Biology Faculty Research Parasitic Mistletoe

Story posted January 13, 2003

While talk of mistletoe may conjure images of waiting expectantly beneath a sprig for a dash of romance, not all types of the plant should bring a smile to the lips.

Dwarf mistletoe, a tricky relative of the plant we associate with kissing, may in fact be forcing us to kiss Maine's spruce forests goodbye.

Barry Logan, assistant professor of biology, and Jaret Reblin, biology laboratory instructor, recently gave a presentation on the subject of Eastern dwarf mistletoe at the weekly faculty seminar.

Logan and Reblin's research on the native parasitic flowering plant began in the summer of 2002, and focused on trees growing on Monhegan Island.

"It's a very interesting way to make a living, being a parasite," said Logan. "The dwarf mistletoe makes its living withdrawing water, food, and needed resources from its host. It has to figure out how to penetrate the host."

Dwarf mistletoe is very successful at figuring it out. White, red and black spruces -- all of which are in Maine forests -- are the primary hosts.

Reblin states there is a 90% probability of infection in Maine's spruces.

If it weren't bad enough that the dwarf mistletoe sucks the life out of the trees, the way it spreads sounds even more sinister. The dwarf mistletoe fruit on infected trees shoots its seeds out like blow darts to a distance of 15 meters. The seeds, which are very sticky, anchor onto whatever they hit, including the branches and needles of other nearby trees.

Logan and Reblin recounted the bizarre experience of walking through a forest and suddenly being bombarded with the seeds.

After clinging to the trees, the seed embryos start to grow, penetrate the host, tap into the system, and create their network of parasitic activity.

Infected trees are recognizable by the thick clusters of twigs growing abnormally along branches. These twig clusters are known as "witches' brooms" because of their resemblance to the business end of a broom.

Red spruces seem to tolerate dwarf mistletoe better than others, sometimes making it possible for the tree to continue to grow. They "seem to be better at shutting down the brooms," said Logan.

However, Reblin reports that the mistletoe cuts the life expectancy considerably in spruces. "Some trees that should live to be 500 years old die at 100."

Reblin spent the summer on Monhegan Island (Logan worked from the Bowdoin campus) in conjunction with Prof. William H. Livingston, chair and associate professor of forest resources at the University of Maine. Monhegan boasts both red and white spruce on the undeveloped parts of the island: red in the interior, and white on the coast.

All the white spruces on Monhegan are infected. Effects can be seen in needle length, quantity and density. Meanwhile, there is a decreased capacity for photosynthesis in the red spruce.

Why the difference?

"Stay tuned," Logan said about their ongoing research.

And what can be done about the dwarf mistletoe infestation?

"It's a growing problem," said Logan. "There's not that much that can be done. It's a native plant. But the forests won't regenerate without some major manipulation." That manipulation would probably need to include the removal of all infected trees.

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