Hypoxylon Cankers

January-February 2008

IT SEEMS AS THOUGH we are seeing more than our fair share of trunk and branch cankers on deciduous trees in the last little while. One in particular that I’ve been hearing or reading about a lot is Hypoxylon canker (e.g. Hypoxylon mammatum, H. atropunctatum) on deciduous trees. At the early stages of infection, the tissue under the bark is invaded and eventually killed, causing a brown canker on the trunk or major branches after a couple of years. If you cut into the cambium with a sharp knife, you would see a dark mottling of the cambium and outer sapwood surrounding the visible young canker. The canker results in the localized death of the cambium, the conductive tissue that sends water and nutrients to the rest of the tree.

Not surprisingly, latent infection causes wilting, leaf browning (with retention of killed leaves) followed by dieback of the infected branches or side of the tree. Tree mortality can happen quickly, sometimes within a few years. Quite often, we don’t notice Hypoxylon until the canker is more mature. The mature canker is quite dark, covered by a tar-like fungal mat. If you look closely with a magnifying lens, you can see the tiny pimple-like projections on the fungal mats which indicate sporulation. Unfortunately, fungicide treatments are not usually effective, especially after the fungus enters the tree bark.

The fungus is reported to be a significant disease of poplar (Populus spp.) but only an innocuous pathogen of hardwoods such as beech (Fagus) and birch (Betula). For an “innocuous pathogen,” it sure seems to be associated with a lot of cankered beech trees in the Ontario landscape, especially in gardens. Scientists believe that the fungus cannot invade healthy tissue very well but that it requires an entry point created by another organism or event. Trunk injury could be caused by freeze-thaw cycles, insects, rodents, other pathogens and us. Not only do humans damage bark through grass trimming, pruning and weeding, some of us also tend to pile large amounts of compost, triple mix and mulch around tree trunks. When trunk tissue gets buried, it rots and the rotted tissue creates an entry point for Hypoxylon and other pathogens. Not only are we damaging the tree trunks, we are constantly ripping up tree roots in our efforts to cultivate desirable plants underneath the canopy of these beloved trees. Well as you know, beech trees are somewhat shallow-rooted, and they don’t like to have their roots disturbed. When you consider all the damage gardeners can inflict and then factor in the number of summers we’ve had with record high temperatures and record low precipitation, it’s no wonder our beloved trees are at the mercy of weak pathogens such as this.

Weird & Wacky Wonders
During the 2007 growing season, I received some pretty wild photos of pests and tree problems from arborists and landscape professionals across the province. Some I can identify from my own experience, for others I am lucky to be able to draw on the incredible skill sets of the scientists who surround me here at the University of Guelph and OMAFRA. This summer, someone sent me a photo of a soil-inhabiting insect. “What is this?” they asked. I thought it looked like a creature out of a scientific movie and I quickly replied: “It’s ugly, that’s what it is!” Well my reply garnered a response of “LOL: – netiquette for “laughing out loud”– but I did later get confirmation that it was the nymph stage of a cicada species.

Some other interesting photos come with the theme of (what I like to call), “landscape terrorism.” I have been contacted by several property owners or landscape professional who have reported significant plant injury or mortality after returning from a vacation or business trip. I have four sisters, so I know how easy it is to suspect that someone has been messing with your things while you have been gone. And just like with siblings, it is next to impossible to prove that the neighbour’s activities have led to the injury and mortality of your $400 specimen tree. Thankfully, with the advent of email and digital cameras, I have been able to blame winter damage, frost cracks, transplant stress, apple scab, rose slug, nutrient deficiencies and more. Plant pests and physiological or environmental stress are most often to blame for problems in the landscape. But sometimes, there is no concrete explanation and I know how frustrating that can be.

Tobias Effinger of Arboreal Tree Care Professionals in Thornbury sent me some remarkable photos of a sugar maple. For background information, this roadside tree has been suffering substantial dieback for quite some time. The first photo showed some pretty strange, black, curly structures dangling from the bark.

Uh oh, significant dieback and a scary, fungus-like disease emerging from the bark. What is it and should we be worried about it? Well, what do you think? Some of the smaller, low-profile black structures on the bark in the second photo are reminiscent of Hypoxylon canker, but they do not have the characteristic tiny bumps on them that are so indicative of that disease
The black, curly structures are found mainly on the dead or nearly-dead parts of the tree, which would indicate that it may not be the original pathogen but that it just living off the dead portion of the trunk (saprophytic). These types of saprophytic organisms are rarely known to kill living tissue, so it is probably not a pathogen.

The second photo showed little orange and grey leafy structures glued to the bark? Yep, those are lichens; symbiotic organisms consisting of fungi and algae which we often see on tree trunks and outdoor wooden structures. We showed these photos to Dr. George Barron, renowned mycologist at U of Guelph and he confirmed that the pendant, black structures are indeed a type of lichen, but one that is not very common in southern Ontario. I would hazard a guess that the extremely warm and dry summers and the roadside exposure of the site have not been very kind to the shallow-rooted sugar maple in these photos. The lichens have been slowly colonizing the tree bark and simply took the opportunity that nature presented them. Every day brings a new wonder and a new challenge. What a great job I’ve got.

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

Email any questions you have directly to Jen and we'll publish her response.

P: 519-824-4120 ext. 52671 • F: 519-767-0755

OMAF website: http://www.omafra.gov.on.ca/scripts/english/crops/agriphone/index.asp

Nursery Landscape Agriphone: 1-888-290-4441

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