Cankers: what the standards and agencies describe

This page describes what published sources say about one condition in trees. It gives no verdict on any tree and is not an assessment. The sources come from the United States. The parent page, structural defects in trees, compares the conditions that have pages here.

What the sources call a canker

The US Forest Service brochure defines a canker as a localized area on the stem or a branch where the bark is sunken or missing, caused by wounding or disease. Illinois Extension describes the same condition as an area where the bark is sunken, swollen, flattened, cracked, dead or absent, developing from wounds or disease. A University of Kentucky plant pathology fact sheet adds that cankers appear as dead or dying regions on bark, twigs, branches or trunks, and that some are inconspicuous and show only as a flattened area.

What happens inside

The Kentucky fact sheet says the cambium is damaged or killed where a canker forms, which obstructs the uptake of nutrients and water, and that a canker small in the first year of infection can expand. A University of Illinois plant clinic note, written about Russian olive, describes a canker as a dead area that can enlarge and girdle a cane, twig, limb, trunk or root. It says the sapwood under the canker is generally brown or black, and that on tightly barked trees a colour difference in the bark may be all that shows the edge.

Where cankers start

The Kentucky fact sheet lists plants stressed by poor planting practices, improper maintenance, extreme weather, insect damage, mechanical damage or other wounds as being at increased risk of infection by canker-causing pathogens. The Illinois plant clinic note says cankers are generally caused by secondary or stress pathogens and are "merely a clue that something more is wrong". It names one exception for Russian olive in Illinois: a Phomopsis canker fungus that the note calls an aggressive pathogen able to attack vigorous trees. The Pacific Northwest plant disease handbook, in its entry for black canker of aspen, says beetles and flies attracted to fresh wounds carry the fungus, and lists branch stubs, insect attack, fire scars and any other physical injury as the wounds.

How cankers appear on defect lists

Source	What it says about cankers
US Forest Service brochure	A canker raises the chance of the stem breaking near it. A canker that takes in more than half the circumference is listed as a reason to act, as is a canker physically connected to a crack, a weak branch union, a cavity or another defect.
Illinois Extension	If a canker takes in more than half the circumference of the stem, the article recommends immediate action.
ISA Trees Are Good leaflet	Lists cankers and wounds as externally visible defects, item D of A to H. Each could be minor or the start of a significant problem, and the leaflet says further investigation may be warranted.
Pacific Northwest handbook	For black canker of aspen, says infected trees are susceptible to wind breakage and that the disease is slow growing.

The Forest Service brochure also names aspen as a species prone to breakage at a young age because of several factors including decay and cankers. It says a defect matters where a target is within striking distance, and that defects touching one another are the ones to examine closely.

What the sources say about who looks

The ISA leaflet says evaluating the seriousness of defects is best done by a professional arborist. The Kentucky fact sheet is about sampling for diagnosis and says samples go to a county Extension office. Who may apply any pesticide is covered on pesticide licences. For the methods and the credentials, see tree risk assessment methods and consulting arborist credentials. A related condition is on cracks.