January 27, 2011 at 9:01 a.m.
When we think about the holidays we think of the happy images of Christmas with its traditions, including mistletoe. At one time mistletoe was believed to have magical powers to ward off evil, grant fertility, block lightning, and wooing spouses.
The truth is that mistletoe is a parasitic plant capable of killing large sections of forests if left unchecked. The type of mistletoe used as a Christmas decoration does not live in Minnesota, but its relative Eastern Spruce Dwarf Mistletoe does. This parasitic plant most commonly attacks black and white spruce, but has been known to attack almost all conifers. These trees are typically only infected with mistletoe when planted close to groups of spruce infected with the parasite.
Trees parasitized by dwarf mistletoe are most easily recognized by clumps of branches all coming from the same point on a larger branch. These clumps are known as witches-brooms, and the needles in this clump remain green while the rest of the needles turn yellow and fall off. Just before the tree dies, the only needles remaining on the tree are in the witches-broom. Examination of the cluster will show small greenish-brown shoots covered with scales that belong to the mistletoe plant. The plant uses root-like structures buried deep in the bark and wood of the parasitized tree to steal all of the water, nutrients, and sugars needed.
This results in the decline and eventual death of the tree.
The female flower of the mistletoe plant can shoot seeds up to 55 feet away. The seeds are sticky and cling onto almost anything. As it dries, it hardens and acts like a cement around the seed. The seed germinates and colonizes the wood and bark of its new host tree.
There are no symptoms of mistletoe infection until two years after germination when a slight swelling of the branch can be seen at the infection point. Not until four or five years after the seeds have landed on the host tree will you see the first mistletoe shoots growing out of the tree branch. It will keep producing shoots and flowers every year until the tree dies.
It is very difficult to cure trees of a mistletoe infection because the parasite is able to colonize many areas of the trees before the first symptoms can be seen. Pruning branches with witches-brooms and mistletoe shoots may help for a while by improving the vigor of the tree. However, it may reappear in two to three years from the seeds or roots that have already colonized the bark and wood.
If more than 50 percent of the tree's branches have mistletoe, witches-brooms, or have been killed by the infection, the tree should be removed. Removing infected trees will help prevent the parasite from spreading to nearby spruce and other conifers.