Home

Back

Foliage diseases can reduce the aesthetic value of aspen and cottonwood.  Occasionally, a severe disease outbreak causes premature defoliation or dieback of parts of the tree.  If leaves are lost early in the season, a second growth of foliage may occur and tree health will not be seriously affected.  Defoliation in mid-summer with subsequent refoliation, however, may prevent the tree from fully hardening off before cold weather or reduce the amount of stored food.  This leads to increased danger of frost damage, reduced growth and predisposition to other diseases or insects.  If leaves are lost late in the season, the tree will not refoliate or lose much vigor.

Marssonina Leaf Spot

The fungus Marssonina causes the most common foliage disease on aspen and poplars.

Symptoms and Signs: Marssonina leaf spots are dark brown flecks, occasionally with yellow halos, up to about one-sixteenth of an inch (2mm)  in diameter.  Mature spots characteristically have a white center.  On severely infected leaves, in wet weather, several spots may fuse to form large black dead patches.  Spots also may develop on leaf petioles and succulent new shoots.

Disease Cycle: Marssonina survives the winter on fallen leaves that were infected the previous year.  With spring and warmer, wet weather, the fungus produces microscopic "seeds" or spores that are carried by the wind and infect emerging leaves.  Early infections are rarely serious, but if the weather remains favorable, spores from these infections can cause a widespread secondary infection.  Heavy secondary infections become visible later in the growing season and cause premature defoliation of infected leaves.

Ink Spot on Aspen

The fungus Ciborinia causes a leaf disease of aspen commonly known as ink spot.

Symptoms and Signs: The first symptoms of ink spot appear in late spring to early summer as tan to brown areas on the upper leaf surfaces.  Concentric, discolored ring patterns may become visible as the fungus advances through the leaf.  Infected leaves may be totally brown by mid-summer while adjacent uninfected leaves remain green.  Raised black bodies begin to appear on affected brown leaves.  These hard masses of fungal material are oval shaped and nearly one-fourth inch (4-6mm) long.  These are the "ink spots" which give the disease its common name.  In late summer these spots fall out, leaving a characteristic "shot hole" effect on leaves that remain on the tree.  This disease is especially prevalent in dense aspen stands.  Early defoliation may cause reduced growth damage.

Disease Cycle: The hard fungal tissue masses that fall from infected leaves are the overwintering stage of the fungus.  Wet spring weather stimulates spore production.  Spores are blown and splashed from the ground to developing leaves.  Ink spot rarely reaches epidemic proportions because the fungus completes only one infection cycle per year.

Leaf and Shoot Blight

Leaf and shoot blight, causes by the fungus Venturia is a disease affecting young aspen and cottonwood tissue.

Symptoms and Signs: In the spring symptoms first become visible on leaves near shoots infected the previous season.  Brown to blackened, irregularly shaped areas spread through the leaves causing them to dry and become distorted.  Typically, the fungus spreads down through the succulent new shoot, which blackens and curls to resemble a shepherd's crook.  Death of new shoots causes distorted, shrubby growth.

Disease Cycle: The leaf and shoot blight fungus survives the winter mainly on shoots infected the previous season.  Spores are windblown early in the season and infect newly expanding leaves and shoots.  As the season progresses, uninfected tissue becomes more resistant to the disease.

Leaf Rusts

A rust disease caused by the fungus Melampsora is often seen on aspen and cottonwood.  Though common, this disease rarely causes serious problems.

Symptoms and Signs: The disease is recognized easily by small yellow-orange pustules scattered on the lower leaf surfaces.

Disease Cycle: The life cycle of this fungus is somewhat complex because it requires two different tree hosts.  During wet spring weather spores are released from the fungus, which has overwintered on fallen cottonwood or aspen leaves.  These spores infect evergreens, such as Douglas fir, pine, fir or spruce, where they cause very little damage.  After two or three weeks, spores are produced on these evergreen hosts and are blown to aspen or cottonwood hosts, it can multiply rapidly under favorable wet conditions throughout the summer.  Several years of heavy infections can cause some growth losses, especially on younger trees.  Fallen infected leaves shelter the fungus until the next year's disease cycle.

Disease Management

Sanitation is an effective control for some foliar diseases.  Fall removal of infected leaves, twigs and branches can reduce the amount of disease the next spring.  Marssonina leaf spot, ink spot and leaf rust can be reduced by raking and destroying infected leaves.  The shoot blight fungus overwinters in diseased stems and twigs, so it must be pruned out to reduce new infections.

Keep leaves as dry as possible to reduce the incidence of leaf spots.  Water in early morning so leaves can dry out, keep sprinkler patterns adjusted so leaves don't stay wet and space trees apart to reduce humidity to help prevent leaf diseases.

Fungicides, if applied early enough, can prevent foliage diseases.  Spraying will prevent only new infections; it will not cure leaves already infected.  If an infection is developing on particularly valuable trees, or if there is good reason to believe an infection is imminent, the trees can be sprayed with fungicides.  Trees that perennially break and then two or three times during the growing season at 12 to 14 day intervals.  Check with your county Cooperative Extension agent or district forester for specific fungicide recommendations.  Specific recommendations are available in the Landscape Insect and Mite Pest Management Guide.

Courtesy of the Colorado State University Cooperative Extension (10/88)

    by D.P. Miller and W.R. Jacobi