marsonnina leaf spot on aspen (6229 bytes)

Aspen Leaf Spots

By Laura Pottorff, Colorado State University Cooperative Extension agent, horticulture and plant pathology

As aspen trees begin to leaf out, few of us think about disease problems. But, if your tree has had Marsonnina leaf spot or rust disease in the past,  bud break  may be the time to use preventative sprays. Here, however, are some considerations before reaching for the fungicide bottle:

  • Be sure last year's black or orange spots actually were caused by disease. Aspen leaves turn black or yellow or drop off for many reasons -- most of them from poor growing conditions. If Marssonina or rust is present, symptoms will appear in early August to late September. Rust will cause small yellow or orange spots, while Marsonnina will cause round black spots. If these symptoms occur year after year, fungicide application might be warranted. If symptoms don't occur every year, fungicides usually are not necessary.

  • Think about what other types of control strategies you could use in place of or in conjunction with fungicides. Fungicides are widely overused in urban areas and even when their use is justified, they may need a little help. You can minimize Marssonina and rust diseases by cleaning up infected leaves that drop to the ground in the autumn. The fungus overwinters on this leaf debris and if we clean up as much debris as possible, less "inoculum" will be present to begin the disease cycle over again in the spring.

Photo: Judy Sedbrook

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Date last revised: 01/05/2010