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Invasion of the destructive dwarf mistletoe

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By Sylvia Brockner

Editor’s Note: Sylvia’s column will reappear next week. This column is reprinted from Dec. 10, 2008.

Christmas is not far away, and as we do our shopping and baking, our thoughts turn to Christmas decorations. In England and in our Eastern states, mistletoe is an important part of Christmas. The mistletoe common throughout the South grows mainly on oaks. It forms huge clumps or balls, and the whole plant generally is cut from the tree. The sprigs with the white berries are usually sold for Christmas decorating.
All mistletoes are parasites, sinking their roots deep into the host tree on which they grow, securing their nourishment from the tree.
We have several mistletoes in the West. None of them is as large or as pretty as the Southeastern variety, and all of them are much more destructive. Our dwarf mistletoes all belong to the genus arceuthobium, which comes from the Greek and means “juniper living.” Members of this genus were first described in the Mediterranean region, where they grow commonly on juniper trees. We do not have a species that is parasitic on junipers, but we do have four species, each parasitic on a single host tree.
Arceuthobium vaginatum is parasitic on ponderosa pine and is perhaps the most common. Other species are parasitic on lodgepole pines and Douglas fir. Dwarf mistletoe are leafless, flowering plants. The staminate, or male flowers, are born on one plant; the pistillate, or female flowers that produce the seed, are born on another plant.
The dwarf mistletoe develop a huge root system much like a mushroom does, that invades the bark and wood of its host, absorbing nutrients and water. It can live for many years inside the host without producing the aerial shoots that we see. These shoots are contained in a berry-like capsule that splits and contracts when the seed is ripe and literally explodes the seed like a mortar shell. Seeds easily travel 20 to 40 feet and sometimes more than 60 feet.
Coated with a sticky substance call viscin, the seed sticks wherever it lands, sends out a root and grows. If it happens to land on a young tender twig of a suitable host tree, it will survive.
Mistletoe is a slow but sure killer of your ponderosas. It will take many, many years to actually kill a tree, but it also weakens he tree, making it more susceptible to bark beetles, heart-rotting fungi and other diseases. The only effective method of controlling mistletoe is to prune it out. If an infected branch is pruned back 18 inches or more below the mistletoe shoot, the entire plant is usually removed. If it is growing 18 inches or less from the trunk, the absorption system has usually entered the trunk, and there is little that can be done. Since any of the root system, which you have missed, will grow and send out new aerial shoots in about two years, pruning operations should be continued at two-year intervals for eight years. Winter is a good time to prune your ponderosas, for the pine beetles are dormant and will not be attracted by the smell of fresh pitch.