Arceuthobium pusillum is most damaging in black spruce [Picea mariana] stands ... Although white spruce (Picea glauca) and red spruce (Picea rubens) are also highly susceptible to this parasite, eastern dwarf mistletoe is not as common on these trees ... Other hosts include eastern larch (Larix laricina), jack pine (Pinus banksiana), eastern white pine (Pinus strobus), red pine (Pinus resinosa), balsam fir (Abies balsamea), and blue spruce (Picea pungens), but these hosts are only infected when growing near other infected species of spruce ...

[After two or more years of latency, the first indication that a host is infected] is a swelling at the point of infection. Buds proliferate at this point, giving rise to a witches' broom, a compact mass of branches and twigs ... If the tree is vigorous, the infected tissue may have longer internodes than uninfected tissue. Initial growth of the witches' broom may be quite vigorous. Aerial shoots typically appear 4 years after infection, and these produce flowers and fruits [a year later].

Witches' brooms ... live as long as the host remains alive and may reach 3-10 feet (1-3 m) in diameter. Branches in these brooms may have several mistletoe plants on them. Uninfected tissues decline first, until nearly all the foliage is contained in the witches' brooms. and the tree is near death. The upper crown often dies first, so severely infected trees usually have dead tops. After a tree dies and loses its needles, basal cups on branches allow observers to distinguish mistletoe brooms from those associated with spruce broom rust (Chrysomyxa arctostaphyli).

[click below for photos]

- Baker, F.A., J.G. O’Brien, R. Mathiasen, & M.E. Ostry: (2006) Eastern Spruce Dwarf Mistletoe^©

Reference: https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev2_043490.pdf