Cytospora Canker - Field Guide to Insects and Diseases of AZ and NM Forests

Symptoms/Signs: Young cankers on smooth bark appear as brownish-yellow sunken areas. They are often irregular in outline and range from diffuse to slightly target-shaped. Later, the bark often splits at canker margins, the inner bark turns black, and wood beneath the canker is stained brown and water soaked. Black, pimple-like, asexual fruiting bodies (pycnidia) develop within a few weeks after death of the bark. They extrude spores in orange-red tendrils in wet weather. Later, black, pimple-like sexual fruiting bodies (perithecia) originate beneath the bark, which expel white masses of spores during wet weather that collect on the surface of the bark.

Biology: The fungus causing cytospora canker exists in two stages, a sexual stage known as Valsa sordida, and an imperfect, or asexual, stage called Cytospora chrysosperma. The latter is more commonly encountered. The fungus is considered a normal inhabitant of aspen bark microflora, which readily enters and parasitizes bark that has been injured or weakened by any cause. Trunk cankers are formed by a gradual killing of the bark in a more or less circular area over a period of several years. Annual canker growth can be seen by the slight annual callus formation around the perimeter of infection. The fungus fruits readily in the dead outer bark; even when typical canker symptoms fail to develop. The Cytospora stage forms small black fruiting bodies from which sticky spores ooze out in long, coiled, orange-to-dark-red masses called spore tendrils. The “Valsa” stage appears as flask-shaped perithecia formed beneath and in a circle around the old pycnidia. Some ascospores are forcibly discharged from the perithecia; others collect around the openings of the perithecia in sticky white masses on the dead bark. The diseased inner bark remains attached to the tree for 2 or 3 years before turning lighter brown in color and falling off in large pieces.

Effects: Cytospora chrysosperma is the most common fungus found on aspen throughout its range. It is weakly parasitic and normally attacks stressed trees. Cytospora infection is associated with frost cracks and sunscald, elk feeding wounds, tree vigor and damage by fire.

Similar Insects and Diseases: The orange tendrils or spore horns distinguish this fungus from all of the other aspen canker causing fungi.

References: 38, 44, 92