Aspens (Populus tremuloides
) tend to be short-lived trees, as expected from their usual role in forest ecology. In the High Plains or urban landscape, even properly cared-for aspens may not reach 20 years. Lifespans may be further shortened by one or more of the several insects or diseases that attack aspen, some of which have no controls. Ecologically, aspen in natural settings serve as "succession" trees, quickly seeding in where other vegetation is lost because of erosion, fire, logging, insects or disease. They provide cover for seedlings of pine, fir and spruce as they grow; as these become larger, the "nurse crop" of aspen then dies out.
Aspen trees reproduce not only by seed but also by extensive suckering. A grove of aspens in the mountains may have started as suckers off the roots of an original "mother" tree that arrived at the site by seed. This suckering habit can be a nuisance in the urban landscape, coming up in lawns and gardens. A homeowner who chooses to plant aspens, should expect diseases and insect problems. Aspen prefers the moist but well-drained, slightly acidic soil found at higher elevations. Conversely, much of the soil of in Colorado is compacted, poorly-drained alkaline clay. Aspens transplanted to such soils are at a disadvantage, especially considering that a significant portion of the original root system is lost in the digging process.
The thin bark of aspen is damaged and injured easily by lawnmowers or string-trimmers ("weed eaters"), which allows pathogens and certain insects easy entry. Fungal diseases, such as Cytospora
or other cankers that attack the trunk, are common, as are diseases of the foliage such as rusts or leafspots. Of the many insects that attack urban plantings of aspen, oystershell scale, aphids and aspen twiggall fly are most prevalent. Spraying aspen foliage to control leaf diseases or insects often results in injury to the foliage (phytotoxicity). Aspens are notoriously prone to phytotoxicity from insecticides or fungicides. Once an individual tree in a grove of aspen starts developing deadwood and a thin crown, it should be removed to allow new shoots to emerge.
Here are some common problems affecting aspen trees in Colorado:
--Blackened leaves towards the late summer and fall are caused by aphid secretion or a leaf spot disease. Five fungi cause most foliage diseases on aspen, cottonwoods and other poplar species. The leaf spot disease known as Marsonnina
leaf spot or aspen leaf spot causes the most common foliage disease on aspen and poplars in urban and forested areas of Colorado. Septoria
leaf spot is a common foliar disease mainly on cottonwoods and occasionally aspen in urban areas of Colorado. Ink spot is caused by the fungus Ciborinia
found mainly in the Colorado mountains. Leaf and shoot blight is caused by the fungus Venturia
--a disease affecting young aspen and cottonwood tissue primarily in the mountains. A leaf rust disease caused by the fungus Melampsora
is often seen on aspen and cottonwood, but rarely causes serious problems. Sanitation is an effective control for some foliar diseases. Fall removal of infected leaves, twigs and branches, sources of reinfection, can reduce the amount of disease the next spring. Raking and destroying infected leaves can reduce Marssonina
leaf spot, ink spot and leaf rust. The shoot blight fungus overwinters in diseased stems and twigs, so it can be pruned out to reduce new infections. See fact sheet Aspen and Poplar Leaf Spots.
--Orange pimples on the bark indicate the presence of Cytospora canker. Cankered branches need to be removed. See Cytospora Canker.
--Oystershell scale are tiny, quarter-inch-long brown or gray shells that form along the trunk and branches of aspen. This disease is caused by an insect that is difficult to control. See Oystershell Scale.
--Dozens of species of aphids (plant lice) may be found on shade trees and woody ornamental plants in Colorado. Aphids feed on plants by sucking plant sap from the leaves, twigs or stems. See Aphids on Shade Trees and Ornamentals.
--Poplar twiggall is an increasingly common gall found on the twigs and branches of cottonwoods, poplars and, particularly, aspen. This gall is caused by an insect, the poplar twiggall fly. Galled tissues continue to grow and swell, and ultimately become large knots on trunks and larger branches, giving the plants a gnarled, bonsai-like appearance. Although these old injuries produce a permanent disfigurement, they do not seem to threaten tree health. There are no effective chemical controls to prevent this gall. See fact sheet Poplar Twiggall Fly.
--Bacterial wetwood is a common disease that affects the central core of many shade and forest trees. In Colorado, the disease is most prevalent in elm, aspen and willow. Prevention of tree stress is the best management approach. Effective control measures do not exist. See fact sheet Bacterial Wetwood.
Insect Control: Horticultural Oils.
Colorado State University Cooperative Extension also has a for-sale publication that may be helpful.