April 17, 2010

1 - Mistletoe can infest landscape trees.

Yard and Garden - April 17, 2010

Q.

Will you please inform the public of the danger of mistletoe to their trees? I have noticed the last few years that trees all over Las Cruces contain the parasite.

- Sharon C.

A.

Many trees in New Mexico contain mistletoe. Mistletoe plants are parasites and semi-parasites that damage their host plants. While they may not kill the host, they can greatly reduce the health, vigor, and appearance of the host. They can also predispose the host to death due to drought or other stresses that they would have survived if they had not been weakened by the mistletoe.

There are several kinds of mistletoe in New Mexico. One type infests pine trees, another infests junipers, and a third type infests broadleaf trees. The last is the one that you have been observing in Las Cruces. I have seen it there for many years in poplars, cottonwood, and other trees. I have heard of its spread to the bosque in Socorro, and recently saw it on a cottonwood tree in the Albuquerque bosque. A few years ago I saw it infesting a small New Mexico olive shrub in a landscape in Santa Fe. In the case in Santa Fe, it may have been imported with the newly transplanted New Mexico olive shrub, but the others were obviously established and overwintering in trees in Las Cruces, Socorro, and Albuquerque. This adds relevance to your concern. Trees in many parts of New Mexico are subject to damage by mistletoe parasites.

The broadleaf tree mistletoe is itself a broadleaf plant with chlorophyll and can feed itself through photosynthesis, but it redirects water and minerals in the host tree to itself. This causes other parts of the tree to exhibit injury.

Now, the bad news! Once a tree is infested with mistletoe, it must be pruned out. The mistletoe plant's roots (called haustoria) are inside the wood of the host tree and extend a distance down the infested branch or trunk. If the mistletoe is cut from the tree, any remaining haustoria will produce new mistletoe shoots, so it must be pruned out completely. This often results in excessive pruning that reduces the appearance of the tree. If the haustoria have invaded the wood of the trunk of the host tree, the mistletoe cannot be removed and the tree should be replaced.

Wise gardeners will inspect newly purchased trees and shrubs to avoid purchasing plants already infested with mistletoe. Infested plants are not common in the nursery, but it is wise to check. The broadleaf mistletoe seed is spread to uninfested trees by birds. That means healthy plants may develop mistletoe infestations which should be pruned from the tree as soon as discovered.

There are chemical sprays based on the plant growth regulator, ethephon, which can cause branches of mistletoe to fall from the tree. The haustoria then produce new plants, but treatment can prevent production of seeds to spread mistletoe infestations. These products will not cure the mistletoe problem, but reduce the spread of mistletoe parasites to new host trees. These products should be applied in the spring when temperatures are above 65 degrees, but before leaves form on the host trees.

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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