Halifax Tree Project

 Diseases that may cause tree death

Natalie Secen, Halifax Tree Project

2022-03-03

Disease in a host (in this case, trees) is caused by a pathogen, which can take form as a fungus, bacterium, or virus. It can weaken a tree, block the transport of essential nutrients, impact structural integrity, and, given the title of this article series, lead to tree death. There is a huge amount of diversity when it comes to tree diseases and the portfolio of pathogens that impact our urban forest is ever changing, sometimes making it difficult to keep up with proper identification.

So why do we bother? Well, there are steps we can take to reduce the spread of tree diseases, and their identification is important because we absolutely must then consider that the tree is likely to be structurally compromised. In an urban streetscape, this is important given the wide array of targets (pedestrians, roads, power infrastructure, homes, etc.) that will potentially be impacted by a failed tree. Furthermore, some diseases only impact the aesthetic appearance of the tree whereas others can rapidly lead to tree decline and failure. We need to know what we’re working with so that we know how to manage it.

Of course, there is a lag time between onset of disease and tangible evidence that a pathogen has taken hold of a given tree. In an initial assessment of tree health, we look for signs and symptoms. A sign is physical evidence of a damage-causing agent (usually biotic), and a symptom is a change in the growth and/or appearance of the tree in response to this agent. For example, the emerald ash borer (discussed in a previous article) is a sign that we would look for in tree health assessment for ash trees. On the other hand, tar spot, a common disease of Norway maples in Halifax’s streetscape, exhibits a symptom of dead or discoloured spots on a leaf. When identifying tree diseases, we’re often looking for such symptoms.

Before we dive into examples, I want to take a moment to point out the importance of proper tree pruning. When a wound is made to a tree’s bark (e.g., a branch pruned), tissue for disease entry is now exposed, especially for wood-decay fungi. If pruned properly, the tree can, in time, seal the wound with specialized callus tissue, and the point of entry for tree decay no longer exists. Of course, this doesn’t apply for diseases like foliar pathogens and insects. However, hiring a certified arborist to prune your tree is a good way to minimize the likelihood of disease taking hold and spreading throughout its tissue - they know how to prune your tree in a way that will encourage it to seal the wound quickly and effectively.

Ok, enough with the background information, now for some cool examples!

Cankers are caused by fungi and bacteria. Fungi grow on the tree and release spores, which then enter another tree through branch stubs or bark wounds and cause localized dead tissue. Trees are weakened at this localized canker location, so things like wind, snow, and ice are more likely to cause tree failure and eventual death. As you can see in the photos (all taken by me), cankers can look very different across varying species and trees. I found this example of nectria canker (Photo 1) by MicMac Mall, and black knot close to the intersection of Young St. and Windsor St (Photo 2). on the Halifax peninsula. Beech bark disease is a fungus that infects wounds left by the insect beech scale. This example (Photo 3) I found in Hemlock Ravine Park, but unfortunately, it’s very common and afflicts almost all native beech trees in Nova Scotia.

When it comes to wood decay fungi, it’s important to note that there are many species of fungi out there that are beneficial to trees. A common one that might be popping into your mind right now is mycorrhizal fungi, which is part of a mutual symbiotic association between a fungus and a higher plant. When I’m talking about wood decay fungi, I’m referring to those that infect living (and dead) trees by breaking down cell walls and ultimately leading to tree failure and death. A fruiting body on a tree is a sure sign that the decay is present and the fungus has completed its life cycle. But a fruiting body doesn’t always look like your classic mushroom. Take, for example, this black “shoestring” rhizomorph that I found in Hemlock Ravine Park (Photo 4). It attacks the tree’s roots. In contrast, I’ve added an example of a red-belted polypore (Photo 5), a classic-looking fruiting body that attacks the heartwood of a tree.

Foliar and vascular pathogens include things like moulds and mildews, rusts, wilts, needle casts, anthracnose, and blights. These are caused by both fungi and bacteria, and are often the tree diseases that have a mainly aesthetic impact on the tree (but of course there are exceptions and the impact can be more than just aesthetic). As I mentioned earlier, tar spot is common in the streets of Halifax. Photo 6 shows the fungus in its early stages, the spots just starting to emerge.

I encourage you to do some research of your own into the world of tree diseases. Although it makes me sad that there’s such a wide array of tree diseases causing tree death, I must say, tree pathology is a fascinating world to dig into.