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Tree Diseases and Insect Pests

Cooley Spruce Tree Urban environments are extremely hostile places for shade trees to grow and survive because there are many interrelated variables that may cause urban shade tree stress.  Stress may be defined as any environmental pressure that causes a change in a tree's physiology that predisposes the tree to invasion by secondary organisms (disease & insects) that it would otherwise resist.  

Some of these urban environmental stress agents include drought, construction damage, soil compaction, lawnmower & weed whip damage, de-icing salt, vandalism, and air pollution.

Shade trees that are exposed to one or more of these stress agents are much more vulnerable to disease and insect attacks.  In fact, many insect pests will only attack a tree that is under severe stress.  By planting drought tolerant and/or native species and following proper planting, mulching, watering, and pruning techniques, you will help ensure that your tree remains healthy and vigorous and more able to fight off disease and insects.     

Routinely monitor your trees throughout the growing season.  If you notice anything that looks out of the ordinary, contact a licensed, certified arborist.

Don't Move Firewood

Moving firewood will spread insects and disease. Mountain Pine Beetle and IPS Beetle can be brought into Fort Collins from infested wood found in our local mountains. To learn more about the dangers of moving firewood click here: Don't Move Firewood

To learn more about Firewood & Pest Regulations in Colorado click here: Colorado Regulations


Desease Alert - Thousand Cankers Disease of Black Walnut(TCD):

Thousand Cankers Disease is a relatively new disease to Colorado's Front Range. This disease has killed many black walnuts in Boulder and Denver and is spreading. This disease is spread by the walnut twig beetle, which carries the fungus Geosmithia morbida. This fungus spreads and creates cankers in the walnut which eventually coalesce and girdle the tree. Signs and symptoms of this disease include, branch flagging and dieback in walnut. To read more about Thousand Cankers disease click here: Thousand Cankers Disease or here: Thousand Cankers Disease

To learn more about the following tree disease problems, visit Colorado State University Cooperative Extension - Gardening Fact Sheets.  These fact sheets describe disease symptoms and prevention & management techniques.  You will also find a fact sheet on "Environmental Disorders of Woody Plants" that covers sunscald, drought, overwatering & frost injury.

Common Diseases and Host Plants
Anthracnose Sycamore
Bacterial Wetwood Wide range of trees
Cedar-hawthorn Rust Apple, Hawthorn
Chlorosis Silver Maple, Oak
Cytospora Canker, Aspen, Cottonwood, Poplars, Apple, Cherry, Peach, Plum, Birch, Willow, Honeylocust, Mountain Ash, Silver Maple, Spruce and Siberian Elm
Decay Silver Maple, Boxelder
Dutch Elm Disease American and English Elms
Dwarf Mistletoe Pines
Fireblight Apples, Crabapples, Hawthorns, Mountain Ash, Pears
Leaf Spots Aspen, Cottonwood
Powdery Mildew Apple, Crabapples, Pear
Thyronectria Canker Honeylocust


Diseased Pine Tree

INSECT ALERT: Ips Bark Beetles

Ips is a common group of bark beetles that infests pine and spruce trees. Ips beetles rarely attack healthy trees and most problems occur on trees that are under severe stress. Several generations of ips can occur in a season. Most of our urban problems occur in spruce trees. The most noticeable symptom is tree die back from top down. To read more about IPS Bark Beetle click here: IPS Beetle

Mountain Pine Bettle (MPB)
Mountain Pine Beetle is currently affecting thousands of acres of lodgepole and ponderosa pine forests in Larimer County. Beginning in 2007 Mountain Pine Beetle began to show up on pines within the City of Fort Collins. The past several years we have seen an increase of MPB attack in Scotch Pine and Ponderosa pine. Roughly 10% of pines attacked in Fort Collins will die from MPB.

Addition information about MPB click here: Mountain Pine Bettle

Insect Alert: Emerald Ash Borer
Emerald Ash Borer has been detected in Colorado. On Septemeber 23, 2013 Emerald ash borer was found in Boulder, Colorado. Emerald Ash Borer is a very aggressive insect that is responsible for killing an estimated 50 million ash trees in 21 different states. It is important to understand that the movement of this insects to primarily caused by the movement of ash wood. For more information about detection, prevention, and treatment please visit the Colorado department of Agriculture website.

To learn more about the following insects, visit the Colorado State University Cooperative Extension - Insects Fact Sheets. These fact sheets describe the insect & life cycle, signs & symptoms of attack, and management techniques.

Common Insects and Host Plants
Aphids Wide range of plants
Borers Birch, Black Locust, Green Ash, Pinyon Pine
Boxelder Bug Boxelder
Brownheaded Ash Sawfly Ash
Codling Moth Apple
Cooley Spruce Galls Blue Spruce
Elm Leaf Beetle All Elm Species, especially Siberian
Insect and Mite Galls Oaks, Hackberry, Honeylocust
Ips Bark Beetles Pines, Spruce
Leafhoppers Honeylocust
Leaf (needle)miners Birch, Lilac, Aspen, Cottonwood, Ponderosa Pine
Mountain Pine Beetle Pines
Nipple Gall Hackberry
Pitch Nodule Moth Pinyon Pine
Plant Bugs Ash, Honeylocust
Pod Gall Midge Honeylocust
Sawfly on Ash
Sawfly on Pine
Ash, Pine
Scale(s)on Conifers
Scale(s) on Shade Trees and Ornamentals
Ash, American Elm, Pinyon Pine, Scotch Pine, Linden
Spider Mites Honeylocust, Juniper, Spruce
Spindle Gall Pinyon Pine
Spruce Budworm Douglas-fir, Spruce
Tip Moth Austrian Pine, Pinyon Pine, Ponderosa Pine
Tussock Moth Douglas-fir, Spruce, White Fir
Zimmerman Pine Moth Austrian Pine