How to - Pests & Diseases

How to deal with tree fungi

Shelagh McNally

Tips to recognize the different types of blight and what to do about them

 Is your favourite tree turning the wrong shade of colour this fall or developing weird blemishes on its leaves? You’re not alone. This autumn many of our shade trees are suffering from fungal infections.

“We’re getting calls from across North America this fall,” says Jeff Tekavcic, founder and general manager of Tree “The wet, cold summer released a lot of the spores that are usually dormant and as a result there is a lot of fungal outbreaks.”

These infections are tough to fight since there aren’t many warning signs until the telltale marks start showing up on the leaves. While all fungus infections are unsightly, the common ones are not fatal.

Ugly, but not lethal

Powdery mildew: The most widespread fungus this season, powdery mildew begins as light green or yellow spots, giving the appearance of the plant being dusted with flour. If you look closely you’ll notice millions of small black dots, which are the spores that are easily spread. Infected leaves eventually turn yellow and drop prematurely. While it won’t kill anything outright, powdery mildew will weaken your tree or shrub making it more susceptible to other problems. A spray made with baking soda is said to be an effective cure (1 teaspoon to 1 litre water).

Tar spots: The fungus Rhytisma genus creates raised, black spots on the leaves of maples and other deciduous trees. Also known as tar spots–since they do look like drops of tar--this fungus is hard to spot. The first symptoms appear in mid-June as small pale yellow spots and as the season progresses, the spots grow in size, shape and thickness with sometimes indentations or ripples. Although tar spots can ruin the look of the tree, they are rarely serious and won’t prevent new leaves from forming. They can cause an early leaf drop, which can be annoying.

Anthracnose: Commonly known as leaf blight, anthracnose diseases are caused by several closely related fungi that attack most shade trees. The symptoms depend on the tree and strain of anthracnose, but the most general symptoms are brownish discolouration along the veins. There may also be dead areas that follow the main veins out to the edges. New growth may be stunted resulting in short twigs that ruin a tree’s symmetry. It’s rarely life threatening except with sycamore and dogwood anthracnose where the fungus usually moves back into stem tissue. Anthracnose can also cause early loss of leaves, which over many years can leave tree vulnerable to winter injury.

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