Forest Insect

& Disease

Leaflet 147

Revised March 2010

U.S. Department of Agriculture • Forest Service

1

Plant Pathologist, USDA Forest Service, Pacific Northwest Region, Central Point, OR.

2

Professor, School of Forestry, Northern Arizona University, Flagstaff, AZ.

Mistletoes on Hardwoods in the United States

Katy M. Mallams1 and Robert L. Mathiasen2

Most people in North America 

know of mistletoes because of the 

Christmas-related traditions that 

surround these parasitic flowering 

plants. Some may also realize that 

mistletoes parasitize trees and can 

damage or kill their hosts. However, 

many are not aware that these parasitic 

plants are also ecologically important 

components of many forests around 

the world. 

In the western hemisphere, mistletoes 

in the genus Phoradendron are 

parasites of both coniferous and 

hardwood trees and shrubs. They are 

most common in the tropics but there 

are several that occur in temperate and 

desert regions of the United States. 

They are generally intolerant of cold 

weather and their northern distribution 

is probably limited by cold winter 

temperatures. This leaflet focuses 

on the species of Phoradendron that 

affect hardwoods. They are sometimes 

referred to as the “true mistletoes” 

or “leafy mistletoes” and are the 

mistletoes that people often collect for 

decoration at Christmas. 

Phoradendron species that parasitize 

conifers are discussed in FIDL 

#164 (Hawksworth and Scharpf 

1981). Conifers are also parasitized 

by dwarf mistletoes in the genus 

Arceuthobium. Another species 

of mistletoe, Viscum album, the 

European mistletoe, was introduced 

in Santa Rosa, California in 1900 

by Luther Burbank, the famous 

horticulturist. It infects a wide range 

of hardwood trees and shrubs, but is 

found only in the vicinity of the area 

where it was originally introduced.

Figure 1. Phoradendron villosum on Oregon 

white oak.
2

Identification

Mistletoes in the genus Phoradendron

typically form round to oblong clumps 

of leafy, evergreen foliage that emerge 

from the branches or trunk of the host 

(Figure 1). There is often a slight 

swelling where the mistletoe shoots 

emerge (Figure 2). In older trees, these 

swellings may become quite large and 

broken open, exposing decayed wood. 

Mistletoe infection in deciduous trees 

is easiest to observe in winter when 

the trees are leafless. 

In the western United States, there 

are five species of Phoradendron

affecting hardwoods, found mostly 

in the Southwest and California. One 

species, Phoradendron villosum, 

occurs as far north as northern 

Oregon. Only two species of 

Phoradendron occur in the eastern 

United States: Phoradendron rubrum, 

affecting mahogany in southern 

Florida, and Phoradendron serotinum, 

which affects over one hundred 

different species of hardwoods from 

New Jersey south to Florida and west 

to Louisiana, Arkansas, Missouri, 

and then through southern Illinois, 

Indiana, Ohio and much of West 

Virginia. The latter species is also 

found in Virginia, the Carolinas, 

Kentucky, and Tennessee. In general, 

the mistletoe species on hardwoods in 

the United States are best identified 

by their geographic location (Figures 

3 and 4) and host species (Table 1). 

Characteristics of the plants, leaves, 

and fruits can also be used to assist 

with their identification.

Biology

Members of the genus Phoradendron

are green, flowering plants, but they 

are parasites and require a living 

host from which to obtain water and 

minerals. Although they are capable 

of photosynthesis and produce much 

of their required carbohydrates, 

they may also acquire some of their 

Figure 2. Phoradendron villosum shoots 

emerging from slightly swollen branch on 

Oregon white oak.

Figure 3. Geographic distribution of 

Phoradendron villosum, Phoradendron 

californicum, and Phoradendron coryae.
3

Figure 4. Geographic distribution of Phoradendron macrophyllum, Phoradendron tomentosum, 

Phoradendron serotinum, and Phoradendron rubrum.

Mistletoe Hosts and Distribution Remarks

Phoradendron 

serotinum 

Affects more than 100 species in 50 

genera of native and introduced trees. 

Occurs in the eastern United States.

Extends as far west as LA and AR, 

possibly central and south TX. Its 

distribution has not been precisely 

determined.

Phoradendron 

rubrum Affects mahogany in the Florida Keys. Primarily occurs on Caribbean islands southeast of Florida.

Phoradendron 

californicum

Affects trees in the pea family, mainly 

mesquite, palo verde, and acacia in AZ, 

southern CA, and southern NV.

Leaves are small and scale-like, stems 

are reddish to brown, and berries are 

pink to reddish.

Phoradendron 

tomentosum

Affects hackberry and mesquite, 

infrequently affects oak and elm; 

primarily in TX and OK.

Morphologically similar to

P. serotinum, except mature leaves 

are slightly hairy; mature leaves are 

smaller than P. macrophyllum.

Phoradendron 

macrophyllum

Affects more than 70 species in over 30 

genera of native trees. Common hosts 

include willow, poplar, maple, ash, 

walnut, alder, and sycamore; not found on 

oaks. Occurs from central CA through AZ 

into central NM and south along the Rio 

Grande River into west TX.

Mature leaves are large 

(>28 mm long and 18 mm wide). 

Leaves are much larger and smoother 

than P. villosum.

Phoradendron 

villosum

Primarily on oaks, but occasionally affects 

other trees such as alder, aspen, buckeye, 

European birch, locust, manzanita, 

sycamore, walnut, and willow. Southern 

CA into northern OR.

Mature leaves are thick and hairy, 

smaller than those of P. tomentosum, 

and variable in size and shape.

Phoradendron 

coryae

Only on oaks from western TX to 

northwestern AZ.

Leaves are similar to P. villosum, but 

yellow-green instead of dark green. 

Extremely hairy leaves and stems.

Table 1. Mistletoes affecting hardwoods in the United States.
4

carbohydrates from their host trees. If 

the aerial shoots of the mistletoe are 

removed, the root-like structure within 

the host branch can remain alive for 

many years.

These mistletoes are dioecious; there 

are separate male and female plants. 

Female plants produce inconspicuous 

small flowers on spikes. After they 

are pollinated, the female 

plants develop fruits called 

berries which are white, 

pink, or reddish depending 

on the species (Figure 5). 

Male plants also produce 

inconspicuous flowers on 

spikes which produce pollen 

that is transferred to the 

female flowers for pollination 

by a variety of insects. The 

male and female flowers are 

so similar that it is usually 

difficult to determine the sex 

of a plant unless the fruits are 

present on female plants.

Many mistletoes, including 

Phoradendron species, are 

spread by birds that feed 

on the mature berries. The 

berries are round and about 

one-quarter inch in diameter. 

A berry usually contains a 

single, hard seed surrounded 

by a sticky pulp called viscin. 

Birds eat and digest the pulp around 

the seeds and then excrete the seeds 

which can survive passage through 

the bird’s gut. By this means, seeds 

are often deposited on susceptible 

hosts, spreading the mistletoe within 

infected trees or to nearby uninfected 

hosts. A viscous coating and hair-like 

threads on the seeds allow them to 

attach firmly to the host branch. Upon 

germinating, the growing radicle 

becomes tightly pressed to the branch 

surface. Infection takes place by 

means of a specialized structure that 

mechanically forces its way through 

the bark into living host tissue. 

The root-like system of the mistletoe 

grows within the host tissue and 

establishes connections to the water 

conducting tissue (xylem). The aerial 

shoots develop shortly after the 

root-like system has become well 

established. Usually, several years are 

required following infection for the 

shoots to mature and produce flowers. 

Mistletoes on hardwoods do not 

spread rapidly but once a plant is 

established, the root-like system 

slowly extends up and down the 

Figure 5. Shoots and mature fruits of 

Phoradendron macrophyllum.
5

branch. Defoliation or destruction 

of the aerial shoots does not kill 

the mistletoe. New shoots may be 

produced from the root-like system, 

or the mistletoe may survive and 

grow within the infected branch for 

many years without producing aerial 

shoots. Not until the host tree dies or 

the infected branch dies or is removed 

will the mistletoe die.

Impacts

Young or small trees are seldom 

infected by mistletoe. In most cases, 

initial infections occur on large or old 

trees, particularly those growing in 

the open, because birds often prefer to 

perch in the tops of these trees, where 

they excrete the mistletoe seeds. 

Severe buildup of mistletoe often 

occurs within infected trees (Figure 

6) because birds are attracted to and 

may spend prolonged periods feeding 

on the mistletoe berries, roosting and 

excreting the seeds.

In many cases, mistletoe infections on 

hardwoods do not cause significant 

damage in areas where water stress 

is not an issue. However, in some 

situations, trees severely infected 

with mistletoes may be weakened 

and may have greatly reduced growth 

rates. Weakened trees may be attacked 

by insects and often die during 

droughts. Furthermore, the ends of 

infected branches beyond the mistletoe 

infections often die during droughts 

resulting in extensive branch dieback 

throughout the crowns of infected trees.

Branch and trunk swellings or round 

galls often result from mistletoe 

infection (Figure 7). These swellings 

can become entry points for fungi 

that decay and subsequently weaken 

the wood, resulting in breakage and 

Figure 6. Stand of Oregon white oak severely infected by Phoradendron villosum.
6

reduced commercial value. Branches 

with large or numerous mistletoe 

infections may break from the weight 

of the mistletoe plants. Broken 

branches may damage buildings, 

electrical lines, or parked vehicles, 

and injure people, particularly during 

wind or ice storms. In some regions, 

mistletoes damage crops by reducing 

production of fruits or nuts, such as in 

pecans in the Southwest and walnuts 

in the Central Valley of California.

Ecologic and 

Economic Importance

Mistletoes have ecologic, economic, 

and social values. Because mistletoes 

usually occur high in canopies of 

trees, they may have evolved intricate 

relationships with birds that serve 

as the principal dispersal agents of 

mistletoe seeds. Berries are a favored 

food of several bird species, including 

robins, bluebirds, thrushes, cedar 

waxwings, Townsend’s solitaires, and 

phainopeplas, especially during the 

winter when other food sources are 

scarce. Birds will frequently defend 

mistletoe plants to prevent other birds 

from eating the nutritious berries. 

The decayed wood in swollen 

branches and trunks provides 

important habitat for cavity-nesting 

birds and mammals (Figure 8). 

Primary cavity-nesting species such 

as flickers excavate holes in the 

softened wood that are later used by 

many other wildlife species including 

squirrels, bluebirds, wrens, nuthatches 

and small cavity-nesting owls. 

Deer, elk and even livestock 

are known to supplement 

their winter diet with lowhanging mistletoe plants. In 

some areas, livestock are fed 

mistletoe shoots as forage; 

although in other areas, the 

shoots and berries may not be 

suitable for livestock because 

the leaves and berries of 

many mistletoes contain toxic 

compounds.

Mistletoe has been associated 

with Christmas for many 

years, and the harvesting 

and sale of mistletoe plants 

for Christmas decorations 

provides seasonal income 

to service organizations and 

others that harvest large 

amounts of mistletoe. Due 

to the toxic compounds in 

the leaves and berries of Figure 7. Swollen trunk on Oregon white oak resulting from 

infection by Phoradendron villosum.
7

some species, decorations containing 

mistletoe should not be eaten or left 

within reach of pets or young children. 

Although rare, cases of mistletoe 

poisoning have been reported in the 

United States.

Management

Management of mistletoes may be 

needed to reduce the level of infection 

or prevent severe infections from 

developing. Valuable trees near private 

homes, businesses, in recreation 

areas, or in commercial orchards can 

be treated to minimize the effects 

of mistletoe infections, reduce the 

number of infections, or even eliminate 

mistletoe when trees are only lightly 

infected. This can be accomplished by 

pruning infected branches, removing 

mistletoe shoots, or applying selective 

chemical treatments. 

Pruning: Pruning is practical when 

a tree does not have many mistletoe 

infections and they are not on primary 

stems or large branches near the 

main bole. Branches with mistletoe 

infections should be pruned at least 

six inches below the point 

of mistletoe attachment. 

Large branches or main 

stems with mistletoe 

infections should only be 

pruned if their removal 

will not detract from the 

appearance of a tree or 

remove a large amount 

of a tree’s foliage. Such 

drastic pruning for 

mistletoe control may 

be more damaging to a 

tree than the mistletoe 

infections themselves. 

The number of mistletoe-infected 

branches that should be pruned will 

vary from tree to tree depending 

on a number of considerations, but 

removing even a few infected branches 

can increase the overall health of a tree 

by reducing the water stress associated 

with mistletoe infection. Keep in mind 

that birds may re-introduce mistletoe 

into pruned trees, and mistletoe plants 

that were small and difficult to observe 

when the tree was initially treated 

may become apparent, necessitating 

periodic retreatment. 

It is generally recommended that trees 

be inspected every three to five years 

so new infections can be pruned as 

they occur. This will prevent the level 

of infection from becoming severe 

and potentially damaging. Detection 

of mistletoe infections is most easily 

accomplished during the winter on 

hardwood trees because the mistletoes 

are evergreen and can be observed 

when the leaves of the trees have fallen 

off. Mistletoe plants attached to pruned 

branches cannot cause new infections, 

so they do not need to be destroyed.

Figure 8. Hole excavated in decayed wood associated with 

Phoradendron villosum trunk infection and swelling in 

Oregon white oak.
8

Removal of Aerial Shoots: Simply 

removing aerial shoots of mistletoe 

from tree branches can also reduce 

the water stress placed on trees. 

Removing the mistletoe shoots does 

not kill the portion of the mistletoe 

plant inside the branch so eventually 

it will sprout new shoots that will 

need to be removed. Trees should be 

inspected every three to five years 

to find and remove new shoots. 

Removing mistletoe shoots may be 

impractical when trees have many 

infections because of the labor 

involved, but it can be combined with 

pruning infected branches. Small 

branches with mistletoe infections 

can be pruned, and mistletoe shoots 

can be removed from large branches 

and main stems. Combining these 

treatments may prolong the life of 

severely infected trees. 

Another method of mistletoe 

management that has been attempted 

but has not proven effective or 

aesthetically acceptable is removing 

mistletoe shoots from infected 

branches or trunks and wrapping 

the infected area in black plastic or 

tar paper. It was thought that this 

would prevent light from reaching 

the infected part of the branch, 

thereby preventing or slowing the 

emergence of new mistletoe shoots. 

Some arborists still recommend this 

practice but it has not been effective 

in most cases.

Chemical Treatments: Several 

chemical treatments have been tested 

for mistletoes affecting hardwoods in 

the United States but many of these 

have not proven effective in killing 

the mistletoe or they have caused 

unacceptable damage to the host trees. 

One chemical that has been tested 

and proven effective is a growthregulating compound, ethephon (trade 

name Florel TM). Mistletoe shoots 

are sprayed with ethephon to the 

point of saturation. In one to three 

weeks, the mistletoe plants drop off. 

However, the portion of the mistletoe 

plant within the tree branch is not 

affected so new mistletoe shoots will 

emerge in one to three years. In many 

cases, this treatment may provide an 

acceptable level of control. As it is 

relatively more expensive than hand 

pruning, the cost of application of 

ethephon should always be considered 

before initiating treatment.

In the Southwest and California, 

pecans and walnuts infected with 

Phoradendron macrophyllum are 

treated with herbicides during 

the winter when the host trees are 

dormant. Because the mistletoe is an 

evergreen and still physiologically 

active during winter, the mistletoe is 

killed when it absorbs the herbicide. 

The host trees are dormant so they are 

not adversely affected.

A technique that has been applied 

in India on mangoes infected by 

mistletoes is to wrap bands saturated 

with herbicide around the base of 

mistletoe plants that have been 

scored with a knife to expose the 

water conducting elements in the 

mistletoe shoots. This method 

has proven effective in killing the 

mistletoe without harming the host 

trees but it has not been tested in the 

United States. Because of the labor 

required to wrap the bands around 

each mistletoe plant, it may prove 

impractical here.
9

Planting Mistletoe-resistant Trees:

When trees have many mistletoe 

infections throughout their crowns 

and/or on their main stems, it may be 

necessary to remove the tree and plant 

another in its place. Planting trees that 

are not susceptible to mistletoes that 

occur in the area is the most effective 

way to prevent infection and is 

probably the best means of preventing 

mistletoes from becoming established 

around homes and businesses in urban 

areas. Therefore, correct identification 

of the local mistletoes and their host 

species is critical. Tree species selected 

to replace mistletoe-infected trees 

should be those that are suited to the 

location and climatic conditions in the 

area. Arborists, nursery personnel, and 

county extension agents can provide 

recommendations regarding which tree 

species are appropriate to plant.

In wildland forests, mistletoe 

reduction can be accomplished by 

harvesting infected trees, and retaining 

and planting non-host species. Pruning 

is only practical for high-value trees 

in recreation areas, around homes 

and businesses, or in orchards where 

the crop is commercially valuable. 

Removal of infected branches by 

pruning or removal of the plants 

will not insure that trees will not 

be infected again. Because birds 

are the principal dispersal agents of 

these mistletoes, they are likely to 

re‐introduce mistletoe onto previously 

treated trees.

Assistance

Private landowners can get more 

information from County Extension 

Agents, State Forestry Departments, 

or State Agriculture Departments. 

Federal resource managers should 

contact USFS Forest Health Protection 

(www.fs.fed.us/foresthealth/). This 

publication and other Forest Insect 

and Disease Leaflets can be found at 

www.fs.fed.us/r6/nr/fid/wo-fidls/. 
10

References

Hawksworth, F. G., and R. F. Scharpf. 

1981. Phoradendron on conifers. USDA 

Forest Service. Forest Insect and Disease 

Leaflet No. 164. 7 p.

Hawksworth, F. G., and R. F. Scharpf. 

1986. Spread of European mistletoe 

(Viscum album) in California. European 

Journal of Forest Pathology 16: 1-5.

Mathiasen, R. L., D. L. Nickrent, D. 

C. Shaw, and D. M. Watson. 2008. 

Mistletoes: pathology, systematics, 

ecology, and management. Plant Disease 

92: 988-1006.

Scharpf, R. F., and F. G. Hawksworth. 

1976. Luther Burbank introduced 

European mistletoe into California. Plant 

Disease Reporter 60: 740-742.

Sinclair, W. A., and H. H. Lyon. 2005. 

Diseases of Trees and Shrubs. Cornell 

University Press, Ithaca, NY. 575 p.

Wiens, Delbert. 1964. Revision of the 

acataphyllous species of Phoradendron. 

Brittonia 16: 11-54.

Acknowledgments

This is a revision of Forest Pest Leaflet 

147, written by Robert F. Scharpf and 

Frank G. Hawksworth in 1974. 

Helpful reviews were provided by 

Max Bennett, Carolyn Daugherty, and 

David Russell. 

Photos: Figures 7 and 8 by David 

Shaw (Oregon State University), all 

others by the authors.
11

Pesticides used improperly can be injurious to humans, animals, and 

plants. Follow the directions and heed all precautions on the labels. 

Store pesticides in original containers under lock and key–out of the 

reach of children and animals–and away from food and feed. Apply 

pesticides so that they do not endanger humans, livestock, crops, 

beneficial insects, fish, and wildlife. Do not apply pesticides when there is danger of 

drift, when honey bees or other pollinating insects are visiting plants, or in ways that 

may contaminate water or leave illegal residues. Avoid prolonged inhalation of pesticide 

sprays or dusts; wear protective clothing and equipment if specified on the container. 

If your hands become contaminated with a pesticide, do not eat or drink until you have 

washed. In case a pesticide is swallowed or gets in the eyes, follow the first-aid treatment 

given on the label, and get prompt medical attention. If a pesticide is spilled on your 

skin or clothing, remove clothing immediately and wash skin thoroughly. Do not clean 

spray equipment or dump excess spray material near ponds, streams, or wells. Because 

it is difficult to remove all traces of herbicides from equipment, do not use the same 

equipment for pesticides or fungicides that you use for herbicides. Dispose of empty 

pesticide containers promptly. Have them buried at a sanitary land-fill dump, or crush and 

bury them in a level, isolated place. NOTE: Some States have restrictions on the use of 

certain pesticides. Check your State and local regulations. Also, because registrations of 

pesticides are under constant review by the Federal Environmental Protection Agency, 

consult your county agricultural agent or State extension specialist to be sure the 

intended use is still registered. 
12

Published by USDA Forest Service, Pacific Northwest Region (R6), Portland, Oregon

FS/R6/RO/FIDL#147-10/001

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