Community Ag Alliance: Perplexing plant problems |

Community Ag Alliance: Perplexing plant problems

Kristin Mortenson For Steamboat Today

What's killing my trees?

This is a common question we get asked at the Colorado State Forest Service office, but it hasn't been easy to answer with the unique issues plaguing our local trees this past season.  Our tree problems started with a late spring frost and have continued into this wet fall, and is a detective story worthy of Sherlock Holmes.

The red-hue cloaking our hillsides is the fall foliage of Gambel oak, commonly called scrub oak or oak brush.  A deciduous tree with oblong, lobed green leaves, it produces small acorns loved by wildlife and is found in open, dry areas. 

This past spring, many oak hillsides were dressed in brown leaves instead of green.  The culprit: a late spring frost that nipped young buds and killed the new leaves.  Most oak survived the frost, despite the unsettling early die back.

After the late frost came moisture, and lots of it.  Our wet spring created a different problem for other deciduous trees, especially our quaking aspen.  Aspen is a gray-barked tree known for its broad, green leaves that often quake in the breeze and turn brilliant shades of sunset colors in the fall. 

This summer found many aspen leaves discolored, with blotches of brown.  The culprit: fungi. Marssonina blight and ink spot are common aspen diseases caused by fungi that were abnormally widespread this year due to the wet spring.  The fungi can cause early leaf drop, but are rarely fatal. Raking and disposing of those infested leaves in one way to control its survival and spread.

Recommended Stories For You

Following the wet spring came a dry spell and a perplexing problem affecting subalpine firs.  A conifer with smooth, gray bark and flat, blue-green needles with silvery lines, they are often found at high elevations with spruce trees.  This summer, many subalpine firs saw their needles turning red at the middle of the tree, the red moving up the tree with defoliation following. Western balsam bark beetle, the usual suspect, causes the whole tree to turn red and die quickly. 

That is not what was happening and there was no sign of a beetle. A visit to the area by Colorado State Forest Service insect detective, state entomologist Dan West, shed light on the mystery. 

The likely culprit: spider mites. A fan of dry conditions, these tiny pests seek out the damp center of the tree, puncturing plant cells to feed.  A clue to their presence is the protective webbing they spin, though it is often confused with spider webs.  Biological and chemical controls can inhibit spider mites, but many insecticides often favor spider mites by killing their natural predators, so choose a miticide designed to kill only the spider mites, such as Talstar.

After this problematic season for our plants — continuing with the recent wet and heavy snowfall damaging so many trees — should you find yourself needing to replenish or add to your plant stock, the CSFS nursery has just released its 2017-18 seedling tree order form.  The nursery produces low-cost, Colorado-grown seedling trees and shrubs for conservation purposes, and landowners can start ordering plants that will be delivered early May. Contact the CSFS Steamboat District office at 970-879-0475 for more information and visit for the latest order form.

Kristin Mortenson is administrative assistant for the Colorado State Forest Service Steamboat Springs District office.