the same time
Root Causes
What Causes Trees with Pest or Disease Damage?
Emerald Ash Borer Infestation
Emerald ash borer larvae tunnel under the bark of ash trees and cut off the flow of water and nutrients. Rochester has had confirmed infestations since around 2012, and any untreated ash tree in the area should be assumed to be at risk. The woodpecker activity you see on a declining ash is actually the birds digging for the larvae.
The Fix
Infested Branch Removal and Tree Assessment
Removing heavily infested limbs slows the spread slightly but does not save the tree once the borer has reached the main trunk. A full removal and proper disposal of the wood is usually the end result for untreated ash trees.
Oak Wilt Fungal Spread
Oak wilt is a fungal disease that spreads through root grafts between neighboring oaks and through beetles that carry spores to fresh pruning wounds. In Rochester, pruning oaks between April and July is strongly discouraged because beetles are most active during those months and the risk of introducing the fungus through a cut is highest.
The Fix
Affected Branch Removal Outside High-Risk Season
Oak work is done in winter when the fungus-carrying beetles are not flying. Infected trees are sometimes saved by trenching between them and neighboring oaks to sever the root grafts that spread the disease underground.
Cytospora Canker on Stressed Trees
Cytospora canker is a fungal infection that targets spruce and other conifers already weakened by drought or physical damage. Rochester spruces that went through the dry summers of the mid-2010s are showing canker damage now, years after the stress event. The fungus kills branches from the bottom up and leaves resin-soaked dead wood behind.
The Fix
Cankered Branch Removal and Tree Health Support
Removing the infected lower branches stops the fungus from climbing higher. The cuts need to be made outside the infected zone and the tools need to be disinfected between cuts to avoid spreading spores to healthy wood.
Self-Diagnosis
Which Cause Applies to You?
Check the signs you're observing to narrow down the likely root cause before your inspection.
| What You're Seeing | Emerald Ash Borer Infestation | Oak Wilt Fungal Spread | Cytospora Canker on Stressed Trees |
|---|---|---|---|
| S-shaped tunnels visible under bark on an ash tree | |||
| Leaves wilt and brown from the top down in early summer on an oak | |||
| Bottom branches of a spruce are dead while the top is still green | |||
| Heavy woodpecker activity on the upper trunk | |||
| Neighboring oaks are also declining at the same time | |||
| Resin or sap is oozing from dead patches on the bark |
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