Colorado Master Gardener: Fading aspen trees |

Colorado Master Gardener: Fading aspen trees

Vicky Barney
For Steamboat Pilot & Today
Marssonina leaf spot is a fungal disease that occurs in aspen and cottonwood trees.
Vicky Barney

When the weather finally warmed up this spring, my aspen trees leafed out seemingly overnight.  Their new green color quickly matured into a darker green and after a brief snowstorm or two, summer arrived.  But now in the height of summer, their rich color is slowly fading away.  Are my trees dying? 

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Colorado State University Master Gardeners are available to answer questions from 10 a.m. to 1 p.m. Thursdays through the gardening season at the CSU Extension Office, 136 Sixth St. in Steamboat Springs. Contact 970-870-5241 or with questions or to schedule a site visit.

Quaking aspen (Populus tremuloides) are native to our area, grow quickly up to 50 feet tall, and live for about 20 years.  They grow best in sunshine, prefer our high elevation, and are cold hardy.  The bark has medicinal value and is easily damaged by animals and human activity.  The trees do not fare well in drought conditions, nor do they like too much moisture as it may lead to leaf disease.  

After looking closely at some leaves, I discovered my aspens are infected with Marssonina leaf spot.  The Routt County Extension office has been flooded with calls about this problem, prompting Todd Hagenbuch, Routt County extension director, to make a radio presentation on this topic.  His talk is summarized below.

  • Marssonina leaf spot is a fungal disease that occurs in aspen and cottonwood trees.
  • The spots are dark brown flecks, often with yellow halos.  In wet weather, the spots may fuse to form large dead patches on leaves.
  • The fungus winters on fallen leaves that were infected the previous year.  Warm, wet weather allows spores to form and travel by the wind to emerging leaves.  
  • Early infections are typically not serious. 
  • Spores from early infection may cause heavy secondary infections if the weather remains favorable, causing premature leaf loss on infected trees and impacting tree health.
  • Fall colors are muted if many trees are infected.
  • The disease rarely kills the tree.
  • Raking up infected leaves will reduce the number of spores that cause infection the following spring but works only if you have a solitary stand and are not near other infected trees.
  • Keeping leaves dry (water the garden early and carefully in the morning) minimizes spread.
  • Fungicides are available to treat tree buds in the spring but are rarely recommended.

Fortunately, my leaf spot infection does not appear serious.  With careful watering, a bit of raking and some help from nature, my aspens will likely make a full recovery and once again green up beautifully next spring. Read more on the “Aspen and Poplar Leaf Spot” factsheet at

Vicky Barney gardens for wildlife and is a member of the Master Gardener Class of 2011. 

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