Eastern spruce dwarf mistletoe, Arceuthobium pusillum, is a flowering plant which parasitizes black spruce, Picea mariana, and is the causal agent of its most serious disease. This parasite enjoys a broad range, thriving throughout the southern Canadian provinces, from Minnesota east to Maine and south to Pennsylvania. However, it is rarely found in New Jersey, Connecticut, Rhode Island or Massachusetts. Although the mistletoe can affect many species, it is most damaging when invading spruce stands. Red and white spruce (Picea rubens and P. glauca, respectively) are also highly susceptible to Arceuthobium pusillum, but they rarely occur in pure stands and therefore endure less damage.
Dwarf mistletoes are small, parasitic plants bearing seeds. Eastern dwarf mistletoe has green to brown aerial shoots, no secondary branching and leaves reduced in size to basically scales. They produce flowers from late March to early June, with the peak being April and May. It is a dioecious plants; male and female flowers are produced on separate plants, pollination of female flowers is accomplished via wind and insects. While dwarf mistletoe do have chlorophyll for nourishment, they obtain the majority of their nutrients endophytically from the host plant. The parasite’s endophytic system is a network of cortical strands (root-like) which grows within the tree’s bark and wood, it will remain alive as long as the host plant is alive.
The most easily identifiable and common symptom of an Arceuthobium pusillum occurrence is the presence of witches brooms which may reach 3 to 10 feet in diameter. Branches bound in these brooms may be host to several parasitic plant individuals. Counterintuitively, uninfected tissues begin to decline first, leaving the tree with only witches brooms and near death. Trees with severe infestations will have dead crowns before the undergrowth dies. Trees succumb very quickly to mistletoe infestations, it is estimated that, uncontrolled, the parasite will grow 2.4 feet per year and will spread through a stand at an alarming 4.7 feet per year. Local spread results from explosive seed discharges and is aided by wind, insects and animals.
Arceuthobium pusillum infection increases tree mortality, severely decreases vigor and resistance, reduces growth rate, diminished lumber values, reduces cone and seed production, increases trees’ susceptibility to other damaging diseases and pests. By siphoning off water and nutrients from the host tree, mistletoe reduces the available resources need by the tree for growth, defense and reproduction. Removal of infected trees is currently the most common management plan; due to the rapid killing mechanism of the mistletoe “mortality centers” are created within spruce stands. The mortality center will continue to grow unless regenerating mistletoe populations are removed. If you suspect a tree on or near your property has a mistletoe infestation or would advice on disease prevention, contact your arborist and visit https://www.savatree.com/tree-disease-treatment.html.
F. A. Baker, J.G. O’Brien, Robert Mathiasen and M.E. Ostry: Eastern Spruce Forest Mistletoe Insect and Disease Leaflet, NA-PR-04-06 (US Department of Agriculture Forest Service)