Pests AND environmental PROBLEMS of High Altitude Landscapes

Jessica Hendryx B.S. in Horticulture Science with a Landscape Design Option ~ Montana State University, Bozeman

Aspen Canker   (Cytospora)

Susceptible Plants to Cytospora Canker: Quaking Aspen  (Populus tremuloides); Robusta Poplar  (Populus x robusta)

Less Susceptible Plants: Lance-Leaf Cottonwood  (Populus x acuminata) (native to Montana); Narrow-Leaf Cottonwood  (Populus angustifolia)

Alternate Plant Selection: Tatarian Maple  (Acer tataricum)

The Quaking Aspen (Populus tremuloides) is a very common tree among the high altitude landscapes. Even if it is not planted within the landscape it can be found surrounding residences located in the mountains. When designing a landscape near or in the forest  be aware of the surroundings before planting. These forest trees can host many diseases that a newly planted tree is likely to inherit. Take note if there are stands of Aspen that appear to be unhealthy and try to avoid plants that will later host that same disease. There are several types of cankers affecting Quaking Aspen. Identification of each canker is fairly evident from the symptoms, however check with a local county agent before jumping to conclusions. Table 1 briefly describes the diseases commonly occurring on Aspens in the west. Cytospora, the most common canker in this area, is described in more detail to follow.

Table 1:

 A detailed description of each disease can be located at: http://www.na.fs.fed.us/spfo/pubs/fidls/q_aspen.htm

What is Cytospora Canker?

Cytospora Canker is the most common canker affecting aspens in the western landscape. Cytospora Canker, also known as Leucostoma Canker, is a fungus and occurs on more than 70 species of trees and shrubs combined. There are varying species within the pathogen that infect these different plants and they appear to be host specific. For instance, aspens and cottonwoods are both attacked by Cytospora or Chrysosperma.

How is it spread?

During wet weather the spores from the fungus ooze out of the trunk or branches and are transported by rain, insects, and humans. The spores then land on open wounds on larger branches or on the leaf  scars on younger twigs. The fungus becomes established in the dead bark and then grows into living tissue. It spreads through the bark cells and then  into the outer rings of the wood. This causes a girdling affect, which restricts water and nutrients needed by the plant. During the winter the fungus overwinters in the infected tissue of the plants.

Aspen trees are known for their soft wood which allows them to be easily wounded. Markings left by humans and animals can be found all over Aspen stands. Humans will often carve into an aspen tree and over time a dark callus will form causing the wound to stand out against the light colored bark. Mechanical injury can also be caused unintentionally. Weed trimmers and regular maintenance often leave wounds. These, among other methods, are primary areas for the spores to infect the Quaking Aspen. Overall, the trees most likely to be infected are those that are under stress. Cytospora rarely infects healthy trees, mainly plants stressed by drought, insects, or other factors.

Symptoms:

The Cytospora Canker has a unique characteristic that sets it apart from the rest. However, often, entire branches may die before any visual evidence of the canker becomes present. Eventually the canker appears as a gradual circular killing of the bark. The infected area becomes brownish and sunken and is surrounded by raised callus tissue.  In the spring  an orange colored ooze, the characteristic Cytospora Canker symptom, can be visually seen penetrating out of the wounds (See Figure A).  Shriveled bark may fall off exposing the dead inner wood beneath. Each year the cankers become progressively worse. Branches die because of the girdling caused by the fungus and eventually the tree begins to weaken. It is then susceptible to a variety of other problems. Large branches may break during storms causing a hazard for humans.

What can you do about it?

The main reason this disease occurs is because the plant is already weakened in some way. The stress can include trees that are in drought, flooded, and even trees that are damaged by insects. Aspens that have root damage are also vulnerable. This includes trees that are in a construction area as well as newly transplanted trees. Cytospora is often found located on a host that has already been infected by a fungi or some other disease.

Keeping the trees healthy is one of the key methods of prevention and control of this disease. Good cultural practices such as watering and fertilization help keep the trees in good vigor. However, do not over water and fertilize because that may cause even greater damage. Over watering and fertilization in the early fall may stimulate succulent growth, which does not allow the tree to harden off efficiently for the winter. If a plant is not hardened off, winter injury may occur. The excessive water and fertilization also inhibits abundant root growth, combined with rapid shoot growth and delayed hardening for winter, which then makes the trees more susceptible to any stress. Avoid wounding and severely pruning the trees.  Remove the young cankers from the trunk and branches only during dry weather. Cut carefully away at the young canker until the diseased bark is removed and only live tissue remains, keeping in mind to make all cuts cleanly, leaving no jagged edges. If the canker is large do not attempt to cut away at the trunk or remove callus tissue. Simply remove infected branches and loose bark pieces. Remember to disinfect the tools used after each cut with a liquid solution of 1 part bleach to 9 parts water or rubbing alcohol. Do not apply wound dressings, such as tar, merely let the wound dry out after pruning. Dispose of the pruning material properly; either by burning the infected material or bagging the material and disposing. Make sure that the the bags are tightly secured to prevent further spread of the disease.

In nature trees will die and the same goes for landscape plants. Try to use less susceptible varieties when designing in a natural environment and expect to loose some of the trees transplanted. Some slightly less susceptible varieties include: Lance-Leaf Cottonwood - Populus x acuminata (native to Montana) and Narrow-Leaf Cottonwood - Populus angustifolia. These do not stop the disease from occurring, however, they help to prevent the fungus from spreading into the landscape. If cytospora is a local problem avoid using these plants: Quaking Aspen - Populus tremuloides and Robusta Poplar - Populus x robusta. Although Quaking Aspens are a highly desired landscape plant try using an alternative plant such as Tatarian Maple - Acer tataricum. The maple is similar to the aspen in size, form, and adaptable conditions. 

Figure A - Cytospora ooze occurring in the spring on Populus tremuloides.

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Reference:

Agrios, G.N. Plant Pathology. Harcourt Academic Press: 1997 Pgs 321-324.

Montana State University Extension Service 

Designed and researched by Jessica Hendryx - B.S. in Horticulture with a Landscape Design Option
For problems or questions regarding this web contact [martha@montana.edu].
Last updated: May 08, 2002.