Mistletoe

Mistletoe

There is a curious decoration used during the holidays to elicit a kiss from that special someone. It is bright green with translucent-whitish berries and often adorned with a bow and hung over a doorway. Chances are, you may have some of this decoration in one or more of your trees as well.

Though it is clearly being put to good use as a decoration, its adornment in your trees is not such a festive and heart-warming occasion.

Leafy mistletoe is a parasitic plant. It derives half or more of its water and solutes from the host tree. Further, it is an endophytic organism—meaning it can sustain itself without any exterior branch growth.

It can often sustain lower transpirational rates than its host to survive droughts, and in cases of infestation, it can continue drawing water to itself by increasing its size and quantity of leaves even after the death of the branch beyond its infestation. You may notice that when the mistletoe has been growing for a while in one area, that area tends to form a gall—or imploded area—where the tree is attempting to grow tissue quickly over a zone of mistletoe infestation in order to successfully conduct water and minerals past the mistletoe to maintain its branch canopy. Its seeds are ingested primarily by birds and then, after traveling through their digestive tract, the pulpy substance around the seed becomes a substance with glue-like properties which adheres the seeds in place on the tree branch.

Mistletoe is a very determined and resourceful parasite.

Local native species that are most prone to mistletoe infestations are the Hackberry, Mesquite, Cedar Elm, and Spanish Red Oak. There are a number of factors that impact a tree’s likelihood of infestation: 1) the density and depth of bark, 2) the density of the wood tissue to resist the growth of the mistletoe root-seeking to penetrate to the cambial tissue, 3) the roosting/resting preference of bird species and occasionally that of the mammals (primarily possums) within the tree, 4) a tree species’ genetic capacity in the creation of polyphenolic compounds which induce formation of periderm cells directly around the invading root of the mistletoe, in effect compartmentalizing it from further invasion.

So what can be done about this Christmas decoration? First, remove all exterior growth of the mistletoe when acquiring your decorations! Second, you can secure black plastic wrap (industrial-grade trash bag type) over the area of infection and at least a foot to either side of that area for a year or two (best utilized in trunk infestations). Third, and the most effective approach where practical, is full branch removal or at least removal back to a scaffolding branch 1/3 the diameter of the infected branch and at least one foot from the infested area. Mild herbicides or plant growth regulators can be applied during winter with some success, but it is inevitable that you will either toxify or inhibit growth to your tree. It is extremely difficult to gauge the correct concentration and amount of the solution to apply, therefore I don’t recommend this approach.

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