Cytospora Cankers

Many Cytospora spp. cause cankers on many hosts, including species in the genera Populus, Salix, Acer, Ulmus, Alnus, Fraxinus, Prunus, Sorbus, Cydonia, Amelanchier, and many others.  Collectively, Cytospora canker is the poster child for stress-related diseases.  Normally, in natural stands, their impact is minor. But in some circumstances these diseases can be substantial:

• Trees growing out of native range or off-site
• During and following drought
• Severely pruned or wounded trees
• Cuttings in storage or in propagation beds

Pathogens

Pathogens are in the genus Cytospora (Ascomycota, Pezizomycotina, Sordariomycetes, Diaporthales, Valsaceae).  The genera Valsa and Leucostoma have been used for the sexual forms.  It is not yet clear which genus will be used for the “one name“, but Cytospora has been proposed ​[4]​.

Pycnidia are very abundant and appear as pimples in the bark surface where the bark is thin. They have multiple chambers in a sort of pycnidial stroma, join up at the pore (like chestnut blight pathogen, to which it is closely related). When wet, they extrude spores, which often form a tendril, called a cirrhus. (May look like pus oozing out, completing the pimple analogy!) It looks like a curly thread or can become just a tiny, orange, formless mound.

Latent Infections

Although some Cytospora species that cause cankers may infect and initiate pathogenesis and kill tissues immediately, others may cause latent infections, establishing and growing to some extent in healthy, living tissues and causing disease following some stimulus ​[1]​.

These species colonize living tissues asymptomatically, perhaps later inducing symptoms when conditions become conducive.  Picea pungens inoculated with C. kunzei was colonized regardless of stress, but only stressed plants exhibited canker symptoms ​[5]​.  The fungus was also present 5-6 centimeters from the inoculum point after a month .  Although not common, Cytospora species have been isolated from bark and xylem of asymptomatic hosts in a number of species ​[1, 2, 8]​.

See alder heat canker for evidence of latent infections in a canker caused by Cytospora umbrina.

Cytospora Canker of Spruce

This disease, caused by Cytospora kunzei (also known as Leucostoma kunzei), is an interesting one.

Hosts include mostly various Picea spp.  The pathogen has also been found on Larix spp. and Pseudotsuga menziesii.  In the latter host it causes a disease known as pitch-girdle canker.

Symptoms vary greatly, depending on the host and environment.  In native Picea engelmannii (Engelmann spruce) in Colorado, it causes conspicuous trunk cankers ​[3]​.  These cankers are persistent, perennial, target cankers, which is unusual among the Cytospora cankers. The concentric callus ridges are usually quite narrrow.  Cankers are resinous, sunken, and roughly diamond-shaped with flaring margins.  Such cankers can also form on branches.  The disease is apparently absent in many stands but can be locally severe, causing “infection centers” ​[3]​.  Small, pole-sized trees can be killed. The disease is also somewhat unusual is that it does not appear related to host stress.

On spruces planted outside their native range, especially Picea pungens (blue spruce), the disease can be more widespread and symptoms can be quite different ​[6, 7]​.  Lower branches are attacked and may be girdled.  It may spread to other scattered branches and up the tree.  The cankers on branches are difficult to discern except for resin exudation.  Older resin is white or light blue, and might be mistaken for bird droppings.  Cankers are not sunken, and one can best delimit the canker margins by cutting away the outer bark.

Latent infections have been clearly demonstrated, and are triggered into pathogenesis by a stress threshold ​[5]​.

Several large Cytospora cankers on Picea engelmanii (Engelmann spruce).

Cytospora canker of Picea engelmanii (Engelmann spruce).

Cytospora canker of Picea engelmanii (Engelmann spruce). Note the vertical stem crack in the center of the canker.