Arceuthobium

Arceuthobium is a genus of small, evergreen parasitic shrubs commonly known as dwarf mistletoes. They are obligate parasites in the Santalales order, frequently placed in the family Viscaceae (though some classifications place them in Santalaceae). The genus comprises numerous species that specialize on conifer hosts, forming broom-like clusters on the branches of pines, firs, spruces, or hemlocks. Unlike many mistletoes, Arceuthobium species are dioecious, with separate male and female plants.

Arceuthobium plants have haustoria that penetrate the host’s bark and cortex to access xylem. They obtain most of their water and minerals from the host, while maintaining photosynthesis in their own green shoots. After flowering, each female produces a tiny, fleshy fruit that ripens into a piercing, pressure-loaded capsule. When mature, the capsule explosively dehisces, ejecting the seed several meters away. Seeds adhere to host twigs with a viscid substance, germinate, and establish a haustorial connection. Infected trees develop witches’ broom, reduced growth and vigor, and may suffer branch dieback or tree mortality in severe cases.

Ecology and management: Arceuthobium infestations can alter forest structure and timber value. They are widely distributed in boreal and montane forests of North America and Eurasia. Management in forestry involves monitoring stands, removing infected branches or trees, and sanitation harvesting to limit spread. There are no practical broadscale chemical controls; prevention relies on early detection and silvicultural practices that reduce host susceptibility. The genus includes many species, each often host-specific, contributing to diverse ecological interactions in conifer ecosystems.