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Word of the Week: Ascomycete

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Welcome to Word of the Week! Stay tuned for a new word each Friday to amp up your nature vocabulary!

Ascomycete [as-kuh-MAHY-seet] (mycology noun): Any fungus of the phylum Ascomycota, whose spores develop within asci, which are microscopic sac-shaped structures in which sexual spores are produced. The ascomycetes include most molds, mildews, and yeasts, the fungal component of most lichens, and a few large forms such as morels and truffles.

From summer all the way through to fall, you may have noticed dark spots on some of the High Park maple tree leaves. These dark spots are the result of a tree disease, known as “Tar Spot,” which is caused by the Ascomycete fungus, Rhytisma Acerinum.

This time of year, the fallen leaves with the tar spots are getting ready to overwinter. When it starts to warm up again in the spring, the dark spots will begin producing spores internally. After lots of rainfall and many damp days, the dark spots absorb moisture, which causes the spore producing bodies to open up and eject tiny, thin sticky spores that will be carried by wind to the newly expanded maple leaves. These tiny spores enter through the stomata and infect the new leaves. After a few weeks, leaf spots that are light green to yellow green will start to form. Those yellow spots, often unnoticed by most, will then expand slowly over time. It isn’t until the summer that the spots develop that dark eye-catching colour on the upper leaf surface.

Although its appearance seems quite concerning, Tar spot is actually weakly parasitic on the living leaves. It can cause some leaves to drop early, but the number is generally not significant and so, little to no harm is done to the host tree.

Keep and eye out for maple leaves with Tar Spot on your next visit to High Park!

FUN FACT: The dark spots are stroma, which are masses of fungal tissue with spore-bearing structures embedded in or on them.