I frequently receive e-mail inquiries about controlling mistletoes (Phoradendron species) growing on cultivated trees. Many of these questions are from people in the southeastern U.S. where Phoradendron serotinum (= P. leucarpum) is common. The most common mistletoe that causes problems on oaks from California is P. villosum. The following table (modified from Scharpf & Hawksworth 1974 and Hawksworth & Scharpf 1981) lists the Phoradendron species found in the U.S.
| Phoradendron | Hosts | Distribution | Remarks |
|
P. serotinum [= P. leucarpum, = P. flavescens] |
More than 100 species of hardwoods | Southward from central Texas to S. Illinois and New Jersey | Often locally restricted to particular host species |
| P. tomentosum ssp. tomentosum | Hackberry (Celtis) and mesquite (Prosopis) | Texas and Oklahoma | Similar to P. serotinum. Leaves less than 28 mm long |
| P. tomentosum ssp. macrophyllum | More than 60 species of hardwoods (but not oaks) | California to west Texas | Leaves greater than 28 mm long |
| P. villosum ssp. coryae | Oaks (Quercus) | West Texas to western Arizona | Minor differences (hairs) between this and other subspecies |
| P. villosum ssp. villosum | Oaks, manzanita (Arctostaphylos), buckeye (Aesculus) and others | California and Oregon | Leaves hairly, 1.5-4.5 cm long by 1-2.2 cm wide. |
| P. californicum | Legume trees and shrubs | Southwestern U.S. | Leaves reduced to small scales |
| P. rubrum | Mahogony (Swietenia) | Florida Keys | Caribbean; rare in U.S. |
| Phoradendron | Hosts | Distribution | Remarks |
| P. juniperinum ssp. juniperinum | Juniper (Juniperus) | Oregon, Utah, Colorado south to Mexico | Leaves reduced to small scales (appearing leafless). Forms globose clusters |
| P. juniperinum ssp. libocedri | Incense cedar (Calocedrus decurrens) | Oregon to Baja California, Mexico | Leaves reduced to small scales (appearing leafless). Pendulous |
| P. densum | Cypress (Cupressus) and Juniper (Juniperus) | Southern Oregon to Baja California; outlier in central Arizona | Leaves 1-2 cm long by 3-5 mm wide |
| P. boleanum ssp. pauciflorum | White fir (Abies concolor) | Central California to Baja California, Mexico; outlier in southern Arizona | Leaves 1.5- 3 cm long, 5-8 mm wide |
| P. capitellatum | Junipers (Juniperus) | Arizona and southwestern New Mexico (rare) | Leaves hairy, 8-14 mm long by 1-2 mm wide |
| P. hawksworthii | Junipers (Juniperus) | New Mexico, west Texas to Mexico | Leaves 6-20 mm long by 1.5-3 mm wide |
Although control of Phoradendron in forest situations is very difficult, homeowners with a few infected trees do have some options.
Pruning. Removing entire trees that are severely infected is an option, albeit an undesirable one in many cases. Removing infected branches is effective, but may not be practical or esthetic because of the size of the tree, number of branches to be pruned, etc. The shoots (stems and leaves) of the mistletoe can also be removed which, if done before seeds are set, will help reduce the number of new infections originating from this seed source. Mistletoe shoots will regrow from the pruned infection area, so this method is only a temporary solution. It is also important to remember that this method does not prevent the introduction of seeds from other infected trees (your neighbor's!).
Covering Infected Areas. As well as removing the shoots, it has been suggested that the affected branch can be covered with an opaque material (tar paper, creosote, duct tape, black polyethylene, etc.). These methods have not been shown to be particularly effective and are certainly not esthetically pleasing.
Resistant Trees. A very sound approach to control is to plant trees known to be resistant to the Phoradendron species that is causing problems in your local area. Of course this does not help when an existing infected tree is already present, but should be considered for new home landscaping. I've noticed that different trees respond differently to mistletoe pressure. For Phoradendron serotinum, there have been extensive publications (by T. E. Hemmerle, mainly in Tennessee) about host races. This means that there are locally adapted strains of the mistletoe that prefer (say) maples over elms, even though elms may be heavily infected in other locations. In residential areas of Norfolk Virginia, Lytton Musselman pointed out to me cases where maples were severely infected (and killed) but the surrounding infected oaks were doing much better. So, keep this in mind when planting new trees.
Chemicals. In California, the plant growth regulator ethephon (Monterey Florel brand) has been used to control mistletoe in dormant host trees. The important word here is dormant. If this chemical is used on trees with leaves present, one runs the risk of killing the tree. The chemical spray must thoroughly wet the mistletoe foliage to be effective. Spraying provides only temporary control, because after the shoots fall off, the mistletoe will regrow from the infection sites. Remember that you should be very cautious when using chemicals of this type (read the labels, get advice from local extension agencies, etc.).
It is important to assess the situtation with your tree(s) on a local scale. Is the mistletoe actually killing the tree? In many cases, a tree may support a mistletoe population, but it does not seem to show major pathological effects. When you see a large, mature tree with a sizeable number of mistletoes growing on it, bear in mind that this tree has likely been infected for many decades and has not died. If the tree dies after living 100 years, can we place the blame on the mistletoe or is this within the range considered normal for the lifespan of that tree species? As a plant biologist interested in parasitic plants, when I look at a tree heavily infested with Phoradendron, I say "Wow, look at that wonderful population of Phoradendron!" When others look at it, they may say "Oh, that poor tree!" It's a mindset thing. I know that there is not much one can do about a tree with all that mistletoe, so I try to appreciate the marvelous coevolutionary relationship that is taking place.
For another web site that describes the mistletoe life cycle and control measures, see:
Scharpf R.F., Hawksworth F.G. 1974. Mistletoes on hardwoods in the United States. : Rocky Mt. Forest and Range Experimental Station, Ft. Collins, Colorado; Forest Pest Leaflet 147. U.S. Government Printing Office: 1974 O-547-468. 7 pages.
Hawksworth F.G., Scharpf R.F. 1981. Phoradendron on conifers. : Rocky Mt. Forest and Range Experimental Station, Ft. Collins, Colorado; Forest Insect and Disease Leaflet 164. U.S. Government Printing Office: 1981 O-353-985. 7 pages.