The Leaves They Are A’Fallin’

Posted by: Mary Small, Colorado State Master Gardener Coordinator and Jefferson County Extension Office

Time to get out the rake....

Okay, can I just say I am sick of aspen and cottonwood dropping their leaves already?!

We’ve received numerous leaf samples from these trees and nearly all of them have been infected with either Marssonina or Septoria leaf spot diseases. It’s so severe in some places that some trees are already defoliated or darn near. It’s causing a lot of concern and unfortunately we can’t do much for the trees or their owners!

Caused by fungi, these diseases develop readily in cool, wet weather (like we had this spring).  Fungal spores are released from last year’s infected leaves.  Spores (the “seeds” of the fungus) are then blown or carried by splashing water to susceptible new tissue, causing infections. 

Septoria leaf spot on cottonwood

Although leaves are infected in the spring, symptoms usually don’t appear until mid to late summer. Leaves will drop prematurely.  Trees can be stressed during severe outbreaks due to reduction of photosynthesis and food production.

Symptoms vary between these two diseases. Marssonina leaf spots are dark brown dots with yellow halos.  In prolonged wet weather (which we’ve also had), smaller spots may fuse to form larger black patches. Septoria spots form under similar conditions and often occur alongside Marssonina. Septoria start out as dark sunken flecks that may enlarge and form larger dead areas or spots are tan and circular with dark margins. 

Marssonina leaf spot on aspen

One of the best times to manage the diseases is now, from late summer into fall. Where it is practical (such as in a landscape, rather than a forested setting), rake up fallen leaves and dispose of them in the trash. The diseases over-winter in the dropped leaves and raking and disposing removes that inoculum from the site for next year. (Unless of course, infected leaves/spores blow in from somewhere else!)

In the landscape, space plants according to recommendations for the species.  Trees planted too closely increase the humidity within the canopy, creating conditions ideal for infection. Keep sprinkler water out of leaves, so they stay dry.  Splashing water from sprinklers can also spread spores between leaves, causing secondary infections.  Prune to thin trees. This increases air circulation, keeping leaf surfaces dry. 

I could say we might hope for a drier year in 2016, but I won’t. Sorry, aspen and poplar.