California Parasitic Plants

The most recent flora of California listed 45 species of parasitic dicotyledons that are native or introduced to the state, including the introduced European mistletoe, Viscum album. In Los Angeles County, species of two groups are most commonly seen: the twining vines of dodder (genus Cuscuta) and the woody mistletoes of the genus Phoradendron. In coniferous forests you may find the golden-colored dwarf mistletoes of the genus Arceuthobium (12 species). Much more difficult to find are the secretive root parasites of the Orobanchaceae and Lennoaceae, which are only rarely seen, and only botanists are ever likely, if persistent enough, to find Pilostyles thurberi, very uncommon stem parasite of a leguminous desert shrub. The flora also lists about 70 species of hemiparasites occurring in California, all but one species belonging to Family Scrophulariaceae. Of the hemiparasites, owl's clover and Indian paintbrush (Castilleja) are very common herbs. This combined list of parasites and hemiparasites comprises less than 2% of the total species in California, but collectively they provide a fascinating life style.

There are five species of Phoradendron that can be easily observed during casual walks and drives through Southern California.

  1. Attacking the California sycamore (Platanus racemosa) on the UCLA campus and throughout the urban region and canyons is the big-leaf mistletoe, Phoradendron macrophyllum, but this species also occurs on many other deciduous trees, including alder (Alnus), cottonwood (Populus), and walnut (Juglans).
  2. On woody stems of oaks (Quercus) of oak woodland can often be found oak mistletoe, Phoradendron villosum, which has green leaves approximately the same size as in the big-leaf mistletoe.
  3. Much narrower leaves characterize fir mistletoe, Phoradendron pauciflorum, which occurs often around 2000 meters in yellow pine forest growing on white fir, Abies concolor.
  4. On the eastern, desert-facing slope of our mountains occurs pinyon-juniper woodland and in neighboring Joshua tree woodland. Here the California juniper, Juniperus californica, is a dominant plant species, and often this gymnosperm is heavily infested with juniper mistletoe, Phoradendron juniperinum. This stem parasite has shorter and narrow leaves, and from a distance may appear like the host plant, but the host has instead very tiny, scale-like leaves.
  5. In the deserts of Southern California are tree legumes, notably the species of mesquite (Prosopis), palo verde (Cercidium), ironwood (Olneya), and catclaw acacia (Acacia), which can be infected by desert mistletoe, Phoradendron californicum. Desert mistletoe, like many desert species, has very much reduced, scale-like leaves on its reddish stems.

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