Invasive Fungus Threatens Beloved Evergreens at Bookworm Gardens

Horticulturist says simple pruning can help save infected white pine trees

Published on Feb. 20, 2026

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An invasive white pine blister rust fungus from Asia has been damaging the white pine trees at Bookworm Gardens in Sheboygan, Wisconsin for years. Horticulturist Charlie Parks says the disease starts by infesting the ends of pine branches, cutting off the tree's food and water supply. Parks explains that the browning on branch tips is a sign of infection, and once the spores reach the main trunk, the tree will eventually need to be cut down. However, Parks says there's a way to save infected trees before they're beyond help - using simple pruning techniques to amputate infected branches.

Why it matters

White pine trees are a beloved part of the Bookworm Gardens ecosystem, and the spread of this invasive fungus threatens the health and longevity of these native evergreens. The gardens are a popular community attraction, and preserving the trees is important for maintaining the natural beauty and educational value of the space.

The details

The white pine blister rust fungus was first introduced to the U.S. in the late 1800s or early 1900s. It starts by infesting the ends of pine branches, cutting off the tree's food and water supply. The browning on branch tips is a sign of infection, and once the spores reach the main trunk, the tree will eventually need to be cut down. Bookworm Gardens loses at least one or two white pines to the disease each year. Parks says the spores make their way to the surface and create blistering on the bark, then attach to shrubs native to the area which serve as alternate hosts. To save infected trees, Parks demonstrates a simple pruning technique using pliers to amputate the infected branches, leaving a bit of a collar at the base.

  • The white pine blister rust fungus was first introduced to the U.S. in the late 1800s or early 1900s.
  • Bookworm Gardens loses at least one or two white pines to the disease each year.

The players

Charlie Parks

A horticulturist who has spent nearly a decade caring for the plantlife at Bookworm Gardens in Sheboygan, Wisconsin.

Bookworm Gardens

A public garden in Sheboygan, Wisconsin that features a variety of native plants and trees, including white pine trees that are threatened by the invasive white pine blister rust fungus.

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What they’re saying

“This is a great time to walk the property and look for that kind of discoloration.”

— Charlie Parks, Horticulturist (tmj4.com)

“Just giving that cut. And you want to leave the bit of a collar at the base. That's where nature puts the first aid kits.”

— Charlie Parks, Horticulturist (tmj4.com)

“Do not be dissuaded from planting white pine. It is a magnificent native tree.”

— Charlie Parks, Horticulturist (tmj4.com)

What’s next

Parks plans to diversify the ecosystem at Bookworm Gardens to help stop the spread of blister rust, as he explains that "it just dilutes the impact of a pathogen like that." He encourages other property owners to apply the simple pruning techniques to save infected white pine trees.

The takeaway

The invasive white pine blister rust fungus poses a serious threat to the beloved white pine trees at Bookworm Gardens, but horticulturist Charlie Parks has demonstrated that simple pruning techniques can help save infected trees before they are beyond repair. This highlights the importance of proactive management and community efforts to preserve native ecosystems in the face of invasive species.