Both fungi survive winter on fallen infected leaves. In the spring, the fungus produces spores that are wind blown or rain splashed from infected leaves to newly emerging leaves. Infection begins in the lower branches and progresses upward. Initially, symptoms appear as small, dark colored flecks on the leaves. Later, these flecks may expand into spots of various sizes and shapes. Some spots may eventually develop white to tan centers with dark margins. Infected leaves often turn yellow and drop from the tree.

Fungal reproductive structures in the center of the leaf spot produce spores, which are rain splashed to nearby leaves throughout the summer. These spores cause new infections and spread the disease in an upward direction. If infection is severe, spots may coalesce causing entire leaves to brown and drop prematurely. Occasionally, severe infections by Septoria musiva may produce cankers (discolored and sunken areas) on the branches or trunk. These cankers mainly affect poplar hybrids.

There are no chemical controls available for Marssonina or Septoria leaf spots. However, these diseases generally do not cause serious harm to healthy trees. Trees that are severely defoliated for several years in a row are more susceptible to dieback and winter injury. Both diseases can be managed with sanitation and proper tree care. Rake leaves in the fall and prune dead or dying branches. Properly dispose of all infected material. Water trees during dry periods, mulch, and fertilize as needed to maintain tree vigor.

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