Dwarf Mistletoe in Kern County Forested Land

John Karlik

UCCE Farm Advisor, Kern County

Environmental Horticulture/Nursery

August 23, 2002

In the higher foothills and mountains of Kern County dwarf mistletoe occurs, and the incidence of this parasitic plant seems to be increasing. Dwarf mistletoe is not as obvious as leafy mistletoe, but is more injurious to susceptible trees, causing limb dieback and often tree death.

Dwarf mistletoes, Arceuthobium spp., are found infesting conifers throughout the western U.S. In Kern County dwarf mistletoe is found mostly on pines, including digger, jeffrey and ponderosa pines. An infestation begins when a mistletoe seed falls on a branch or stem of a susceptible tree. The seed germinates and root-like structures gradually penetrate the vascular tissues of the tree, obtaining nutrients, sugars and water. These "roots" continue to grow for 2-5 years, after which shoots of the mistletoe erupt from swollen areas of bark. Dwarf mistletoe shoots are golden to yellow-green, resembling the scaly leaves of a juniper. In 1-2 more years the plant matures, producing small inconspicuous flowers which give rise to seeds. The seeds mature in midsummer to late autumn, depending on species, and are ejected by a spring-like mechanism. Most seeds land within 15 feet of the source, but seeds can travel as far as 50 ft. Birds and tree squirrels can also disperse seed to distant trees and treetops when the seed sticks to feathers or fur. Once a mistletoe plant develops high in a tree crown, susceptible understory trees will likely be infested.

Dwarf mistletoe stresses trees by using water and consuming sugars. It causes wood to be weak and distorted. Trees infested with mistletoe may show symptoms of dieback, broom-like growth patterns, and reduced vigor. Heavy mistletoe infestation predisposes trees to attack from insects and diseases. The resulting accumulation of dead wood constitutes a fire hazard.

The most effective method of mistletoe control remains pruning to remove infested branches and stems. When pruning, be sure to cut a foot or two below the clump because mistletoe roots grow some distance within the branch. Cut to a lateral branch or remove the branch entirely rather than leaving a stub. Cutting only the mistletoe will give temporary control; however, mistletoe shoots will regrow from the roots lodged in the host tree. If a tree is heavily infested, it may be best to remove the whole tree to prevent spread to surrounding trees in the forest. There are other strategies for management in forested land where number and size of trees make individual attention difficult, but these strategies are variations on the same themes of pruning branches and roguing heavily infested trees.

Ethephon is a chemical which releases ethylene, a naturally occurring plant growth regulator. Ethephon is the active ingredient in FlorelTM, and that product has been registered for dwarf mistletoe control in conifers. Ethephon can be applied in autumn to dwarf mistletoe as berries ripen, causing the berries to fall before seed dispersal occurs. However, on tall trees coverage with a sprayer may be difficult or impossible, so treatments are usually limited to small high-value trees. Ethephon cannot be expected to kill the dwarf mistletoe plants outright, especially with only one application.