Marssonina leaf spot is a fungal disease of aspen, poplar and cottonwood. Infection begins in the spring at bud break when spores are blown or splashed to the tender, newly-developing leaves. Brown spots with yellow halos begin to appear on leaves in mid to late July. Many of these spots eventually merge together, creating large brown blotches. Leaves may drop prematurely. To manage the disease, rake up fallen leaves in the autumn and dispose or compost. If there is a history of severe infestations, the fungicides Daconil may be used in the spring beginning at budbreak. Fungicides are of no value by the time symptoms appear. The disease does not kill trees, but stresses them and may weaken them over time.
For more information, see the following Colorado State University Extension fact sheet(s).
For more information, see the following Planttalk Colorado™ script(s).