Trees infested with the sap-sucking parasite would like to kiss the Christmas novelty goodbye.

And that may become easier -- even without holiday harvests -- due to new research at the Texas Agricultural Experiment Station.

"Mistletoe is unsightly and adversely affects the health of trees," said Dr. Todd Watson, Experiment Station urban forest researcher.

Watson completed the first year of a two-year study aimed at eliminating the pest from urban landscapes and found promising results with at least one new treatment -- a plant hormone.

The problem with mistletoe is that it stays with the tree until the tree dies. Spread by birds who eat mistletoe, the parasitic plant grows from seed deposited in bird feces on tree limbs. Watson said mistletoe left unchecked can cause die-back of tree limbs and occasionally the death of the tree, especially in drought conditions.

"Mistletoe grows into the wood of the tree, drawing water and minerals out," he said. "Mistletoe is a plant, so it makes its own nutrients from photosynthesis, but it is the tree's water that it pulls from and that weakens the tree and causes stress."

Yet that's oft overlooked. For hundreds of years, mistletoe has been associated with various cultures in countries around the world as a plant symbolic either of peace or of romance. Its yuletide custom of suggesting a kiss underneath suspended mistletoe apparently is linked to English tradition.

But arborists have a decided lack of love for this parasite, stemming from the fact that mistletoe is especially hard to kill without harming the tree in the process, Watson explained.

"One can repeatedly cut off mistletoe to prevent it from making seed and therefore spreading, but that is very labor intensive," he noted. "Or one can prune the infected branch, but that is only affective if there are a small number of branches with mistletoe.

"Some have covered affected limbs with black plastic to kill mistletoe by cutting it off from the sun," Watson added, "but that is as unsightly as the mistletoe and very labor intensive. A chemical currently labeled for use against mistletoe has not been very effective in totally eliminating mistletoe in one application when used at the recommended label rates."

To find a more reasonable, effective way to eliminate mistletoe, Watson tested eight different treatments in elm trees on the Texas A&M University campus. There was a controlled, untreated, group of trees; a group where mistletoe was pruned out; a group where entire branches were pruned; a group treated with the labeled chemical; a group in which the mistletoe infestations were covered with dark caulking; a group sprayed with glyphosate; a group treated with 2-4D; and a group treated with a specific plant hormone.

Watson said the trials were looking for at least 90 percent control in the 25 mistletoe plants treated in each group to be considered successful. The plant hormone yielded better than 90 percent control, he said.

The trial will be continued for another year and additional twists on the tests, such as developing formulations to improve the effectiveness of the plan hormones will be implemented, Watson said. If successful, the research said, the method likely will be patented for use on mistletoe throughout the United States.

Note: If no author is given, the source is cited instead.

• Trees
• Botany
• Invasive Species
• Nature
• Seeds
• Pests and Parasites

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