Mistletoe

Mistletoe is a plant parasite of trees – both deciduous and conifer. Mistletoe on deciduous trees typically attach to stems, are capable of photosynthesis, and are parasites of the xylem. Mistletoe on conifers produce scales instead of leaves; the seedlings are capable of photosynthesis, but the adult plants are dependent on their host, parasitizing both xylem and phloem. The mistletoe causes distortions in the conifers, called witches’ brooms. Mistletoes are dioecious, meaning they produce male (pollen) and female (berries) reproductive structures on separate host trees.

Mistletoe in a sycamore

The mistletoe is easiest to see in deciduous trees, when they have lost their leaves in the fall. The mistletoe is evergreen and appears as a leafy mass hanging off a branch. Here in the Bay Area, we frequently see mistletoe in landscape trees. Broadleaf mistletoe (Phoradendron macrophyllum) infests alder, Aristocrat flowering pear, ash, birch, box elder, cottonwood, locust, silver maple, and walnut. P. villosum infests oaks. Arceuthobium spp. infests firs and pines.

Detail of mistletoe in a sycamore tree

Birds feed on the berries and excrete the partially digested seeds, which fall on branches. When a seed germinates, it sends haustoria into the bark. The haustoria “takes root”, tapping into the xylem and/or phloem transport systems and consuming nutrients and water. The mistletoe grows slowly at first, but becomes embedded deep in the branch. The host tree may not look as healthy as uninfected trees, since it is sharing its nutrition with its parasite. A tree can support a mild infestation, but individual branches may weaken; a heavily infested tree may eventually die, especially when the tree is stressed by environmental conditions such as drought.

For management tips, see the Integrated Pest Management (IPM) site. To manage the problem you must contain its spread as quickly as possible. Prune infested branches at least one foot below the point of attachment. For infestations on a branch or trunk, this may be a problem. The IPM site recommends wrapping the area with wide, black plastic to cut off the light source for the mistletoe, causing it to eventually die. Chemical treatment with a plant growth regulator (ethephon, or Monterey Florel Brand) is possible. IPM recommends planting resistant tree species, including Bradford flowering pear, Chinese pistache, crape myrtle, eucalyptus, ginkgo, golden rain tree, liquidambar, sycamore, and conifers such as redwood and cedar.