There are several genera and species of mistletoe. In Arizona we have 5 species of Phoradendron (leafy mistletoes) and 3 species of Arceuthobium. The latter, called dwarf mistletoe, infests only conifers. Leafy mistletoes are considered as "hemi-parasites", which means that they produce some or all of their own energy through photosynthesis but depend on the host for water and minerals. Mistletoes elicit a disease response from most hosts and are considered a pathogen. However, mistletoe seldom kill healthy hosts (except dwarf mistletoe, which can cause severe damage in coniferous forests). Severely infested trees usually have been subjected to other stresses that increased their susceptibility such as drought, flooding, soil compaction, nutrient deficiencies, etc.

The "root" of a leafy mistletoe is directly connected to the host's xylem (that part of the plant's plumbing that conducts water and minerals from the roots to the leaves). The "root" of a dwarf mistletoe is connected to the host's phloem as well as the xylem. The phloem conducts sugars and other products of photosynthesis from the leaves to other parts of the plant. So the dwarf mistletoe is highly parasitic, depending on the host of for photosynthate as well as water and minerals.

Leafy mistletoes can occur on several hundred host species. Mistletoe creates a drain on host resources that reduce growth, decreases vigor, and increases susceptibility to other diseases and insect pests. Local symptoms can include dieback, formation of witches' broom, and weakened branches. Dwarf mistletoe, in particular, can cause spiketop, where the entire host crown dies. Dwarf mistletoe also causes witches' broom which increases the diversion of water, minerals, and nutrients to the site of infestation.

Removal of the branch below the mistletoe remains an economical and fairly efficient method of mistletoe "control". However, control by pruning requires diligence. Birds eat mistletoe berries and spread the seed which is unaffected by the digestive tract. So any fruiting mistletoe that survives pruning is a source for reinfestation. Chemical treatment has not been effective to date. In one study 2,4-D was injected in infected eucalyptus trees. The chemical killed 70% to 100% of the mistletoes, but partially defoliated all trees and killed 5%.

Recent research efforts have focused on interrupting fruit set with hormone sprays combined with pruning. Resistant cultivars and biological control remain as possible long term solutions.

Wrapping the mistletoe and infected branch with black plastic to block light and increasing heat has proven somewhat effective. The plastic must remain around the branch for several months.

Paine & Harrison. 1992. HortTechnology 2:34-330.

Author: Robert E. Call, Extension Agent, Horticulture