Mistletoe, Phorandendron spp.

Leininger, T.D; Solomon, J.D.; Wilson, A. Dan; Schiff, N.M. 1999. A Guide to Major Insects, Diseases, Air Pollution Injury, and Chemical Injury of Sycamore. Gen. Tech. Rep. SRS-28. Asheville, NC: USDA Forest Service, Southern Research Station. 44 p.

Importance.-Extensive infections and mortality are not common in sycamore trees. Infections usually occur in openly grown trees. Phoradendron plants are commonly used as greenery in Christmas decorations.

Identifying the Injury.-An affected branch may have a gall or swelling at the site of infection, and multiple infections may retard growth. Stunting and dieback may occur beyond where the mistletoe plant is attached to its host.

Figure 38a. - Closeup of a mistletoe plant.

Identifying the Parasite.-Mistletoe has stout, woody, green stems and darkgreen, leathery leaves (fig. 38a). It is seen most easily in winter growing on scaffold and minor branches and twigs of the host (fig. 38b). The plant has opposite branching, inconspicuous flowers, and produces white berries in the fall and winter.

Biology.-Mistletoe is a perennial evergreen plant that grows on tree branches from which it draws water and nutrients. Its seeds are covered with a sticky, gelatinous coating and are spread by birds and animals. Seeds germinate where they lodge on young branches, and a rootlike structure penetrates the branch and produces a mistletoe plant. One species, P. serotinum, affects many broadleaf tree species in the South and East but is limited by temperature in its northern range (Kansas to New Jersey). Several other species of Phoradendron occur in the West.

Figure 38. - Mistletoe infections in a mature sycamore.

Control.-Control of mistletoe in sycamores is normally not needed but can be done by cutting the host branch at least 30 cm below the point of infection.