Drought Adds to Plants’ Vulnerability
MANHATTAN, Kan. – Damaging and sometimes deadly cankers are beginning to show up in high-value trees and shrubs. The basic reason: drought’s stranglehold on central U.S. landscapes.
“Our trees and shrubs have been experiencing long-term stress. Last year was dry. Winter was dry. Spring was dry, too. Things are pretty crispy out there now, even in areas that got several decent rainfalls,” said Megan Kennelly, K-State Research and Extension plant pathologist.
Most plant diseases thrive in wet conditions, Kennelly added. For the pathogens that cause cankers, however, drought-weakened woody plants can be prime places to attack.
Cankers are localized dead areas in bark, she explained. They result after a canker pathogen gains access to inner bark through some kind of wound – much as an infection gets into a human scrape or cut. Once in, the pathogen colonizes. It can work on twigs, stems, branches and trunks.
“Many types of injuries and environmental stresses can foster colonization,” Kennelly said. “Drought-stressed ornamentals, for example, can have root dieback, as well as brittle branch cracks.”
On thin-barked plants, cankers often look like sharply defined and slightly sunken (depressed) areas that are off-color – a dark shade of red, brown or black.
“On thick or rough-barked trees, cankers can be harder to detect. To get a look at them, you may have to shave off suspicious-looking outer bark with a knife -- being careful not to injure the plant’s inner tissues,” she said.
While diseased bark is dying, it also can look water-soaked, be resinous or exude a foul-smelling sap.
Later on, canker pathogens may form spore-producing structures on top of dead bark. Their appearance can range from black pepper-like spots to small, red coral-like clusters.
“Healthy woody plants will produce a relatively light-colored ring of callus tissue that’s designed to contain the canker – prevent it from spreading. The tissue is a built-in, natural control,” Kennelly said. “But, some canker pathogens are more aggressive than others. And, stress interferes with plants’ ability to fight back.”
Certain types of tree, for example, can end up with a target-shaped canker that’s perennial. It slowly enlarges as the disease pathogen persists, colonizing each year’s new ring of callus tissue.
In contrast, sometimes a pathogen’s speed is aggressive. Trees can be disfigured or killed in a single year if a pathogen colonizes so rapidly that cankers totally girdle a branch or the plant’s trunk.
Fortunately, unless other factors are playing a part, most tree trunks recover -- even from a fairly good-sized canker wound, she said. In general, trees’ odds for survival become unlikely only when a canker infection is still active and has already damaged more than a third of the trunk’s circumference.
“You can remove a branch that’s exhibiting active, serious disease if you can prune at least 2 to 3 inches below the canker margin and down at the base of the branch, to avoid leaving a stub,” Kennelly said. “Branch pruning won’t necessarily eliminate future canker development, but it can help.”
Disposing of diseased limbs limits the number of pathogens available to infect healthy tissues, she said.
“In the end, though, the best way to manage cankers is by preventing them in the first place,” Kennelly said. “That can include avoiding mower and trimmer injury. This year, it includes providing adequate irrigation.”
Advice about what’s “adequate” is available at any county or district extension office, she said. Facts about watering mature trees and shrubs also are online at Watering Established Trees and Shrubs. Advice about watering newly-planted ornamentals is at Watering Newly Planted and Young Trees and Shrubs.