Dutch Elm Disease

Description: Dutch Elm Disease (DED) is a fungal pathogen that affects Elm trees and can kill entire trees quickly. DED was officially confirmed in Cooper Park in 2024 and infected trees have been observed throughout the city. Chemical treatments are the only effective prevention and once more than 5-10% of a tree shows symptoms/signs, the tree must be removed. 

Image 1: Dieback in the canopy of an Elm tree.

Due to the severity of this disease and its recent detection, the City of Bozeman has partnered with Montana State University to publish a factsheet for further information. Some key items for managing DED are summarized below:

Symptoms/Signs: Affected leaves and branches display “flagging”, quickly wilting and turning an orange/brown color. This can happen suddenly and occur throughout the tree canopy. Flagging progresses into dieback of major branches. Underneath the bark, beetles that spread the fungus display feeding “galleries” in a distinctive pattern.

Image 2: Flagging leaves in the upper canopy of an Elm tree. 

Image 3: Galleries created by one of the introduced Elm bark beetles with larvae at the end of the tunnels.

Image 4: Bark tunneling by the native Elm bark beetle.

Treatment: Trees displaying mild flagging (less than 5-10%) should have those branches removed and considered for chemical treatments. Trees with more than 5-10% flagging are not viable for chemical treatments and should be removed promptly. Specialized trunk injections, containing the active ingredients thiabendazole or propiconazole, are the only effective treatments and should only be administered by a professional ISA Certified Arborist.

Additional Info: Elm trees should not be pruned during the growing season; aside from removal of deadwood or infected branches, save maintenance trimming for winter when the leaves are off the tree. Removed trees have special considerations for proper disposal to reduce the spread of DED (See our factsheet under “Tree Removal and Disposal”).

Emerald Ash Borer

The Emerald Ash Borer (EAB) is a looming threat to Bozeman’s ash tree population. The non-native, invasive pest quickly kills entire trees and has the potential to wipe out ALL of our Ash tree population. With recent outbreaks in nearby states, the Forestry Division is making preparations. Bozeman City Commission recently adopted an EAB Action Plan to combat this threat to our urban forest.

What is Bozeman doing to prepare for EAB?

• We’re gradually reducing our Ash tree population in parks & boulevards. Trees in poor health, conflicting with infrastructure, or having a history of storm damage are great candidates for removal & replacement.
• Ash trees are no longer an approved species in our Street Tree Guide. Planting them is not allowed in parks & boulevards and we don’t recommend planting new Ash trees in your private landscape, either.
• We’re planting a wide variety of new trees each year! Increasing the species diversity of our urban forest makes our city more resilient to any pest or disease threat!
• Forestry Division staff are constantly looking for signs of EAB. We work on Ash trees year-round, and any pruning or removal work is a chance to spot trouble.

What can you do at home?

• Seek an ISA Certified Arborist to assess the trees in your landscape. These local tree experts can help you form a plan unique to your site.
• Gather information on potential treatment options, but do NOT start chemical treatments for EAB yet. State & Federal agencies recommend waiting until an official infestation is declared.
• If you have multiple Ash trees on your property, consider removing & replacing some of them proactively.
• Install wood chip mulch rings around all trees, and water even mature trees regularly through the summer. Water and Mulch are two of the most important ways to keep trees healthy.

Description: The Emerald Ash Borer (EAB) is a non-native boring beetle. Adult beetles are a shiny, emerald green color. The adults deposit eggs underneath the bark of ash trees. The eggs develop into larvae that tunnel through conductive tissue, severing the flow of nutrients and water within the tree. Larvae develop into adults and emerge in mid-summer, usually July. 

Image 1: The adult form of EAB.    Image 2: The larvae feed under the bark.

Symptoms/Signs: Infested trees will start to die "from the top down". Look for thinning foliage and/or dieback in the upper canopy. This dieback is followed by lush new growth at the base of the tree. Severely declining trees will have "D-shaped" exit holes where adults have emerged. Removal of outer bark will expose "S-shaped" tunneling of the larvae.

Images 3-4: Heavy feeding from EAB will thin the canopy, starting from the top branches. 

Treatment: We do NOT recommend ANY TREATMENT at this time, as EAB has not been detected in our area. It is best to wait until State and Federal entities declare an official outbreak. The most effective treatment is a trunk injection of chemical pesticide. This method provides direct uptake into the tree with minimal environmental or safety concerns for the public. This pesticide will need to be re-applied every 2 years until the EAB population is eliminated. Smaller trees (10" trunk diameter or less) should be considered for removal and replacement.

Bozeman EAB Course of Action Plan

The Forestry Division has a plan for EAB's arrival, including early detection, treatment options, and necessary tree removals. To learn more about the Emerald Ash Borer, read the Course of Action Plan from the link below.

EAB Course of Action Plan - Bozeman (PDF)

Fire Blight

Description:  Fire blight is a bacterial disease which affects members of the Rose (Rosaceae) family, including apple, pear, hawthorn, and mountain-ash. The disease targets new growth in flowers, fruits, and young branches. If allowed to progress, fire blight can lead to severe decline and eventually death.

Images 1-2: Fire blight is recognized by "flagging" - obvious dieback in smaller sections of new growth.

Symptoms/Signs:  Affected tissues experience wilting and turn brown or black. Opaque white or amber "droplets" may form under wet conditions, and infections spread to larger tree limbs, darkening the color of the bark.

Images 3-4: Damaged tissue is easy to spot. Pruning to remove the infection and proper disposal off-site is key to managing fire blight.

Treatment: The fire blight bacteria is ever-present in North America. Chemical options are only preventative and not practical for our urban forest. The best way to manage trees that are infected is to prune out the damaged tissue during dormancy, and remove it from the site.  Caution should be used as the bacteria moves into the tree through open wounds or cuts, especially in warm, wet weather.  Susceptible trees can be replaced with cultivars that are resistant to fire blight.

European Elm Scale

Description: While we are fortunate to not have Dutch Elm Disease in our area, Bozeman's elms can be heavily affected by scale insects. These pests are soft-bodied and mobile when young, then they develop a tough armor coating into adulthood.

Image 1: Scale insects are visible to the naked eye, and can cover branches on infested trees. 

Image 2: Trunks are stained black from honeydew and the Sooty Mold fungus.

Symptoms/Signs: Scale insects feed on the leaves, twigs and bark of elm trees, sucking sap with their mouthparts. Scale excrete "honeydew", a sticky waste product that causes a nuisance for surfaces under the tree. The presence of honeydew promotes a secondary fungal issue known as Sooty Mold, which stains trunks to a dark brown-black.

Image 3: Scale produce "honeydew", a sticky waste product that costs everything beneath the tree.

Treatment: Timing is critical for treating scale insects. Foliar sprays and soil drench applications of insecticides are most effective in the Spring (April-May). The 'Brandon' Elm cultivar is highly susceptible to this pest. Heavily infested trees should be considered for removal and replacement. There are several varieties of Elm that show high resistance to scale.

Honeylocust Cankers

Description: Cankers are lesions of bark decay caused by fungal pathogens. They often enter the tree through wounds on the main trunk. Honeylocust trees are susceptible to several types of cankers, which often lead to severe decay and death.

Images 1-2: A healthy tree will heal over pruning cuts, while trees with canker will struggle and show continued dieback.

Symptoms/Signs: Cankers are characterized by sunken, discolored areas of trunk tissue. Bark deteriorates to expose inner wood and heavy callous tissue. Heavily affected trees may show early fall color and leaf drop.

Cankers often start at the base of the trunk, from mower or string trimmer damage. 

Treatment: Preventing wounds is critical, especially on the main trunk. Pruning branches should always be done with proper technique and clean, sharp tools. Healthy trees stand the best chance - ensure your honey locusts have a mulch ring, trunk protection, and adequate water. The 'Sunburst' cultivar is particularly vulnerable, while 'Imperial' and 'Skyline' cultivars are resistant to cankers.

Rough Bullet Gall

Description: Galls are portions of the tree that have been warped by a pest, in this case a small wasp. The Bullet gall wasp ONLY affects Bur oak (Quercus macrocarpa), forming rounded, bullet-shaped egg sites from young branches. In most cases, Bullet galls are a cosmetic issue for Bur oak, stressing the tree but rarely leading to death.

Galls are readily visible on affected Bur oaks. This pest damage is mostly an eyesore. 

Symptoms/Signs: The brown, rounded galls can be easily spotted. Secondary pests like bees and other wasps can be attracted in the fall by the developing larvae. Adults emerge in late October/early November, with females depositing new eggs into dormant buds.

Treatment: There isn't currently an effective insecticidal treatment for this pest. Start with good site conditions - ensure your oaks have a mulch ring, trunk protection, and adequate water. Developing galls can be removed by hand. Check for exit holes - there is little purpose in removing empty galls aside from aesthetics.

Exit holes show the wasp has already left. Removing old galls only serves an aesthetic purpose. Look for young, light-colored galls with no exit holes. Removing these by hand will also remove developing wasp larvae.