Parasitic plants establish a unique relationship with other plants, deriving sustenance directly from their hosts. This biological interaction allows them to obtain water and nutrients they might not otherwise access. One such plant, mistletoe, attaches to trees and extracts resources, significantly impacting host health.
Meet the Tree Parasite
Mistletoe is an obligate hemiparasitic plant that grows on trees and shrubs. It often appears as a dense, spherical clump of evergreen foliage, particularly noticeable on deciduous trees once their leaves have fallen in winter. Mistletoe species vary, but generally have smooth-edged, oval, evergreen leaves and waxy, often white, berries produced in clusters.
Mistletoe attaches to its host tree using a specialized root-like structure called a haustorium, which penetrates the host’s bark and vascular tissues. This haustorium anchors the mistletoe and absorbs water and nutrients from the host. The seeds, often dispersed by birds, contain a sticky substance that helps them adhere firmly to tree branches, allowing the haustorium to develop and establish the parasitic connection.
How Mistletoe Drains Tree Health
Mistletoe extracts water and mineral nutrients from its host tree through its haustorium, which connects to the host’s xylem. While many mistletoe species perform some photosynthesis, they rely on the host for water and inorganic nutrients. This continuous drawing of resources negatively impacts the host tree by siphoning off vital supplies.
The presence of mistletoe can result in reduced growth and vigor in the host tree, with infected branches sometimes displaying symptoms such as yellowing leaves and wilting. Severe infestations can weaken tree branches, making them more susceptible to breakage, particularly during storms or heavy snowfall. Over time, this drain on resources can increase the host tree’s vulnerability to other stressors like drought, disease, and insect infestations. In instances of heavy infestation, especially in trees already stressed, mistletoe can contribute to significant decline or even mortality of the host.
Understanding Mistletoe’s Toxicity
Mistletoe is widely recognized for its toxic properties, particularly its berries, though the severity of poisoning can vary depending on the mistletoe species and the amount ingested. American mistletoe contains toxic proteins, while European mistletoe contains viscotoxins. All parts of the plant, including leaves, stems, and berries, are considered toxic.
Symptoms of mistletoe poisoning in humans and pets typically involve gastrointestinal upset, such as nausea, vomiting, and diarrhea. While unpleasant, severe poisoning from ingesting small amounts of American mistletoe is considered rare. However, European mistletoe is generally regarded as more toxic, with reports of more serious effects from concentrated doses. It is always advisable to contact a poison control center if ingestion is suspected.
Controlling Mistletoe Infestations
Managing mistletoe infestations primarily involves mechanical removal, which is considered the most effective method for control. Pruning affected branches is a direct approach, requiring cuts to be made at least one foot below the visible mistletoe attachment point to ensure the removal of the embedded haustoria. If the mistletoe is located on a main limb or trunk that cannot be pruned, cutting off the visible plant and then wrapping the area with black polyethylene plastic to exclude light can be effective, as broadleaf mistletoe requires light to survive.
Consistent removal of visible mistletoe growth, even without complete branch removal, can reduce the spread by preventing the plant from producing seeds. Severely infested trees, where a large portion of the canopy is affected, may require extensive pruning over several years or, in some cases, complete removal to prevent further spread to other trees. To prevent new infestations, planting tree species that are less susceptible to mistletoe can be a useful long-term strategy.