A canker is a sunken, dead area on a branch, stem or trunk. It is a symptom like leaf spot, wilt or dieback, and it is caused by one or more disease causing agents. They attack tissue cells and cause decay.
These disease causing agents are either fungi or bacteria.
Fungi reproduce by spores, and these spores are the main way a fungus spreads to new hosts. These spores will travel through the air. Once a fungal spore lands on a plant, it lies dormant until conditions are suitable for it to infect the plant. Moisture is a key ingredient for fungus to develop. You’re probably also aware of this for turf fungi.
Many plant fungi can survive in the soil or on plant debris, whereas bacteria pretty much hangs out just on infected plant debris. This is why it’s good practice to clean up fallen leaves from infected plants and discard them in the trash.
The fungus or bacteria can enter the plant either through a “wound” or even through natural openings. One type of natural opening, and maybe you’re familiar with the term, would be stomata. These are the tiny openings on the underside of leaves.
Wounds Are An Open Invitation To Disease
A plant’s bark is just like our skin. It protects the “innards” from outside harmful things. When an opening occurs the plant becomes vulnerable.
The canker is obvious in the picture of the maple trunk. Michael Hirsch, our plant health care adviser, says the canker developed because a pathogen found its way into a wound at the trunk.
It’s very likely this wound or “split” in the bark occurred from sunscald. This splitting of the bark happens usually in late winter or early spring when severe cold is followed by a quick thaw. Also, sunscald is usually seen on the west side of the plant where afternoon sun causes the thaw.
To protect against sunscald and the wounds it creates you can wrap the trunk with paper tree wrap in late fall. Just remember to remove the wrap in the spring because insects and other organisms can develop and wreak havoc under there.
It’s also a good idea to shape the the wound into an “ellipse” using a sharp knife. Cornell University has a great short article on bark splitting on trees with recommendations on how to deal with them.
Wounds can certainly be caused by other things besides sunscald.
Most wounds in the landscape are man-made. Examples are: poor pruning cuts, damage from improper handling and planting, lawn equipment, etc. Sometimes animals like rodents and even deer cause damage.
What Can Be Done
There are some proactive things you can do to minimize tree wounding. You know I’m going to say practice good pruning and plant handling techniques, but also install adequate mulch rings around trees to avoid mower and line-trimmer damage. Mike Hirsch also recommends using tree wrap for a short while on a new wound to help prevent a pathogen from settling in. Once callusing starts you can remove the wrap.
In many cases canker will not kill the tree, but could structurally weaken it and its ability to cope in the years to come.
Mike oversees the plant health care on many of my projects. And although he’s always there to diagnose and advise, clients rely on him to not just treat the symptom but discover why these problems are happening. Every landscape contracting business should have a plant health care specialist on staff or one within there network.