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
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
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
Home | Back to Desert Gardening
| Back to Newsletter