Western Dwarf Mistletoe

Another witches’ broom seems like an appropriate Halloween post.

Witches’ brooms are caused by a variety of agents, rust fungi (“Juniper Witches’ Brooms” on 03-10-14), mildews, viruses and mistletoe plants, among others. Mistletoe is host specific. I previously wrote about mistletoe on California black oak (“Mistletoe” on 12-22-2011).

Members of the genus Arceuthobium are known as dwarf mistletoes. The pictured witches’ brooms, photographed on ponderosa pines (Pinus ponderosa) in Lassen Volcanic National Park (Shasta County CA), are western dwarf mistletoe (A. campylopodum). A parasitic plant, western dwarf mistletoe grows on two needle and three needle pines in British Columbia, the Pacific Coast States, Idaho, Montana and Nevada.

Western dwarf mistletoe causes elliptical, integral stem swellings and witches’ brooms on the host pines. The plants are dioecious (male and female reproductive organs are on different plants). The witches’ brooms are composed of succulent, jointed, bright orange-yellow (female) or brownish (male) stems. The leaves are reduced to scales. The fruit contains a single seed that is covered in a sticky, glue-like substance called viscin. As the seed matures the hydrostatic pressure in the fruiting body builds up until fruit explodes, forcefully ejecting the seed. If the seed lands on and sticks to a suitable host it will cause a new infection.

As a parasite, western dwarf mistletoe draws its nutrition from its host. The mistletoe haustoria (tip of root) penetrates into the vascular xylem and phloem tissues and draws nourishment from the infected pine. Western dwarf mistletoe has no connection to the soil and water. In addition to carbohydrates and minerals, this parasite also takes water from the host plant by transpiring faster than the host. With less water pressure in the mistletoe, the water moves toward the mistletoe.

Western dwarf mistletoes do have a small amount of chlorophyll and can fix carbon. However, studies show that their photosynthetic rates are very low.

Western dwarf mistletoe can harm its host by stunting tree growth, reducing cone and seed production and making the host more susceptible to insects and pathogens.

Happy Halloween!

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