Desert Mistletoe[1]

By: John R. Leeper

There is a slow, silent killer in Vail.  It sucks the living juices from the vascular system of its victim much like a vampire. But it’s not a vampire, it’s a plant. It’s mistletoe.

There are at least seven different true mistletoe species in the genus Phoradendron in Arizona. Adding to that, there are eight dwarf or false mistletoes in the genus Arceuthobium. While the true mistletoes infect angiosperms (flowering plants), false mistletoes attach themselves to gymnosperms (coniferous plants). Another important difference between the true and false mistletoes is how their seeds are disseminated. However, with both the true and false mistletoes, it may take up to 50 years or more for the plants they infect to succumb to their infection.

Our focus here is on the native mistletoe found in the dryer parts of the Sonoran Desert and commonly referred to as desert mistletoe or mesquite mistletoe. Common names in Spanish include visco (birdlime), secapalo (dry stick), injerto (graft), and chili de espino (spine chili).

The desert mistletoe is known scientifically as Phoradendron californicum.  The generic name, Phoradendron, is a combination of the Greek words “phor” for “thief” and “dendron” for “tree” and well describes mistletoes’ habit of stealing the life juices of the plants they infect. The specific name, californicum, recognizes the state from which this particular species was first described.

Desert mistletoe is most commonly seen as hanging masses of jointed, brittle, green to rust red colored stems infecting mesquite.  It is also known to attack other desert legumes such as ironwood, palo verde and Acacia species, as well as non-leguminous desert hardwoods such as the creosote bush and desert buckthorn.

P. californicum is native to the Sonoran and Mojave Deserts at elevations below 4,000 feet. It is a perennial that adapted to the desert environment by reducing its leaves to mere scales. This adaptation helps the plant limit moisture loss through transpiration.

P. californicum is dioecious, with separate male and female plants. Very small petalless greenish-yellow flowers are reported to bloom from late January through March and emit an amazingly fragrant aroma for their very small size. However, it was not until the second week of March that I could confirm bloom during my weekly excursion into the Cienega Creek Natural Preserve. Confirmation came through observing bees foraging on the nondescript flowers. And, I failed to detect the reported perfume that was attracting the bees to the blossoms. Male flowers produce bright yellow pollen that nectar foraging bees carry to pollenate the female flowers. Pollenated flowers develop throughout the summer months into berries that ripen in the late fall and early winter when they can be seen in rosy clusters.

A cluster of ripening desert mistletoe berries.

Here, we must digress from our main topic to introduce a key player in the perpetuation of the desert mistletoe. While a number of birds may feed on desert mistletoe berries, a crested silky flycatcher by the name of Phainopepla (Phainopepla nitens) is by far the most important. This bird gets its generic name from the Greek words “phain” and “pepla” meaning “shining robe”, which describes the shiny black plumage of the male. While the female has dull-gray feathers both genders possess brilliant ruby red eyes. The berries of the desert mistletoe are one of Phainopepla’s favored winter foods. The berries contain indigestible seeds surrounded by a thin layer of sticky flesh. The Phainopepla consume the entire berry and after passing through the alimentary canal the guano coated seeds are defecated  onto the branches of host plants. Thus the Phainopepla is the principle vector in the dissemination of the desert mistletoe.

A male Phainopepla (Phainopepla nitens) and close-up of head showing ruby red eye.

Desert mistletoe seeds deposited on a mesquite limb by a Phainopepla.

The deposited desert mistletoe seeds germinate and produce root-like structures called haustoria.  The haustoria penetrate the young tender bark of the host plant and enter the host’s xylem where they tap moisture and part of its nutritional needs from its host. With time, the host plant typically develops a swelling around the point of entry.

A swollen mesquite limb around the desert mistletoe point of infection.

P. californicum is not a complete parasite. It is what botanists call a hemiparasite because its green stems contain chlorophyll that conducts photosynthesis and contributes additional nutritional value to the desert mistletoe.

Mistletoes contain toxic proteins, viscotoxin and phoratoxin, that in concentration can slow the pulse, increase blood pressure, and cause convulsions and cardiac arrest. Eating the green parts of any mistletoe may cause gastrointestinal upset and diarrhea. Despite the toxic qualities of the vegetative parts of P. californicum, some desert creatures have evolved or adapted immunity to the toxins effects.  For example, desert mistletoe is the obligate host to larvae of the great purple hairstreak (Atlides halesus) and it also provides winter forage for deer.

While the white berries of some mistletoe species are considered toxic the berries of P. californicum are not. The Native American cultures of the desert Southwest harvested and consumed the translucent ripe desert mistletoe berries from mesquite, ironwood and acacia while they considered berries from palo verde and desert buckthorn to be bitter tasting and inedible. This is not an endorsement to try eating desert mistletoe berries.

Desert mistletoe is an endemic, non-threatened species to the area and plays an important role in the Sonoran Desert ecosystem. Although it is not an aggressive parasite, heavy infestations can and do kill their host plants, especially under xeric or unirrigated conditions. Infestations around the home can be best managed by pruning back infected limbs of the host plant.

A mesquite tree succumbing to a heavy infestation of desert mistletoe.

For those interested in more information on desert mistletoe, conduct a web search on Phoradendron californicum.

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