Common Diseases for Aspen Trees in Colorado

Marssonina Leaf Spot

• Marssonina leaf spot is cited by the University of Colorado extension as the most common leaf disease on aspen trees both in the forests and urban regions of Colorado. Marssonina fungus causes the disease which is characterized by the appearance of yellow bordered, brown spots on tree foliage.

  In the early stages, the spots are white centered. In severe infection, the spots join together to create large dead areas that also spread to leaf stems and shoots. The fungus spores overwinter on fallen leaves and start to spread in the warm spring weather. Management includes keeping trees clear of fallen foliage and debris. Keep foliage dry and prune affected tree areas. Use recommended fungicides to keep the disease from spreading.

Leaf and Shoot Blight

• Leaf and shoot blight is a common disease of aspen trees growing in the Colorado mountains. The fungal disease is most prevalent during spring and starts with the appearance of brown to black, irregular areas on the foliage growing near stems. The disease causes the leaves to distort and dry.

  As the fungus spreads it creates cankers on stems that result in curled stem tips. The new shoots on diseased trees die and grow in a distorted form. The spores of the fungus overwinter in previously infected shoots and infect new growth in spring. Management includes the use of recommended fungicides. Prune previous year diseased tree areas to keep fungus from spreading to new growth.

Leaf Rust

• Leaf rust is a common disease seen on the aspen and cottonwood trees in Colorado. Melampsora fungus causes the fungal disease which appears as yellowish-orange, spore filled pustules on the undersides of foliage. The disease is most apparent between late summer and early fall.

  Leaf rusts usually spreads to aspen trees from evergreen trees like spruce, pine or fir. The growth of the fungus is very rapid in wet conditions. The fungal spores overwinter in the fallen foliage under the trees. Management includes the use of recommended fungicides such as mancozeb chlorothalonil.