Parasitic Plants

Thousands of plant species parasitize trees. Most do the dirty deed in the soil, infecting the host at root-root contacts or parasitizing their mycorrhizal fungi. For most of them, although the botanical aspects have been studied (anatomy, physiology, evolution, etc.), not much is known about the damage and growth loss they cause. Others, the mistletoes, parasitize aerial stems and branches and are better known.

Direct root-root parasites

• Most direct root parasites are in the tropics and subtropics.
• There is a wide spectrum of host dependency:
  • Some have no chlorophyll and are entirely dependent on the host for food and water.
  • Others are green and can produce some or all of their own food. Some are even capable of reproducing without ever having a host. The green ones can be tricky – there is no obvious indication that they are root parasites.
• Most direct root parasites do not seem to be very host-specific.

Santalales

Santalales is a large order of mostly root parasites (and also includes mistletoes; see below). Most are hemiparasites, meaning they are green and can make most of their own food, but rely on the host largely for water and nutrients. Many are shrubs and even trees themselves, such as Santalum spp. (sandalwood).

Scrophulariales

The order Scrophulariales also has some interesting root parasites. In southeastern North America, from Texas to Virginia, and in the Bahamas, Seymeria cassioides (black-senna) is one of the most important parasitic plants. It is a hemiparasite of Pinus spp., especially P. taeda and P. elliottii, loblolly and slash pines. Although it is an annual plant, it can have severe impacts ​[1]​. Young trees can be stunted and killed. Up to 50% mortality has been reported in young plantations. Older trees can suffer growth loss.

Another member of the order in eastern North America, Conopholis americana (American cancer-root), is somewhat unusual in several respects. It is not restricted to the subtropics, but extends as far as its hosts into northern Canada. It is a holoparasite, without chlorophyll and producing no food for itself. It is usually hosted by Quercus spp. or Fagus grandifolia (American beech).

Epifagus virginiana, beech drops, is also a holoparasite but a more petite member of the group. The genus name means “upon beech,” and indeed it always infects F. grandifolia. Its geographic range is similar to C. americana.

Mycotrophs

The plants above are direct plant-plant parasites. Mycotrophy (deriving nutrition from a fungus; also called myco-heterotrophy) is an alternative way of life for parasitic plants. About 450 species, 1/8 of parasitic plants, are fully mycotrophic throughout their lives ​[4]​. Many more, perhaps 10% of all plants, use this nutritional mode primarily during seedling or sporeling establishment, or to supplement their own photosynthesis.

Of course it was obvious to early botanists that plants without chlorophyll could not make their own food ​[2]​. So they looked at the roots. Some species were not attached to another plant! So an early theory was that these were saprophytes, plants that extract their food from dead organic matter in the soil. You can still find this and other confused explanations on the interwebs.

Now we know that such plants actually connect with and are minimally invaded by fungi. The fungus usually produces a mantle over the surface of the root, and other hyphae grow through the soil. It looks like a mycorrhiza, but is functionally very different from mutualistic mycorrhizae – the plant gets all organic nutrition, mineral nutrients, and probably most water from the fungus. As far as we know, the fungus gets nothing in return. The plant is fully parasitizing the fungus! In fact, in cases that have been studied, it is an obligate parasite.

So where does the fungus get all these goodies? In almost all cases, the fungus is also mycorrhizal with a living tree ​[2]​. That end is a proper mycorrhiza – the fungus provides water and nutrients, and is given organic nutrition in return. So this poor fungus is providing water and nutrients to both its ‘partners’, and is funneling organic nutrition from one to the other.

The mycotrophic plant is sometimes called an epiparasite, because it is a parasite of a parasite. Yes, strictly speaking, in a normal mycorrhiza, the fungus is a parasite of its host plant, even though the symbiosis is mutually beneficial.

The order Ericales has one of the most interesting and best studied cases. Monotropa is a small genus of achlorophyllous species. The one I am most familiar with, M. uniflora, is a stately plant of ghostly, translucent white. It always partners with members of the Russulaceae, most commonly Russula spp ​[2]​. Another widespread species, M. hipopytis, always goes with Tricholoma spp. Moreover, local populations of Monotropa spp. appear to be species-specific with their fungal host, always parasitizing the same fungal species. In contrast, mutualistic mycorrhizae are generally nonspecific.

Generally the fungus between the two plants forms ectomycorrizae, as in Monotropa and its ultimate host plant. However, some cases involving endomycorrhizae are also known ​[3]​.

Mistletoes

Those root parasites and mycoparasites are fascinating, but, at least in North America, mistletoes (specifically dwarf mistletoes) are the 800-pound gorilla of the group. They have far greater impacts on forest ecology and productivity.

The term “mistletoe” generally refers to aerial parasites in the order Santalales. The only genera in most of North America are Phoradendron and Arceuthobium (family Viscaceae, though sometimes lumped into Santalaceae). In much of the rest of the world, Viscum is an important genus, but there are many other genera in several families.

The pages on these groups:

• Dwarf mistletoes (Arceuthobium)
  • Dwarf mistletoe ecology
  • Dwarf mistletoe damage
• True mistletoes (Phoradendron and Viscum)