Plant Diseases - Flowering Plant Parasites

For the next several posts, I’m taking a look at more diseases that affect plants. Previously we looked at bacteria; this time we're looking at flowering plant parasites that live off other plants in the Kingdom Plantae! For sources I’m using Essential Plant Pathology, the U.C Davis Integrated Pest Management (IPM) website, and information from my plant diseases course at Merritt College, taught by Dr. Ann Northrup in Fall 2012.

Plant parasites are categorized as eukaryotes, and have a fairly normal life cycle, in that seeds germinate and develop into plants with flowers that are pollinated and produce seeds. The difference is that instead of roots, the plants develop an absorption structure called haustoria, which enters an opening in a plant’s root, stem, or branch, and grows as a parasite. Seeds can exist in the ground for decades, until they are in proximity of an appropriate host.

Mistletoe growing on a Palo Verde stem
in Joshua Tree National Park

There are two main categories of flowering plant parasites (some do not fit these categories):

• Hemiparasites – these have green leaves and can produce their own food using photosynthesis. They absorb water and minerals from their host, so are mainly xylem parasites.
• Holoparasites – these do not produce their own food using photosynthesis, so they depend on their host for water, minerals, and nutrients. They are both xylem and phloem parasites.

There are four major types of parasitic flowering plants – broomrapes, dodders, witchweeds, and mistletoes. I’ll focus on mistletoe.