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.

tree canker

Canker On The Trunk Of Japanese Maple

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.

  • Gregg
    5:20 PM, 13 May 2014

    Thank you , Thank You , Thank you again.After 10 years of losing Japanese Maples to a mysterious disease. You solved my problem. I hope and pray.
    I have been seeing trees die just like the picture above.My trees are in the open sun and have endured harsh cold followed by quick warm ups in late winter/early spring.Its sunscald followed by canker after a fungus or bacteria attacks the wound.Damage was on west side until trees died.Paragraph seven above was like a light bulb going off over my head when I read it.Paragraph eight is the solution I hope.You guys are my hero’s today.

  • Carolyn Reid
    7:31 PM, 8 April 2015

    I have a beloved acer palmatum ukigumo in a large pot. It has done very well on my deck in the Seattle area until now. It was knocked off my deck in a windstorm, and broke branches, which I had to remove. A few weeks ago, I noticed bright orange raised dots on one limb, beginning at the crotch to the trunk. I have been moving, and meanwhile, they have spread up the branch, which has not leafed out like other branches. I have removed the limb, and some spores remain attached to the trunk. The problem has been diagnosed as the fruiting spores of a canker. My research thus far does not provide any treatment once this stage is reached.

    Is there anything I can do? I plan to move it under the roof of my new deck to reduce moisture on the bark. I will keep the ukigumo away from my other potted Japanese maple. In old literature there is reference to fungicide use. HELP!

    • Roger
      9:23 PM, 13 April 2015

      I don’t have the answer you need here. And when I come across plant problems I’m not familiar with I send a sample down to Rutgers University. They are the NJ university associated with the NJ Cooperative Extension Service.

      Every county in every state has a Cooperative Extension Services office. Through your respective office you’ll be able to get information on sending a sample of your maple to your state’s affiliated university. Here’s a link to the webpage I found for Cooperative Extension Offices in Washington.

      Rutgers diagnostic lab never lets me down. You should have the same positive experience through the university your extension office directs you to.

  • Lenore
    1:01 PM, 28 August 2015

    H E L P. 201-321-1170 my Japanese maple

    • Roger
      3:12 PM, 30 August 2015

      Please comment or ask your question in the comments box so everyone can benefit.

  • martin
    10:09 PM, 9 November 2015

    what can i do with borers in my maple tree

    • Roger
      11:59 PM, 9 November 2015

      You should contact a tree expert (certified arborist) to identify exactly what insect you have and how to treat it.

      If you’re seeing entrance/exit holes and other signs of borers, my guess is it’s a type of flathead borer. From my experience, using imidacloprid (a systemic insecticide) would be the control. But you really do need professional guidance on this.

  • Robink
    2:14 PM, 25 April 2016

    Hi Roger – I posted four photos of our sick little Acer palmatum. You can see another one that is doing well right next to it. They are about 15 years old. Can you tell if this is a canker or what? We’d noticed the bark getting darker but thought the tree was just maturing…this spring it bloomed and began to leaf out, and stalled. It’s looked like this for a couple of weeks now.

    Thanks so much for taking time to help us all out here.

    • Roger
      9:47 AM, 26 April 2016

      It’s hard to say what’s troubling your maple. It could very well be a type of canker. Is there an arborist or plant health care specialist in your area you could contact?

      When I come across plant problems I’m not familiar with, I send a sample down to Rutgers University. They are the NJ university associated with the NJ Cooperative Extension Service.

      Every county in every state has a Cooperative Extension Services office. If you look your’s up perhaps you could send them these pics, or get other specific advice.

      Here’s the USDA’s map of extension partners. Just click on your state to find who you should contact.

      Sorry I couldn’t be more helpful.

  • Michael Fleming
    12:02 PM, 21 November 2016

    Hi – I have a large maple tree. I’ve noticed 2 things. One appears to be lichens, i.e. green splats on the bark, which are very evident when the tree gets wet during a rainfall. The other involved cluster of black dots on various parts of the bark, i.e., branches. Can you tell me if either of these are dangerous to the tree and if so, is there any way to limit the damage they might cause?

    • Roger
      6:20 PM, 25 November 2016

      In different environments I’ve seen all sorts of interesting marks, blemishes and even growths like lichens on maples. It’s hard to say what those black dots you describe are, or whether they pose a threat to the plant.

      If they’ve been there for awhile and the tree does not seem to be bothered, I’d say they’re harmless. Of course if you have a real concern, I’d contact an experienced, certified arborist to take a look.

      Sorry I couldn’t be more helpful.

  • Don Loos
    1:38 PM, 31 May 2017

    Can you use a fungicide on canker and apply tree wound?

    • Roger
      10:55 PM, 31 May 2017

      From what I’ve read, fungicides have not proven effective in controlling cankers. I’ve found that the tree, if generally healthy and sited in good growing conditions, will compartmentalize the area and begin to callus. I would not recommend applying tree wound of any kind.

  • Jane
    3:14 AM, 6 October 2017

    My Japanese Maple has callused exactly like the tree in the picture above, from what I believe now removal of a major branch a few years back without proper handling of the cut wound. I am very worried that we may be losing this decades-old tree. What can I do now so it does not get worse and possibly heal?

    Thank you, Roger. The explanation is really helpful.

    • Roger
      8:45 AM, 7 October 2017

      It’s good you’re aware of the wound on the maple. But it would be difficult for me to give advice on this. A licensed arborist or plant health care person in your area would be better qualified. They can look at the plant’s condition, observe the surroundings and ask you questions about the plant and its past.

      You could call a “professional” tree service in your area to see if they have a licensed arborist on staff. You could also ask at a reputable nursery or garden center if they could recommend someone. Sorry I can’t be more helpful. 🙂

  • Dan Gould
    7:32 AM, 4 May 2018

    Hello Roger,

    We have a Japanese Maple that I estimate to be about 78 years old (that how old the house is) . I would say that 85 percent of the branches are still getting leaves and generally it looks pretty healthy to a homeowner. I have cut off a few dead branches over the last 5 years and I believe that I have done that correctly. The top of this tree is probably 30 feet or a little better. It kind of looks like an old man that is losing his hair in places but still has most of it. There are a lot of green splotches (calling it that since I don’t know what it is) all over the tree bark (the leaves seem fine). Kind of looks like green aliens attached to it. It is spring and it leaf infill is pretty strong. Can you tell me about those light green splotches?

    • Roger
      10:55 AM, 4 May 2018

      I’m not able to identify the green splotches you’re seeing, but it doesn’t sound like canker to me.

      It’s interesting your timing with this comment. Just the other day I met with an arborist to look at a group of older Japanese Maples that he has been caring for.

      I was concerned about certain “things” I was seeing on their trunks and branches. Most discoloration he dismissed as harmless. What he took notice of was earlier signs of possible canker where he could see the bark texture changing (the best way I can describe it). He’s starting a trunk injection regimen of some kind of (disease) control.

      If you have a certified arborist in your area, and you’re concerned about what you’re seeing, it would pay to have him/her take a look.

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