After hearing some reports about aspen losing their leaves early from the Tahoe and seeing a lot of damaged stands on the Lassen, Modoc and Plumas , I wanted to share these publications on aspen leaf diseases.
Most aspen stands that I have visited appear to be suffering from Marssonina Leaf Spot (or Blight). This is the disease that is causing most of the trees to lose their leaves right now. I have also observed leaf and twig blight (Venturia sp.) and aspen/poplar rust.
As far as rust is concerned, most NE California areas also have willow species that are heavily infected with rust, causing yellow foliage and foliar dieback.
All of this is related to our wet spring and summer that has created ideal conditions for these foliar pathogens to thrive. The good news is that these types of diseases generally do not cause tree mortality and affected aspen (and willow) will likely survive and look normal next year (unless we experience the same conditions again). The potential bad news is that some of these stands are already in a poor condition, or are growing on drought impacted sites, and this is just one more stressor. That being said, please let me know if you see any additional health problems next year in some of your disease impacted stands.
Daniel R. Cluck
US Forest Service
Forest Health Protection
NE California Shared Service Area