Mistletoe is a distinctive plant, particularly noticeable in winter when its evergreen clumps stand out against bare tree branches. It is an obligate hemiparasite, meaning it must grow on another plant, its host, to survive. Mistletoe attaches directly to the host tree or shrub, extracting water and various nutrients. Over 1,000 species exist globally, each adapted to different environments and host plants.
Preferred Host Trees
Mistletoe species exhibit diverse host preferences, targeting specific tree species. European mistletoe (Viscum album) is a widespread species capable of parasitizing over 200 different tree and shrub species. It is commonly observed on broadleaf trees such as apple, lime, poplar, sycamore, ash, hawthorn, rowan, and willow.
In North America, the native American mistletoe (Phoradendron leucarpum) frequently infests oak trees. It also thrives on other deciduous hardwoods, including maples, ashes, elms, birches, cottonwoods, hickories, and black walnuts. Dwarf mistletoes (Arceuthobium species) primarily target coniferous trees. These species show high specificity; for example, a dwarf mistletoe on a ponderosa pine typically only infects other ponderosa pines or closely related conifers like firs, junipers, and hemlocks.
How Mistletoe Attaches and Feeds
Mistletoe establishes itself on a host tree through a specialized structure called a haustorium. After a mistletoe seed germinates on a tree branch, the developing plant produces this root-like organ. The haustorium then penetrates the host tree’s bark and cambium layers, reaching its internal vascular system.
Once inside, the haustorium forms connections primarily with the host’s xylem, which is the tissue responsible for transporting water and dissolved minerals from the roots to the rest of the tree. This allows the mistletoe to draw the water and inorganic nutrients it requires directly from the host. While mistletoe is hemiparasitic, performing some photosynthesis for its own organic compounds, it relies on the host for water and mineral intake. Some mistletoe species, particularly those classified as holoparasites, can also tap into the host’s phloem, which transports sugars and other organic nutrients.
Effects on Host Trees
The presence of mistletoe can have varying impacts on its host trees, depending on the extent of the infestation and the health of the tree. For mature and otherwise healthy trees, a light infestation of mistletoe may not cause significant harm. However, heavy or prolonged infestations can lead to a decline in the host tree’s vitality.
Such infestations can result in reduced growth, stunting, and weakened branches over time. Trees heavily burdened by mistletoe may also become more susceptible to other environmental stressors, including drought, diseases, or insect attacks. In severe situations, especially with aggressive species like dwarf mistletoes, the infestation can cause branch dieback or, in some cases, lead to the eventual mortality of the entire tree. The parasitic action diminishes the host tree’s photosynthetic capacity and can reduce its carbohydrate levels, further compromising its health.