Mistletoe - It's Kiss of Death!

As, we all know, mistletoe commonly inspires a kiss between those that happen to pass under it. In fact, the tradition is often that any two people who meet under hung mistletoe during the holidays are obliged to kiss. Now, I'm guessing that this tradition actually fails to apply if two heterosexuals of the same sex, or two homosexuals of opposite sexes, meet under it. But, at kinkier parties, well,.. who knows!

In any case, those kisses are usually not intended to be lethal. At least, not at any parties I've ever been to. But mistletoe does have a darker side. And that's where I'm going with this article. Don't ya just love the title? Did it suck you in to read more? Well, good!! Let me tell you about the Deadly nature of Mistletoe.

Mistletoe is the common name for a group of hemi-parasitic plants in the order Santalales (Class Magnoliopsida, Division Magnoliophyta, Kingdom Plantae) that grow attached to and within the branches of a tree or shrub. So what the heck does "hemi-parasitic" mean? Answer: A Hemiparasite is a plant that is parasitic under natural conditions and is also photosynthetic (uses chlorophyll for photosynthesis - converting sunlight to energy) to some degree. Hemiparasites may just obtain water and mineral nutrients from the host plant. Many obtain at least part of their organic nutrients from the host as well.

Okay, so mistletoe is a parasite - it lives by feeding on it's host. Hmmm,... now that changes my feelings about mistletoe a little bit already. But, at least it does a little photosynthesis by itself, so,... that's a good thing. Right?

Well, maybe not so much.

From TheScientist.com: For the white spruce tree (Picea glauca), mistletoe is the kiss of death.

Barry Logan, an associate professor of biology at Bowdoin College, began studying the interaction between white spruce and the parasitic eastern dwarf mistletoe (Arceuthobium pusillum) 10 years ago in coastal Maine forests. The parasite causes trees to grow twisted, tangled branches called witches’ brooms. These misshapen branches are dead giveaways that mistletoe is wreaking havoc on the tree’s growth and development. [Just take a look at the photo at left!]

While Washington and Oregon western hemlocks show evidence of having weathered infections of hemlock dwarf mistletoe for 80 years or more, Logan says that white spruce, succumb to eastern dwarf mistletoe in 15 or 20 years.

Okay, so now I'm getting a whole 'nuther perspective on Mistletoe! Immediately below is a photo of Mistletoe in a Lebanese Oak tree in winter. [photo by Wikipedia user Elie_plus]

The photo below shows European Mistletoe in a Silver Birch tree. [photo from Wikipedia by Andrew Dunn (www.andrewdunnphoto.com)]

And here's the wrap-up that ought to give you at least a tiny bit more pause for pondering: (also from TheScientist.com)

Trees typically respond to parasites and pathogens by shedding infected branches and sending resources to unaffected limbs. Other tree species respond to mistletoe in this manner, and white spruce, too, react this way when attacked by other pathogens. When plagued with dwarf mistletoe, however, white spruce ship water, nutrients, and sugars to the infected branches, at the expense of the uninfected boughs. “Something about the mistletoe is overriding the white spruce control mechanisms,” says James Lewis, a plant ecologist at Fordham University, who has been collaborating with Logan for the last 4 years.

The mistletoe, it seems, may be meddling with the tree’s hormones. In a not-yet-published study, Logan and his team discovered that needles on infected white spruce branches have twice the concentration of cytokinins as do uninfected branches. These growth-promoting hormones trigger branching and direct the movement of resources into the branch. Infected branches also have significantly reduced concentrations of abscisic acid, a stress-related hormone that some studies have linked to the shedding of old branches. “All of this comes together nicely,” Logan says, to explain how witches’ brooms form and thrive.

So, the way that Mistletoe is killing these trees is quite unique I think. The white spruce actually has a maladaptive response to the mistletoe because the mistletoe is manipulating the tree's hormones somehow. And if you keep in mind that mistletoe's attack on the white spruce is considered an infection, well,...

Let's just say that I'll never think of mistletoe in quite the same way again! I leave you with an oldie but goodie - a mistletoe postcard circa 1900.

Happy Holidays!