February 28, 2015

1 - Leafy mistletoe, a tree parasite, is marching up the Rio Grande and may require mechanical or chemical pruning to slow its invasion.

Yard and Garden February 28, 2015

Some of my trees have recently become infected with parasites. These look like clusters or masses of small green leaves that are attached to the branches of the trees by stalks (roots or stems?). The leaves have an "opposite" branching pattern. They are small, slightly oblong and have a smooth surface and perimeter. The only trees affected are cottonwoods. I live close to Candelaria Farms and the bosque and I see the same thing on some cottonwoods.

What is the name of this parasite? I am told they harm/kill the trees and am very concerned as I have many cottonwoods on my property. What can I do about this?

-Jeanne S.



The parasite you described sounds like "leafy mistletoe" which attacks cottonwood and other deciduous trees. This is a pest that has been moving northward up the Rio Grande for years. It produces a (toxic to humans) white berry that is eaten by birds. The seeds pass through the bird's digestive system and are "planted" on a tree when the bird "deposits" it along with a little fertilizer. They may also plant it as they wipe sticky residue containing seeds from their beaks on new trees.

Northern New Mexico has long known about dwarf and false mistletoes which do not form the pronounced leaves of leafy mistletoe. These are found on piñon and juniper trees throughout New Mexico. The leafy mistletoe is the plant is several species and genera known in much of the country and in Europe as a pest of trees, but used as the "kissing" decoration for Christmas parties. (If you are standing under the mistletoe you are likely to be given a kiss.)

This plant is a parasite in that it does harm the host tree, especially by redirecting water and nutrients to itself from other parts of the tree. It is green, containing, chlorophyll, so it can photosynthesize and feed itself, but it still steals from the host tree causing decline and eventual death.

Mistletoes grow by inserting their "haustorium" (root) into the sapwood of the host tree and through such haustoria mistletoe plants steal from their host. The haustorium may grow for considerable distances under the bark of the host in the sapwood, making it difficult to remove the mistletoe. The host often shows some swelling of the branch where the haustorium is thick, but may not reveal all the haustorium. Removal of the host branch with all haustoria will result in removal of the mistletoe, but if a small amount of haustorium remains, the mistletoe will regrow. Often removing enough of the host branch/trunk to remove all haustoria destroys the beauty of the tree and total tree removal becomes necessary, especially if the mistletoe continues to reappear.

Removing the exterior manifestations (leaves and stems) of the mistletoe to prevent fruit formation can help prevent the spread of mistletoe from your trees to other trees. This may be done by mechanical pruning, or use of a chemical spray that causes the exterior parts of the mistletoe to fall from the tree. This spray contains a chemical called ethephon, it produces ethylene gas, a plant hormone, that causes the mistletoe to self-prune. This is temporary, but effective in preventing formation of berries and spread of the mistletoe. Reapplication is necessary as the stems and leaves reappear. Be sure to read, understand, and follow label directions for your safety, maximum effectiveness, and to minimize damage (defoliation) of the tree and damage to nearby plants.

Select a product is labeled for control of mistletoe. Some are also labeled for causing dropping of unwanted fruit from ornamental trees. This is a warning, if you have desirable fruit trees near the cottonwood trees, you may lose your fruit crop if you do not protect the desirable fruit trees from the spray.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.


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