Mistletoe is an obligate hemiparasitic plant, entirely dependent on a host tree for water and mineral nutrients. The range of trees it can successfully colonize is limited by the compatibility between the parasite and the host’s internal structure. The host tree provides the necessary resources for the mistletoe to grow its dense, evergreen clumps high in the canopy. The specific species of mistletoe and its geographic location determine which trees it targets for this partnership.
Primary Host Species
Mistletoe species target specific groups, predominantly broadleaf deciduous trees and certain conifers. The most recognized American species, Phoradendron leucarpum, frequently infests hardwoods across the eastern and southern United States. Trees such as oaks (Quercus species) are highly susceptible and are often the namesake for the common name “oak mistletoe.”
Beyond oaks, this leafy mistletoe readily colonizes other deciduous hosts, including elms, hickories, pecans, ashes, and sycamores. These trees provide the parasite with a reliable source of water and dissolved minerals, particularly during the winter months when the host has shed its leaves. Trees with dense, hard wood or unique chemical defenses are rarely colonized by this common type of mistletoe.
Regional Variations in Mistletoe Types
The identity of the host tree is determined by the specific mistletoe species present, which are often geographically distinct. In North America, the two primary groups are the leafy American mistletoe (Phoradendron spp.) and the smaller dwarf mistletoe (Arceuthobium spp.). The American variety favors broadleaf trees in the eastern and southern regions, while dwarf mistletoe is a concern for conifer forests in the West.
Dwarf mistletoe is highly specialized, with individual species often restricted to a single genus of conifer, such as pines, firs, or junipers. For instance, the species attacking Ponderosa Pine rarely spreads to Douglas Fir. European Mistletoe (Viscum album), native to Europe and western Asia, frequently colonizes fruit trees, cultivated apples, poplars, willows, and hawthorns. This European species also has subspecies that specifically target conifers, such such as V. album subspecies austriacum on pines and V. album subspecies abietis on firs.
The Parasitic Mechanism
Mistletoe establishes its parasitic connection through a specialized root-like structure known as a haustorium. When a sticky seed lands on a host branch, it germinates and develops this organ, which penetrates the bark and grows into the host’s vascular tissue. The haustorium fuses with the tree’s xylem, the tissue responsible for transporting water and dissolved mineral nutrients from the roots to the canopy.
As a hemiparasite, mistletoe performs its own photosynthesis to create sugars but steals water and minerals directly from the host’s xylem. Mistletoe often exhibits a higher rate of transpiration than the host tree, which creates a stronger water pressure gradient. This effectively siphons resources from the host and requires the parasite to select a host with a compatible and reliable internal plumbing system.
Effects on Tree Health and Management
The parasitic draw of mistletoe places continuous stress on the host tree, especially when infestations are heavy or the tree is weakened by drought or disease. The most noticeable effect is a reduction in growth and overall vigor, as the parasite monopolizes water and nutrients. Severe infestations can cause branch dieback, known as “flagging,” where portions of the branch distal to the infection slowly perish.
Dwarf mistletoe infections often result in dense, tangled masses of wood and branches called “witches’ brooms.” These deform the tree’s crown and make it more susceptible to wind damage. The most effective physical control involves pruning the infected branch at least 12 inches below the point of attachment to ensure complete removal of the internal haustorium. Chemical control, such as the growth regulator Ethephon, can be applied during the host’s dormant season to cause the mistletoe to shed its leaves. However, chemicals do not kill the embedded haustorium, leading to eventual regrowth.