Recent heavy rains in the Thunder Bay area could cause problems for green ash trees.

Prolonged moisture can cause a fungus — technically known as anthracnose — to grow on leaves, causing them to fall off the tree.

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A fungus is overtaking ash trees in Thunder Bay. It can cause the trees' leaves to fall off. (Jeff Walters/CBC)

Vince Rutter, an arborist in the city, said although trees can lose leaves because of too much water, there can be a benefit as well.

"It's kind of a lose-win situation," said Rutter. "More water means more leaf loss, but more water (also) means more root growth and tree development."

Rutter says some species — including birch trees — can thrive after heavy rain. He said the trees that can overcome the fungus end up being stronger.

However, another wet spring will cause more problems for ash trees in the city. Significant leaf loss can open up the door for another disease or pest to get onto the trees, which could kill them.