Gardening: Weak or damaged trees susceptible to Cytospora fungas

Cytospora canker is caused by a number of different Cytospora fungal species. Some are host specific; others can attack numerous hosts. Susceptible trees and shrubs include: aspen, cottonwood, poplar, apple, cherry, peach, plum, birch, willow, honeylocust, mountain ash, silver maple, spruce and Siberian elm.

Cytospora is a weak pathogen that attacks stressed or wounded trees. Trees with root damage caused by construction or transplanting are highly susceptible. Other factors that can invite an infection by the fungus include mechanical injury from mowers and trimmers, drought, herbicide damage, sunscald, insect damage to the bark and improper pruning cuts. Healthy trees usually will not be affected by this fungus.

The Cytospora spores enter wounds in the bark or the roots and moves through the tree into the xylem (which moves water and minerals up in the tree) and phloem (which moves food down in the tree). The fungus reduces the flow of water and food between the roots and the upper portion of the tree, weakening the shoots and branches and causing sunken patches called cankers. Eventually, the tree will die due to girdling of the bark.

Symptoms
Cytospora chrysosperma infects aspen, cottonwood, poplar, birch, willow, honeylocust, mountain ash, silver maple, and Siberian elm. The bark on infected branches and trunks becomes yellow, orange, or black. As the fungus matures, black pustules form, which contain spores. The pustules rupture in damp weather, releasing orange, thread-like coils. A clear ooze is often present at the site of the canker.

Leucostoma persoonii attacks stone fruits such as cherry, peach, apricot, nectarine, almond and plum. This is often called “gummosis” due to the dark, amber colored ooze present in affected areas. The bark eventually dries out and will peel away.

Gummosis is a serious threat to peach growers in Colorado. Curtis Swift, former CSU Horticulture Extension agent states, “In Colorado at least one-third of the producing peach trees have Cytospora cankers on trunks, scaffold limbs, or in the fruiting wood. Within a couple of seasons these infections can cause severe dieback; eventual death of the tree is likely. In highly susceptible peach varieties, the canker can weaken an entire orchard in three or four years.” (Visit CSU Extension Tri-River area website at http://www.coopext.colostate.edu/TRA/PLANTS/gummosis.shtml for more information.)

Cytospora kunzei is the pathogen that affects spruce. Symptoms include depressed, gummy areas surrounded by callus, sometimes resembling a gall. Branches at the base of the tree usually die first. Cankers can be hard to find due to the large amount of sap leaking from the wounds. The pitch is clear at first, later turning white and hardening. The needles turn grayish green to brown and can persist or fall off.

Avoidance
Cytospora canker is infectious and has very few effective treatments. Maintaining plant vigor is the best defense. Reduce stress to the tree by watering properly. Under or over-watering will lower the trees’ resistance to the fungus. Watering in the winter in dry years will increase plant health.
Avoid wounding the tree with mowers, trimmers, excavation and improper pruning cuts. Avoid pruning in wet weather; the moisture encourages the dispersal of the spores. Apply appropriate fungicides to pruning wounds on peach and cherry. Plant resistant varieties when available and carefully inspect nursery stock before purchasing new trees. Follow the guidelines for properly transplanting trees (visit the Colorado Master Gardener website at www.cmg.colostate.edu and read GardenNotes #635,”Care of Recently Planted Trees” for more details).

Control After Infection
Remove all infected limbs with smooth cuts; avoid jagged edges or tearing of the bark. Cut back into unaffected tissue. Clean pruning tools with ethyl alcohol or a bleach mixture (1 part bleach, 9 parts water) after every cut. If the infection is on the trunk, remove dead bark at least 1” into unaffected tissue with a sharp knife and treat with disinfectant (as per Curtis Swift, CSU Extension). Do not compost infected branches. The spores will remain viable and spread the contamination.

For further information, please visit www.ext.colostate.edu and read Fact Sheet #2.937 on “Cytospora Canker”.