Community Agriculture Alliance: Unhappy aspen |

Community Agriculture Alliance: Unhappy aspen

Kristin Mortenson
For Steamboat Pilot & Today

STEAMBOAT SPRINGS — What are those spots on my aspen leaves?  Why are the leaves suddenly dropping before they have turned their autumn gold?  Why are my aspen unhappy? 

The snowy winter and wet spring that we experienced earlier this year provided a much-needed boost for the flora of our area struggling to deal with years of drought.  However, the abundant moisture not only supported our trees and plants, it also provided the perfect breeding ground for fungus — especially Marssonina, the fungus that causes the most common foliage disease affecting aspen in our area.

Marssonina causes leaf spots that are dark brown flecks, often with yellow halos.  The spots may fuse to form large, black dead patches.  Since aspen clusters are often clones, composed of genetically identical trees that sprouted from the root system of one “mother tree,” what affects one tree affects them all.  

Ciborinia is a fungus that commonly affects mountain aspen, causing Ink Spot disease.  With Ink Spot, leaf discoloration occurs in late spring/early summer as brown spots, often with concentric ring patterns.  Infected leaves may turn totally brown by midsummer, then sport raised black masses of fungal material — the ink spots that give the disease its name. 

In late summer, these spots fall out, leaving the characteristic shot hole effect on leaves.  The hard fungal matter that falls from infected leaves are the overwintering stage of the fungus, which is stimulated into spore production by wet spring weather.  

How can I make my aspen happy again?  The best treatment for fungal infected aspens is to water your aspen regularly and remove infected leaves, twigs and branches.  Raking and destroying infected leaves will help reduce the spores available the next spring. 

Fungicides, if applied early enough, can prevent foliar diseases but will only prevent new infections, not cure leaves already infected. A year of heavy infestation will not kill your tree, just lower its productivity. Several years in a row of heavy fungus foliar infestations can kill a tree, so preventive measures, such as raking the leaves and watering the tree, are recommended, especially during the following growing season. 

What about the aspen gold?  While the above fungal diseases affect many aspen, there are happy, healthy “quakies” ready to bathe the hillsides in yellows, oranges and reds.  Higher elevation stands and those farther north will color first, their brightness determined by health and climate.  A wetter growing season followed by a dry, sunny autumn with cool but frost-free nights results in the brightest colors. 

These colors are produced when the chlorophyll in leaves decline as the tree readies itself for winter and nutrient flow to leaves decrease, with leaves sending sugars to the roots before dropping. The carotene left in the leaves causes them to appear yellow while anthocyanin causes the orange and red colors—the pH of the sap determining the shade of red. 

For more information on aspen, aspen diseases and fall foliage, visit or drop by the Colorado State Forest Service Steamboat Springs Field Office. Enjoy the autumn gold!

Kristin Mortenson is the administrative assistant at the Colorado State Forest Service Steamboat Springs District office.

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