Many plants in the Colorado landscape are affected by diseases every year with some causing little to no damage while others may produce a sudden decline or death. Chemical controls have been proven to be largely ineffective against most of these pathogens. The best defense against any of these diseases is to ensure plants are healthy, fertilized and properly watered. Prevent injury to the plant and damage to the roots. Removal of diseased wood may be required to prevent the spread of disease. If you think that a disease may be harming your trees or shrubs, then meet with an Elk Creek representative to have it diagnosed correctly the first time.
Front Range Plant Bacterial Diseases
Fireblight is a bacterial disease that most commonly affects crabapples, apples, pears, mountain ash and at times, other stone fruit trees along the Front Range. This bacterium is introduced through open wounds or from pollinators transferring the bacteria to flower blossoms. Warm, wet conditions during spring and early summer will encourage the spread of fireblight. Trees with fireblight will develop curled and blackened tips extending down the branch towards the trunk. Leaves will remain on branch after they have died turning brown or black and severely affected branches appear burned. Pruning of diseased wood is recommended while the tree is in dormancy; any pruning done during the growing season can transfer the bacteria with each cut so this is generally not recommended. Elk Creek recommends a foliar copper spray applied twice to flower blossoms in the spring to help control the spread of fireblight.
Bacterial wetwood is a common disease that affects many of our shade trees including aspens, elms, cottonwoods, maples and willows though it may be found in a variety of trees. Areas of infection will be moist or wet with a yellow-brown discoloration on the bark. Internal gas pressure from the bacteria will cause slime to ooze from the trees’ central core which is slimy and has an unpleasant odor. This slime will also attract insects and can be a host for fungi. Bacterial wetwood will inhibit new growth in infected areas and may eventually kill the tree. There is no effective control for wetwood.
Front Range Fungal Diseases
Marssonina Leaf Spot is the most common foliage disease of aspens and cottonwoods in the urban corridor. Infections will occur after bud break during warm and wet springs. Damage to the leaf first appears as small brown flecks with a yellow halo and as it progresses, spots will merge to become large black areas and leaves will fall prematurely. This damage is mostly aesthetic and contained to the leaves, although repeated outbreaks year after year will stress the tree. Removing fallen leaves from the landscape will help to prevent the spread of this disease in the spring.
Septoria Leaf Spot is more commonly associated with cottonwoods but will also affect aspens. Damage may be irregular but can begin as tan spots with a black border progressing to large brown or black areas. As with other leaf spots, the damage is mostly aesthetic. We do not recommend the use of fungicides to control leaf spots.
Shoot Blight is a fungus that affects new growth tissue on aspens and cottonwoods. This fungus will be spread during warm, wet conditions at any time of the growing season. Symptoms first appear on the leaves as brown or black areas causing distortion and dead leaf tissue. Affected leaf shoots will turn black and begin to curl resembling a shepherds’ crook. The removal of fallen leaves as well as the pruning of dead branch tips during dormancy will help to control this disease.
Cytospera Canker affects many trees and shrubs in the Front Range landscape. The fungus will attack plants or parts of plants that are injured or highly stressed, especially affecting plants with root damage. Cankers will form on the bark of the limb appearing as a blister and may ooze sap. Leaves on affected limbs will turn yellow and may fall prematurely. As the canker girdles parts of the plant, new growth will be inhibited and dieback may occur above the canker. No effective chemicals are available to control the fungi that cause canker. Pruning of diseased wood is recommended to control the spread of this disease.
Apple Scab will affect crabapples as well as apple trees. It will create leaf blotches and cause yellowing of the leaves, leaves may drop early. This fungus will also damage the fruit causing dark, soft spots and reduce the quantity of the crop. Cultural practices are the best defense against this disease. Avoid irrigation hitting the leaves of the tree, prune think trees to enhance air circulation and drying, remove all leaves from the ground in the fall and prune diseased wood from the tree during dormancy. For repeated outbreaks, a fungicide may be applied at bud break in the springtime.
Anthracnose causes mainly aesthetic problems for sycamores and oak trees in our region though repeated outbreaks year after year will stress the trees. Symptoms appear during the summer but the damage was done at bud break during wet, cool springs. Leaves will develop irregular brown patches and distortion is common. Leaves will drop early and there may be some tip dieback. Fungicides are not effective for this disease. Remove all leaves from the ground and prune deadwood from the tree.
Dwarf Mistletoes are common throughout the western U.S. These leafless, parasitic plants will commonly infect ponderosa pine, pinyon pine, lodgepole pine, and Douglas fir. These plants will actually root into the bark stealing water and nutrients from the host tree causing yellow needles, reduced foliage and witches broom. This will cause a gradual decline in the trees health and will make susceptible to disease and insects. Mistletoe should be removed from the host tree by pruning.