Extension > Garden > Yard and Garden > Trees and Shrubs > Diseases of spruce trees in Minnesota > Eastern spruce Dwarf Mistletoe
Eastern spruce Dwarf Mistletoe
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Photo by J.O'Brien USDA FS
Bugwood.org
Dwarf mistletoe is a parasitic plant (Arceuthobium pusillum) that most commonly attacks black spruce (Picea mariana) in northern Minnesota, although white spruce (Picea glauca) are also highly susceptible to the parasite. Dwarf mistletoe can also attack white pine, red pine, jack pine, eastern larch, balsam fir, and Colorado blue spruce. These trees are typically only infected with dwarf mistletoe when planted close to groups of spruce infected with the parasite.
Photo by J.O'Brien USDA FS
Bugwood.org
Identification
- Witches' brooms, a clump of small weak branches arising from one point on a larger branch, form in infected trees.
- Needles within the witches' broom remain green, needles on the rest of the tree yellow and fall off, typically from the top of the tree down.
- Short (1/2 - 1 inch) brown to orange dwarf mistletoe stalks can be seen during the growing season but fall off after seed dispersal in August or September. This occurs only after 4-5 years of infection.
Photo by J.O'Brien USDA FS
Bugwood.org
Biology
Dwarf mistletoe is a parasitic plant that lives its entire life within the canopy of the tree. Female flowers shoot seeds, coated in a sticky layer, up to 55 feet away. Seeds can also stick to birds or other wildlife and be carried to trees much farther away. If these seeds land on a spruce branch, they germinate and colonize their new host. Root-like structures develop within the tree branches and rob the tree of nutrients and water. Witches' brooms develop on infected branches, and as the infection continues, the dwarf mistletoe continues to steal nutrients, and the spruce tree begins to decline. After 4-5 years the first dwarf mistletoe shoots appear on the infected spruce branches. These short (less than 1 inch) branches are yellow orange in color and have only small scale like leaves.
Management
- Witches' brooms can be pruned out to improve the look of the tree and reduce the spread of the dwarf mistletoe. Often however, the dwarf mistletoe reappears in other parts of the tree within 2-3 years from deep root-like structures or new infections.
- Trees infected with dwarf mistletoe with over 50% dead branches should be removed.
- Do not plant spruce or other susceptible conifers near infected trees.
