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. 

The Mid Summer Leaf Drop Blues

Posted by Mary Small and Curtis Utley, Jefferson County Plant Diagnostic Clinic

Ah, sweet mid-summer! Long, warm evenings spent on the veranda listening to the sounds of leaves dropping. Huh?!  leaf drop in late July/early August?

Yep, you read that right. We’ve been getting inquiries and emailed photos about, and observing this very phenomenon.  Well, “phenomenon” probably isn’t quite the right term when you consider we see it every year, usually beginning mid-July.  Several situations cause this by themselves or combine with others into a “perfect storm”.  Tree leaf drop is probably the number one problem we encounter in our clinic every summer. So why do trees drop leaves in the summer?

Trees often form more leaves in the spring than their systems can support later in the season.  The hotter, dryer weather of summer signals plants to drop leaves as a defense mechanism against water loss.  There may not be enough available or transpiration rates are so high that trees just can’t keep up. So they drop leaves to cut water losses and keep the rest of the system going.

Drought stress causes leaf drop.  Some of the tree canopy is shed to balance water intake and “outgo”, similar to the above situation.  And, even though it looks like a lot, as much as 10% of leaves can drop without causing serious injury.

Newly transplanted trees (including those planted last year) frequently drop leaves to compensate for root loss and reduced water absorption.

Overwatering, poor soil drainage and too-deep planting results in root oxygen deficiency. Poorly functioning or dying roots can’t absorb needed water, so again the tree drops leaves to reduce transpirational losses.

Insect pressure causes leaf drop. It’s not unusual for so-called “healthy-looking” trees to drop leaves all of a sudden.  Lilac-ash borer and the banded ash clearwing are common culprits.  These insects spend a good chunk of their lives tunneling in and through wood underneath the bark a couple of inches deep.  In addition to physically weakening the tree, the activity can interfere with the flow of water, stressing the tree and inviting attack. Leaves are shed as (you guessed it) trees try to cut their transpirational losses.

Then there’s hail and wind that physically knock leaves (and branches) out of trees. Sometimes the full extent of damage doesn’t show up until a bit after the storm, when damaged petioles and leaf blades succumb to dehydration following tears or dings. 

Aspen trees begin their Marssonina-infected leaf drop in mid to late July. Infection occurred in spring, so there’s nothing to do until leaf raking season.

Trees lose leaves for many reasons in late summer but the question behind the question for most folks is, “is the tree okay”?  Check the quality and quantity of new growth by looking at the annual growth increments, the general size of the leaves and remaining canopy density. If all of these things check out, then the tree is probably going to survive to see another spring…and summer.