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Thursday, October 21, 2021

Mistletoe: the kiss of death?

On a busy weekend in December, Jerry Seifert, owner of the Silveyville Pumpkin and Christmas Tree Farm, can sell up to 75 bunches of mistletoe a day. Families drive out to Seifert’s farm in the country to pick up a bushy Christmas tree and a bunch of mistletoe, a bright green sprig of cheer to combat the gloomy days of winter.

“It’s a traditional item at Christmas time,” said Seifert. “It dates back many years in history.”

She’s right. The tradition of kissing under mistletoe goes back to the days of ancient Europe. Mistletoe was associated with the Norse goddess Frigga, a goddess of love. The Druids of England even thought the plant was an aphrodisiac.

Early Christians saw mistletoe as a symbol of the Virgin Birth of Christ – this might have something to do with how mistletoe can grow through the winter, a beacon of life in leafless trees.

This plant has been romanticized for years, but few know mistletoe’s deep, dark secret: it’s a pest. Mistletoe infestations can weaken, even help kill trees.

Mistletoe is what is called a hemiparasite. Most species use sunlight to photosynthesize, but they get water and nutrients from the tree. Some species of trees, like oak and walnut, are more susceptible than others.

“They tend to be somewhat host specific – certain species [of mistletoe] will attack certain types of plants,” said Joseph DiTomaso, a weed specialist with the department of plant sciences at UC Davis.

DiTomaso described the life cycle of a mistletoe plant. First, a hungry bird eats a berry from an inviting mistletoe branch. The berries are sticky, and as the bird feeds, the berries stick to its feathers. That bird flies away and lands on another branch. Satisfied with its lunch, the bird preens and poops to its heart’s content. Mistletoe berries from the bird’s poop and feathers fall on the tree.

If a seed-filled berry lands on a thin patch of bark, it’s go time. The seed sends out tendrils called haustoria, which bore their way inside the branch of a tree.

“It’s not really a root because it doesn’t go in the soil,” DiTomaso said.

The tree stands in for soil, and the haustoria exploit the natural water and nutrient pathways used by the trees.

Like most parasites, mistletoe doesn’t want to kill its host. If the tree dies, the mistletoe will die too. DiTomaso said mistletoe can weaken or stunt tree growth, but trees with heavy infestations only die with there are other stressors – like drought – in the ecosystem.

“Obviously, if the plant has to share its water and nutrients with a really heavy infestation of mistletoe, that could be the breaking point for everyone,” DiTomaso said.

DiTomaso warned that while mistletoe looks cute in doorways, it is also extremely poisonous.

“People are welcome to kiss under it – just don’t eat it,” said DiTomaso.

This winter, mistletoe grows strong in the walnut trees at Seifert’s farm. Seifert prunes it every year, but the plant grows back from its hidden haustoria.

“We have way more mistletoe than we could ever use,” Seifert said.

Seifert knows the plant is a pest, but she doesn’t worry about it on her trees.

“If we want to kiss, we can stand under the trees in our yard,” she said.

For more information on mistletoe control go to cagardenweb.ucdavis.edu.

MADELINE McCURRY-SCHMIDT can be reached at science@theaggie.org.

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