Tree Diseases & Pests in Colorado

Colorado's trees face a variety of threats from diseases and pests, some native to our region and others that have arrived more recently. Understanding these threats is essential for Douglas County homeowners who want to protect their property's trees and maintain a healthy landscape.

Destructive Insect Pests

Several insect pests pose significant threats to trees in Douglas County. Early identification and treatment can often save trees, while delayed action may result in tree loss.

Emerald Ash Borer (EAB)

The emerald ash borer is an invasive beetle that has devastated ash trees across North America. First detected in Colorado in 2013, EAB has been confirmed in the Denver metro area and poses an ongoing threat to ash trees throughout Douglas County.

Signs of infestation:

• D-shaped exit holes in bark (about 1/8 inch wide)
• S-shaped tunnels under bark created by larvae
• Crown dieback starting at the top of the tree
• Increased woodpecker activity
• Epicormic sprouting (new shoots growing from trunk or branches)
• Bark splitting or cracking

Treatment options: Preventive trunk injections with insecticides can protect ash trees, but treatment must be ongoing. Some homeowners choose to remove ash trees preemptively due to the high cost of long-term treatment. Consult with a certified arborist to determine the best approach for your specific trees.

Mountain Pine Beetle

Mountain pine beetle is native to Colorado and has caused massive die-offs in our mountain forests. While primarily affecting higher elevations, this pest can also impact ponderosa pines in Douglas County, particularly in areas like Larkspur, Castle Pines, and The Pinery.

Signs of infestation:

• Pitch tubes (popcorn-shaped masses of resin) on the trunk
• Boring dust in bark crevices and at tree base
• Needle color change from green to yellow to red
• Blue-stained wood under the bark
• Woodpecker activity stripping bark

Prevention: Healthy, well-watered trees are more resistant to beetle attacks. Thinning dense stands of pines can also reduce susceptibility. Preventive spraying is available for high-value trees.

Ips Beetle

Also known as engraver beetles, Ips beetles attack stressed pines and spruces. They're particularly active in Douglas County during dry summers when trees are drought-stressed.

Signs of infestation:

• Reddish-brown boring dust in bark crevices
• Small, round exit holes (about 1/16 inch)
• Y-shaped or H-shaped galleries under bark
• Fading or yellowing needles, often starting in upper crown

Prevention: Proper watering during drought, prompt removal of dead or dying branches, and avoiding pruning pines from March through October.

Lilac/Ash Borer

This native wood-boring insect attacks ash trees and lilacs, creating visible damage and weakening trees. It's often confused with emerald ash borer but has been present in Colorado much longer.

Signs of infestation:

• Sawdust-like frass near holes in bark
• Swelling or scarring on trunk and branches
• Branch dieback
• Wet areas on bark during active boring

Common Tree Diseases

Cytospora Canker

Cytospora canker is one of the most common and destructive diseases affecting Colorado blue spruce in Douglas County. The disease causes branch dieback, typically starting on lower branches and progressing upward.

Symptoms:

• Dead branches with brown or purple-brown needles
• White or bluish resin flow on affected branches
• Sunken, discolored cankers on bark
• Progressive dieback from bottom to top of tree

Management: There is no cure for cytospora canker. Management focuses on pruning infected branches, improving tree health through proper watering and avoiding wounding the tree. Severely affected trees may need removal.

Rhizosphaera Needle Cast

This fungal disease affects spruce trees, particularly Colorado blue spruce. It causes premature needle drop, typically starting on inner and lower branches.

Symptoms:

• Purple or brown needles on inner and lower branches
• Black fungal fruiting bodies (visible with magnification) in rows on needles
• Progressive needle loss starting near trunk
• Trees appear thin and sparse from inside out

Treatment: Fungicide applications in spring (May-June) for two or more consecutive years. Improving air circulation through pruning can help.

Fire Blight

Fire blight is a bacterial disease that affects apple, crabapple, pear, and other related trees. It can spread rapidly during warm, wet spring weather.

Symptoms:

• Blackened, wilted shoots that curl into "shepherd's crook"
• Leaves appear scorched or burned
• Amber-colored bacterial ooze on infected areas
• Cankers on branches and trunk

Management: Prune infected branches at least 8-12 inches below visible symptoms during dry weather. Disinfect pruning tools between cuts. Avoid high-nitrogen fertilizers that promote susceptible new growth.

Dutch Elm Disease

Dutch elm disease has devastated American elm populations across the country. While less common in Douglas County than historically, it remains a concern for remaining elms.

Symptoms:

• Wilting and yellowing leaves on one or more branches
• "Flagging" - dead branches with brown leaves still attached
• Brown streaking in wood when bark is peeled back
• Rapid progression from first symptoms to tree death

Environmental Stress Conditions

Colorado's climate creates additional stress on trees that can make them more susceptible to diseases and pests:

Drought Stress

Douglas County's semi-arid climate means trees often don't receive adequate water, especially during summer months. Drought-stressed trees are more vulnerable to beetle attacks and diseases.

Signs of drought stress:

• Wilting or drooping leaves
• Leaf scorch (brown leaf edges)
• Early fall color and leaf drop
• Sparse canopy
• Branch dieback

Prevention: Deep watering during dry periods, mulching around the base (but not against the trunk), and avoiding excessive pruning during drought.

Winter Injury

Colorado's temperature swings can damage trees, particularly during late winter when warm days followed by freezing nights cause bark splitting and cell damage.

Susceptible trees: Thin-barked species like maple, ash, and fruit trees are most vulnerable. Southwest-facing trunks are typically affected first.

Prevention: Tree wraps on young trees in fall, proper watering before winter, and avoiding late-season fertilizing.

Chlorosis (Iron Deficiency)

Colorado's alkaline soils often cause iron chlorosis in trees, particularly in silver maple, red maple, pin oak, and some fruit trees.

Symptoms:

• Yellow leaves with green veins (interveinal chlorosis)
• Symptoms typically appear on newest growth first
• Stunted growth
• Leaf browning in severe cases

Treatment: Soil amendments, trunk injections with iron, or chelated iron applications. Consider planting species better adapted to alkaline soils.

When to Call a Professional

Many tree health issues require professional diagnosis and treatment. Contact an arborist if you notice:

• Rapid decline in tree health
• Signs of boring insects (holes, sawdust, pitch tubes)
• Widespread needle or leaf loss
• Fungal growths on trunk or branches
• Oozing sap or unusual discharge
• Multiple trees showing similar symptoms

A certified arborist can properly identify the problem and recommend appropriate treatment options. Early intervention often provides the best outcomes for saving affected trees.

Prevention Best Practices

Healthy trees are naturally more resistant to diseases and pests. Follow these practices to keep your Douglas County trees healthy:

• Water properly: Deep, infrequent watering is better than shallow, frequent watering
• Mulch correctly: 2-4 inches of mulch in a ring around the tree, not touching the trunk
• Avoid compacting soil: Don't park vehicles or place heavy materials under tree canopies
• Prune properly: Make clean cuts at the branch collar, avoid topping trees
• Time pruning correctly: Avoid pruning pines in spring and summer, prune oaks in winter
• Monitor regularly: Inspect trees seasonally for signs of problems
• Choose the right tree: Plant species suited to Colorado's climate and your specific site conditions

Local Resources

For more information about tree diseases and pests in Colorado:

• Colorado State Forest Service - Information on forest health and pest management
• CSU Extension - Fact sheets on specific diseases and pests
• Colorado State University Plant Diagnostic Clinic - Can analyze samples to identify problems