Myriad insects attack all the varieties of aspens, though few of them are fatal unless the tree is already ill. Aphids are a regular pest, leaving tiny pinpoints where they’ve sucked juices from leaves. Leafminers tunnel between the top and bottom of the leaf sucking juices, leaving winding trails. Borers dig into the trunks and suck the sap, and poplar twigall flies will cause unsightly galls. Scale insects will form little tough mounds on the bark and stems, shielding the insects as they suck on the sap. Tent caterpillars make a web enclosure and eat all the leaves within, moving to new leaves as those become exhausted.
Fungal diseases often attack most species of aspens. Marissonina blight is one of the most common fungal diseases, leaving black flecks on the leaves that grow into large dead areas. Blank canker is a fungal disease found on the trunks or stems, leaving gray dead areas with a black outline. Trunk rot can be identified by hooflike outcroppings along the trunk, and is likely to attack trees that are already damaged in some way. Inside the trunk, the heartwood will decay and cause the tree to become unstable.
Physical damage can occur through normal maintenance of the landscape and through more natural means. Weed whackers and lawn mowers are the most common culprits, leaving damaged bark that may invite disease. In the most severe cases, bark will be damaged all around the bottom of a tree, essentially cutting off water and nutrient flow to the tree. These trees will die. Elk and other rutting animals will scrape their horns against trees found in more remote areas and inhibit or stop nutrient and water flow.
Improper Care or Environment
Swedish columnar aspens are a cool-weather tree, native to areas with cold winters and overall cool summers. If they are brought out of these conditions, they will become weak and susceptible to disease. They need even moisture and respond badly to overly dry or wet soils. All aspens tend to produce suckers, and people are often tempted to spray them with herbicide, forgetting that these suckers are attached to the tree’s roots. Spraying the suckers will likely cause damage or death to the tree.