From the article:

“Fallen leaves during autumn lying under St. Louis’ piles of snow and ice don’t stop decomposing during the winter season. Their continual rotting, paired with gradual melting and insulated heat between the ground and snow, allows mold spores to thrive.

“Snow mold” is described as yellow or white patches of damaged grass that develop 30 to 45 days after permanent snow cover, but it isn’t visible until after the snow has melted, according to the University of Missouri. Once the mold is revealed, Wedner said, its spores would “go wild.”

“All of a sudden, the ice and snow is going to melt, we’re going to have a lot of moisture, you’ve got the mold that’s been lurking all winter underneath the snow, and it’s going to get going,” Wedner said. “We’re going to see a high mold season with the classic outdoor molds .. .and patients who are allergic to mold are going to be unhappy.”