What are those spots on my aspen?

Posted by: Todd Hagenbuch, Routt County CSU Extension Agriculture Agent

This summer, our office has been flooded with calls about aspen that are looking poorly.  Most often, the first thing a homeowner notices about these trees is that the leaves are starting to get spots on them.  As the condition persists, the spots can cover the majority of the leaf, and leaves become shriveled.  Often times the leaves look dead, and then they fall off. 

As you can imagine, watching the leaves on your tree go through this transformation can be quite alarming.  Most people call the office saying, “My tree is dying!” When I or one of our Master Gardeners take a look at it, we see that the tree is being harmed by Marssonina leaf spot, an all-too-common fungal disease that occurs on aspen and cottonwood.

Marssonina leaf spots are dark brown flecks, often with golden yellow halos.  Immature spots may have a white center. On severely infected leaves several spots may grow together to form large black or brown  patches. 

Marssonina leaf spot on aspen leaves
Photo courtesy of Routt CMG Vicky Barney

Marssonina is a fungus that is always present in our mountain areas. The spores (or 'seeds') the fungus produces survive the winter on fallen leaves that were infected the previous year.  When spring arrives and the temperatures increase, the fungus produces new spores that are carried by the wind and land on new, budding leaves. Wet weather in the spring increases the number of spores present.

Early infections are typically not serious, but if the weather remains damp, spores from the original infection can spread and cause significant secondary infection. These secondary infections are noticed later in the growing season and create the large, dead patches on the leaves mentioned above. As the leaves are overcome by the fungus, they may fall off the tree. I assure people who have Marssonina in their tress that it rarely kills infected trees; its damage is mostly aesthetic.

As you can imagine, the wet weather we’ve had this summer has produced an ideal situation for Marssonina fungus to thrive, meaning we’ve seen more of it this year than usual. I also suspect we may not have a spectacular fall since many aspen leaves are compromised by the fungus.

Because the fungus overwinters on leaf debris on the ground, rake up your fallen aspen leaves every fall to reduce the incidence of the disease. However, this is only effective if you have a solitary stand of aspen in your yard. If your neighborhood has a lot of aspen or if you’re next to a natural stand, raking up the leaves will not significantly reduce the number of spores that could re-infect the tree next year. 

There are fungicides you can apply to the tree at bud break in the spring time that will control Marssonina. We rarely recommend this because the disease is mostly aesthetic and it doesn’t usually harm the tree. To help reduce the spread of the spores, try to keep leaves as dry as possible to reduce the incidence of leaf spots by measures you can control:  water your lawn and garden in early morning so leaves can dry out, and keep sprinkler patterns adjusted so leaves don’t stay wet.

So, what does next year look like? The reality is, if next spring is wet, then Marssonina be back. If we have a drier spring, then we should get through the season with little impact. Either way, the aspen will still turn gold each autumn and provide beautiful displays, even if the colors are muted this year.