Why are some leaves falling early?

WELLINGTON COUNTY – It’s still about three weeks until the fall equinox, and yet the leaves on many maple trees around Wellington County have already fallen off branches.

Some started losing leaves several weeks ago.

Southern Ontario experienced a wet spring, creating favourable conditions for fungal diseases such as tar spot and anthracnose. 

Alison Morrison, horticulture manager at the Arboretum, said, “trees can change their foliage and shed leaves for myriad reasons, among them temperature changes, but also root problems, insects and disease.”

When it comes to premature leaf loss, Morrison says some common-sense questions to ask might include: “What kind of tree is this? How old is it? And where is it growing?”

A tree may be genuinely responding to seasonal change, but often there are less dramatic reasons a tree might be shedding leaves early.

Morrison contends people often associate the health of a tree with a stereotypically healthy-looking tree, which can be misleading.

Trees also serve as habitat for other biological life, so the presence of a colony of caterpillars, for instance, does not necessarily mean there is something wrong with a tree.

Area residents may have noticed Norway maples, a common maple tree in suburban and urban environments,  shedding leaves early, with their foliage speckled by conspicuous dark spots.

According to The Weather Network, southern Ontario’s relatively wet spring created favourable conditions for the spores responsible for fungal diseases such as tar spot and anthracnose to spread.

Leaves affected by these diseases can eventually be shed prematurely.

While polka-dot leaves might look unusual, they are not necessarily a cause for concern. Problems that persist across more than one yearly cycle, however, can be concerning.

Wet weather

According to Environment and Climate Change Canada, spring precipitation across Canada increased by an average of 2.4 per cent per decade between 1949 and 2023, the strongest increase recorded among the seasons. Ontario’s Provincial Climate Change Impact Assessment also projects increasing spring precipitation across all regions of the province.

If wet spring conditions help fungal diseases such as tar spot become established, could increasingly wet springs make years like this one more common?

For now, Morrison cautions against diagnosing a changing climate from the appearance of an individual tree.

A speckled tree, or one shedding leaves earlier than its neighbours, does not necessarily mean something is seriously amiss. 

When similar symptoms begin appearing across trees and across regions, the more interesting question may be how changing combinations of weather, disease and stress are producing them.