maladie de l'érable - taches noires sur les feuilles

We’re constantly hearing about the emerald ash borer, that invasive parasite that attacks ash trees; however, reports on the diseases that can affect maple trees are scarce. Nevertheless, numerous fungi and parasites can debilitate Canada’s arboreal emblem.

The good news is that not all maple tree diseases are synonymous with a death sentence. Maple trees are particularly robust, demonstrating strong resistance to disease. However, in some cases, you must implement certain actions and treatments to care for them and keep them healthy. Luckily, the growing body of knowledge in the area of arboriculture enables us to identify the various diseases and provide the most appropriate care and preventive measures.

Tar spot disease, verticillium wilt and sooty mold are the principal maple tree infections to be on the lookout for. Since the best form of protection is prevention, we would like to present you with some useful information on these diseases, including their causes, consequences and proven treatments.

Spotlight on the three main maple tree diseases

Tar spot disease, verticillium wilt and sooty mold are definitely three names to remember—and not because of the benefits they offer to the trees!

Tar spot disease

The first item on our list of maple tree maladies is tar spot disease. Caused by fungi of the genus Rhytisma, it’s the bane of the sugar maple, red maple and silver maple, as well as the Norway maple, an ornamental species that’s very common in urban settings. It’s impossible not to notice it since, as the name indicates, it causes the appearance of black, tarry spots up to 3 cm in diameter on the top sides of the leaves.

Present in the majority of countries in the northern hemisphere, it proliferates very quickly, especially when extremely humid weather prevents the leaves from drying out completely. The fungi survive the winter on the infected leaves that have fallen to the ground. In the spring, the resulting spores infect the new leaves, initially covering them with pale or yellowish spots. By mid-summer, the spots turn black and form blisters, as the filaments of the fungi intermingle with the tissues of the leaves. The tissues on the underside of the leaves also end up turning brown or yellow.

While this infection is significant from a purely visual standpoint, it doesn’t threaten the survival of the tree. Of course, the tree may dry out and the leaves may fall prematurely in late summer, but its health is in no danger whatsoever.

Verticillium wilt

When a maple tree is infected with verticillium wilt, a different kind of discoloration! This disease is also the result of a fungus, this time of the genus Verticillium, which causes the leaves, the branches and, ultimately, the whole tree to wilt. This condition isn’t always fatal; however, an infected maple tree is certainly at a greater risk of dying from verticillium wilt than from tar spot disease. Very widespread in New Brunswick, Ontario, Prince Edward Island and Quebec, it generally appears in mid-summer.

It can affect all types of maple trees, but Japanese maples are particularly susceptible. The contamination originates in the soil with a pathogenic fungus that can survive underground for extended periods. The fungi enter through the roots and subsequently make their way up the tree through its vascular system. The substances they produce obstruct the vessels and impede the flow of sap, leading to wilting and a characteristic reddish-brown color.

Sooty mold

We’ll finish our list of maple tree diseases with sooty mold, also caused by a fungus, which literally dries out the tree’s trunk and branches. The symptom that should alert you to the existence of a problem is the yellowing and drying of the leaves, typically after periods of drought, which have become increasingly frequent in recent decades. The following year, the bark will blister and crack, releasing a fine, soot-colored powder, which is actually the spores of the fungus. There’s no happy ending here: by the following spring, the maple tree will be dead.

Caring for maple trees: prevention and treatment

The treatment will vary, according to the disease that affects the trees. Depending on how advanced the infection is, you must also accept the possibility that the tree may already be condemned. If so, the priority should be to protect passers-by and nearby buildings from potential falling branches.

In the case of tar spot disease, there’s no specific treatment. Instead, you’ll need to gather up the dead leaves and dispose of them to prevent further propagation of the fungus. Trimming can also help prevent infection because, by thinning out the branches, you’ll allow the air to circulate more freely, which will help the foliage dry more quickly. While it’s doubtful that this will eliminate the infection, it will significantly reduce it.

There’s no truly effective treatment for verticillium wilt. Nevertheless, removing the contaminated parts of the tree, thoroughly rinsing off any exposed equipment, such as pruning shears, and removing all remnants of affected plants from your yard (preferably burning them) are all good measures to adopt in order to prevent serious propagation. On the other hand, it isn’t recommendable to cut off the wilted branches, as there’s a possibility that they will come back the following season.

Sooty mold generally necessitates cutting down the affected maple tree, as there’s no possibility of it surviving; however, there is an increased risk of falling branches. In no event should you touch an affected tree or the “soot” that emanates from the bark. The spores are extremely volatile and can lead to respiratory problems.
Contact a certified arborist

The diagnosis and treatment of maple tree diseases requires extensive expertise that only a professional arborist can offer. Whether it’s a matter of taking care of your maple trees or felling them properly and securely, the professionals at Émondage SBP can explain the situation in detail and guide you in making the best decision.



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