What's bugging you? August 26, 2015

By Jim Leser


Cottonwood trees are ubiquitous to Colorado. For most folks it is the narrowleaf cottonwood that comes to mind when cottonwoods are mentioned. They line the many waterways that punctuate the landscape of much of the state. They often provide the bright yellow fall leaf color to add to our aspen's golden glow.

But there are many more representatives to the genus, Populus, other than this one. Out in the plains the more common Populus species is the plains cottonwood, a subspecies of the eastern cottonwood. These have larger leaves with a deltoid shape. They, too, tend to follow watercourses. Other members of this genus include lanceleaf cottonwood, Fremont cottonwood, swamp cottonwood and black cottonwood. If we were to add up all their subspecies and cultivars, we would need more than our fingers and toes.

And lest I forget to mention the most planted representative of this genus in Colorado, we also have the aspen. Only the Colorado blue spruce is planted more than aspens.

Male members of the genus Populus produce catkins in the spring to pollinate female trees. But it is the female cottonwood tree that produces the seed with associated "cotton" that fills the air as downy parachutes. Not everyone is a fan of this "cotton." Just make sure you buy a male tree if you want to avoid this seasonal "snowstorm."

Another aggravating trait of this genus is its propensity to produce root suckers. This is okay in the wild but in my lawn? I don't think so! You can cut these off as many times as you like, but they will keep popping up like a proverbial jack-in-the-box. And if you get too aggressive with weed killers, you could damage or lose the whole tree.

One thing that all members of this genus have in common is the incredible number of pests and diseases that find their way to attack our trees. Recently I observed a large webbed tent high up in one of my narrowleaf cottonwood trees. This, of course, was produced by the many caterpillars of the fall webworm, congregating within the protection of this web of silk. If I could reach it, I could prune it out -- but I can't. I could also spray it with a synthetic pyrethroid insecticide with the addition of a wetting agent to aid in web penetration -- but I won't. I'm just going to leave it alone. Not much damage will result.

But it is the disease complex that causes the most problems for cottonwoods. The leaf diseases include Marssonina blight, Septoria leaf spot, ink spot, and powdery mildew and iron chlorosis, which is really a nutrient deficiency rather than an actual disease. Both Septoria leaf spot and Marssonina blight are causing the premature yellowing and defoliation being reported in narrowleaf cottonwoods. Marssonina blight causes spotting and leaf premature fall in aspens without significant yellowing.

Both of these last diseases can be managed by cleaning up all the leaves in the fall. This is where the disease passes the winter. A fungicide containing daconil will provide infection protection in the spring if applied in a timely manner and enough times. I recommend starting when leaf buds begin opening and continuing two more times on a 10-day interval until three applications have been made. Make sure coverage is excellent. There is nothing that can be done now to prevent further leaf loss this year.

Other diseases include wetwood or slime flux and Cytospora canker. None of these diseases has a cure and can eventually kill the tree, but it could take a long time.

I personally get great pleasure from these cottonwood trees and every year look forward to their yellow fall color as it spills down our many waterways. I even have several native narrowleaf cottonwoods on my property, and yes, I have even planted more. But these are all males! Is this considered gender discrimination?

Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.