Parasites of San Diego County

This blog may not be about what you think.

In San Diego County there are two common families of plant parasites that are easy to spot, once you know what to look for. The first type is the mistletoe. Yes, the Christmas mistletoe is a parasitic plant. The second type is called Witch’s Hair.

Probably the most widespread mistletoe in California is Phoradendron, a name derived from two Greek words meaning “tree thief.” Different species of mistletoe tend to favor a single species of tree as its host. The Phoradendron villosum favors oak trees, while Phoradendron tomentsoum likes our native California sycamores. Have you ever looked at a bare tree in winter and wondered why it has some balls of green leaves? Those are mistletoe. They are evergreens.

Mistletoe on a California Sycamore behind Fulano’s house.

Mistletoe seeds are covered with a glue-like substance that sticks to the bills of birds. When birds try to clean their bills, the seeds adhere to the limbs of other trees and shrubs. The seeds also pass through the bird’s digestive tract and are transported from one bush to another in the bird’s droppings. In fact, this probably explains the derivation of the word mistletoe: from two Germanic words: mista (dung) and tan (twig); referring to bird droppings on a branch or stem. Apparently when the word mistletoe was first used in Europe, people were already aware of the dispersal of mistletoe seeds by birds.

Close-up of Mistletoe. Mistletoe are not full parasites as they have
chlorophyll and can manufacture some of their own nutrients
 while relying on the host for the rest.
Mistletoe on a California Sycamore

The other common plant parasite is the Witch’s Hair, a member of the morning-glory family (Convolvulaceae). It is commonly called dodder. There are eight native species of dodder in California, often confined to one or relatively few host shrubs. Dodders resemble tangled masses of orange, spaghetti-like strands twining over shrubs. The orange stems of dodder are without chlorophyll and are non photosynthetic. Therefore, dodder is an obligate parasite that is completely dependent on its host for water and nutrients. The dodder taps in on its host’s nutrient supply with a modified root system that penetrates the stem tissue of the host. It has been estimated that the total length of twining branches produced by a single dodder plant may exceed half a mile.

Witch’s Hair
Close-up of Witch’s hair
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