Several different species of the fungus in the genus Cytospora are prevalent in Western Colorado. Cytospora chrysosperma is a common pathogen of willow, populus species and other deciduous trees and shrubs while Leucostoma persoonii is common on stone fruit trees and shrubs.

The disease caused by Leucostoma persoonii is referred to as Cytospora canker, Valsa canker and perennial canker. Orchardists in Western Colorado often refer to this disease as gummosis "(1). This species of fungus is referred to in the scientific literature as Leucostoma persoonii Hohn., Cytospora leucostoma Sacc., Leucocytospora leucostoma (Pers.) Hohn., and Valsa leucostoma (Pers.:Fr.) Fr. (2).

Leucostoma persoonii Hohn. is the name used and accepted by most pathologists and taxonomists as this is the name given to the sexual (perfect) stage of this fungus. The asexual (imperfect) stage is called Leucocytospora leucostoma (pers.) Hohn. and by its synonym Cytospora leucostoma Sacc. Valsa leucostoma was used in the past as a synonym of Leucostoma persoonii but is no longer considered correct. The confusion stems as far back as 1917 when von Hohnel split the genus Valsa into the two genera Valsa and Leucostoma. The anamorph (imperfect) stage of this fungus was given the genus name Cytospora and Leucostoma.

Cytospora cincta is another species that invades stone fruits. This organism is not found in Western Colorado but is prevalent in other parts of the United States.

Peach Cytospora Canker

Peach Cytospora canker is a serious pathogen of all stone fruit (peach, apricot, plum, nectarine, almond and cherry). Trees as well as shrub-forms are susceptible to this pathogen. Local orchardists refer to this problem as `gummosis' "(1) due to the characteristic gum that oozes from infected peach trees.

The ooze resulting from Cytospora canker in the above photograph is of two types. The dark amber-colored ooze is typical of an infection caused by this fungus when it invades through a wound. This ooze eventually hardens into a varnish-like material.

The lighter colored ooze is most likely caused by an infection that entered the tree along a small wood-boring beetle. Note the stringy ooze half way down the right side of the photograph. Other lighter colored stringy ooze appears elsewhere on tree.

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.

Cytospora is highly infectious and damaging, particularly when trees are under stress. Stress can be caused by too heavy a crop, poor nutrition, drought, winter injury, wet feet or improper watering, spray/insect injury and insufficient renewal pruning. Canker development can take place throughout the year. However, the greatest canker growth occurs in the spring as temperatures warm up just before tree activity resumes.

Symptoms of Peach Cytospora Canker

The fungus enters through wounds caused by winter injury, pruning cuts, insect punctures or mechanical injuries. The infected bark tissue exudes gum. The tissue dries out and breaks away as it is killed.

The fungus over winters in live bark at the edge of a canker. Cankered areas may or may not be surrounded by a roll of callus. Cankers enlarge yearly until the limb or trunk is completely girdled. Gum production is almost always associated with cankers.

The fungus becomes active in the spring as the tree becomes active. Tree growth may callus off the infection (particularly in some less susceptible varieties such as sour cherries). However, as tree growth slows, the fungus invasion may move beyond the callus barrier.

This disease can be confused with damage caused by the Peach Tree Borer (aka Peach Crown Borer). As this insect feeds in the base of the tree a gum is produced. Damage caused by this insect, however, usually does not extend above a foot from the ground. The ooze resulting from the feeding activity of this insect is not the clear amber ooze produced by Cytospora; it contains frass from the feeding activity of the insect.

Small pimple-sized fruiting bodies are produced in the injured tissue under the bark and these produce spores which exude in droplets or in tendrils. Studies show spores are carried primarily by splashing rain, pruning tools, or insects. Moisture and temperatures above 45^o F are required for spore germination and canker development.

The planting of young peach trees (or other Prunus species) near Cytospora-infected trees should be avoided. Other suggestions to reduce spread and help avoid infection include:

• Promote early autumn hardiness of the trees by reducing water in the fall and avoiding the application of fertilizer after mid-summer.
• Prune in late winter/early spring. Avoid pruning in early winter. Prune to encourage the development of the lowest branch on the south west side of the tree to help prevent sunscald.
• Use tree wrap or a reflective paint to help prevent sunscald for the first few years after planting the tree.
  • Tree wrap should be applied at the beginning of November and removed at the beginning of April.
  • Homeowners concerned with the aesthetics of their trees should opt for the annual tree wrapping ceremony as paint will permanently discolor the bark. To prepare a reflective paint, use interior white latex paint diluted 1 part paint to 2 parts water. Apply this to the trunk and lower branches paying particular attention to the south and west exposures.
• Make proper pruning cuts to avoid large wounds. Round the edges of wounds at the top and bottom to enhance and speed wound closure.
• Avoid tree injury.

Cankers are diseased areas within the tree bark. These infections can rapidly spread into the wood, branches and trunk, killing the entire tree. Cankers should be removed to save important limbs and the trunk or main stem of the plant. Removal of infected areas is also necessary to prevent the diseased organism from spreading to nearby healthy trees.

Cankers caused by Cytospora are best removed during the dormant season: December - February. Never prune or make cuts on trees in wet weather; wait until bark is dry. The following steps should be useful in treating Cytospora:

• Small cankers on small branches are best removed by removing the branch 6 inches below any visible discolored inner bark or wood. When removing infected twigs, prune back to the nearest side branch unless that branch is infected in which case the branch will also need to be pruned. In some cases an entire branch may need to be removed back to the main trunk.
• Large cankers (where infection is localized and gumming is limited to a single area) on valuable limbs or trunk (if less than 50% of circumference is diseased) remove the diseased bark (all reddish-brown inner bark and cambium) with a sharp knife down to the wood. Cut 1 inch beyond the actual canker. Tapering the wound (e.g. V-shape) is not recommended. Disinfect the excised area by swabbing with rubbing alcohol. Paint with orange (amber) shellac to provide a temporary dressing. After the first has dried a second coat of shellac is advisable. Amber shellac is available at most lumber yards and hardware stores.
• Disinfect cutting knife, shears and saw with rubbing alcohol, 10% sodium hypochlorite (bleach) or other disinfectant between each canker.

Canker tissue should be removed from the orchard and burned or buried.

(1) Gummosis is typically used to refer to gum produced by a tree or shrubs due to environmental conditions, not due to a pathogen such as Leucostoma persoonii.
(2) Abbreviations following the scientific name refer to the individuals who first identified, named and/or classified the organism.

References Used:

Farr, D.F., G.F. Bills, G.P. Chamuris, & A.Y. Rossman. 1989. Fungi on plants and plant products in the United States. APS Press.
Larsen, Gaus, Zimmerman, Rogoyski (ed.). 1997. Colorado Tree Fruits: Pest and Crop Management Guide. Colorado State University Extension bulletin XCM-41.
Ogawa, Zehr, Bird, Ritchie, Uriu, Uyemoto (ed.). 1995. Compendium of Stone Fruit Diseases. APS Press.