Parasitic Plants of New England

What's a Parasitic Plant?

Generally defined, a parasitic plant is a vascular plant or bryophyte that lives in close physical association with another living organism and obtains at least some of its nutrition from that organism without providing benefits in return.

There are two types of parasitic plants:

1. Species that are parasitic on mycorrhizal fungi. These plants are referred to as mycoheterotrophs or mycoheterotrophic plants. They include certain species of flowering plants, ferns, firmosses, and clubmosses; potentially one species of gymnosperm; and at least one species of liverwort (Merckx and Freudenstein 2010). More information about this type of parasitism can be found here.

2. Species that are parasitic on other plants. These species are referred to as "parasitic plants" or "haustorial parasites." (The term "parasitic plants" used in a strict sense includes only this group of parasites.) All plants exhibiting this type of parasitism develop specialized, multi-cellular structures called "haustoria," which are used to attach to, penetrate, and extract food and water from the host plant. Haustorial parasites may be classified as either root or stem parasites. Root parasites attach their haustoria to the roots (or rhizomes) of the host plant, while stem parasites attach their haustoria to the host's stems and/or leaves. Most root parasites produce haustoria off of their roots (or rhizomes); stem parasites produce haustoria off of their stems (e.g. in Cuscuta), or embryonic roots (e.g. in Arceuthobium). The haustoria are formed after the parasite detects the presence of a host plant in its vicinity. In essence, the parasite is sensitive to a molecule or molecules produced by the host plant. When such a molecule is present, the molecule (or a derivative of it produced via an enzymatic reaction) binds to receptors on the parasite's cells; this ultimately results in the activation of genes involved in the formation of haustoria. Haustorial development is extremely complex—different genes are activated at different stages in the process, and the overall process occurs in response to more than one chemical or physical cue. For example, in the stem parasite Cuscuta, depending on whether the hautoria come into contact with xylem or phloem tissue from the host, the ends of the haustoria will differentiate into either xylem elements (to form a connection to the host's xylem) or phloem cells (to connect to the host's phloem) (Yoshida et al. 2016). Some haustorial parasites, such as Orobanche have seeds that will not germinate unless they fall near an appropriate host. (A molecule produced by the host must be present to trigger germination.)

   Almost all of the known haustorial parasites are flowering plants. One species of gymnosperm (Parasitaxus usta) is also known to parasitize plants. Haustorial parasitism is believed to have evolved in flowering plants on at least 12 separate occasions.

Hemi- vs. Holoparasitic Plants

Haustorial parasites may be described as either hemi- or holoparasitic. Hemiparasitic plants are generally green plants that photosynthesize and are only partially dependent on other plants for food. In contrast, holoparasitic plants lack chlorophyll and obtain most or all of their nourishment from other plants. The distinction between hemi- and holoparasitic plants is not always clear-cut. Some parasitic plants contain minor amounts of chlorophyll and undergo photosynthesis to a limited extent. Because they are capable of photosynthesis, they are usually considered to be hemiparasitic even though they may obtain most or nearly all of their nourishment from a host.

Parasitic Plants Found in New England

All haustorial parasites found in New England are flowering plants belonging to the eudicot clade. They are listed below by family.

Comandraceae (Bastard Toadflax Family)

Plants of the bastard toadflax family contain chlorophyll and are hemiparasitic on the roots of other plants. Two species occur in New England; both are known to parasitize a variety of host plants.

• Comandra umbellata (bastard toadflax)
• Geocaulon lividum (false toadflax)

Covolvulaceae (Morning Glory Family)

The morning glory family includes one genus of parasitic plants: Cuscuta (dodders). Dodders are herbaceous vines that lack roots and have only minute, scale-like vestiges of leaves. They are stem parasites. Some species lack chlorophyll and are considered to holoparasitic, while other species, considered hemiparasitic, have retained a small amount of of chlorophyll and are capable of photosynthesis. Most species of dodder have a wide range of plant hosts. The following 11 species of dodder occur in New England.

• Cuscuta approximata (alfalfa dodder)
• Cuscuta campestris (field dodder)
• Cuscuta cephalanthi (buttonbush dodder)
• Cuscuta compacta (compact dodder)
• Cuscuta coryli (hazel dodder)
• Cuscuta epithymum (clover dodder)
• Cuscuta europaea (greater dodder)
• Cuscuta gronovii (common dodder)
• Cuscuta indecora (collared dodder)
• Cuscuta pentagona (bush-clover dodder)
• Cuscuta polygonorum (smartweed dodder)

Orobanchaceae (Broom-rape Family)

Members of the broom-rape family are parasitic on plant roots. Thirteen genera occur in New England. Four of these genera contain holoparasitic species (Aphyllon, Conopholis, Epifagus, and Orobanche). The remaining species found in New England are hemiparasitic. Species occurring in New England are listed below.

• Agalinis acuta (sandplain agalinis)
• Agalinis maritima (seaside gerardia)
• Agalinis neoscotica (Nova Scotia agalinis)
• Agalinis paupercula (small-flowered agalinis)
• Agalinis purpurea (purple gerardia)
• Agalinis tenuifolia (slender-leaved agalinis)
• Aphyllon uniflorum (one-flowered cancer-root) — holoparasitic on many kinds of plants
• Aureolaria flava (smooth false foxglove)
• Aureolaria pedicularia (fern-leaved false foxglove)
• Aureolaria virginica (downy false foxglove)
• Castilleja coccinea (scarlet painted cup)
• Castilleja exserta (purple painted cup)
• Castilleja septentrionalis (northern painted cup)
• Conopholis americana (squawroot) — holoparasitic on species of red oak (Quercus section Lobatae)
• Epifagus virginiana (beechdrops) — holoparasitic on American beech (Fagus americana)
• Euphrasia micrantha (northern eyebright)
• Euphrasia nemorosa (common eyebright)
• Euphrasia oakesii (Oakes' eyebright)
• Euphrasia randii (Rand's eyebright)
• Euphrasia stricta (strict eyebright)
• Euphrasia suborbicularis (round-leaved eyebright)
• Euphrasia tetraquetra (seacliff eyebright)
• Melampyrum lineare (cowwheat)
• Odontites vernus (red bartsia)
• Orobanche minor (lesser broomrape) — holoparasitic on many kinds of plants
• Pedicularis canadensis (common lousewort)
• Pedicularis furbishiae (Furbish's lousewort)
• Pedicularis lanceolata (swamp lousewort)
• Rhinanthus alectorolophus (European yellow rattle)
• Rhinanthus minor (little yellow rattle)
• Schwalbea americana (chaffseed)

Viscaceae (Christmas-mistletoe Family)

Members of the Christmas-mistletoe family are parasitic on the branches of trees. One species occurs in New England.

• Arceuthobium pusillum (eastern dwarf mistletoe) — hemiparasitic on the branches of black spruce (Picea mariana). It may also be found on white spruce (P. glauca) and red spruce (P. rubens), and occasionally on other conifers from the pine family (Pinaceae). This parasite lives inside the branches of its host. It produces external shoots about four years after infection, with flowers and fruits appearing off of the shoots about a year later (Baker et al. 2006). Although this species is capable of photosynthesis, its photosynthetic rate is very low (even in full sunlight), thus it is strongly dependent on its host for its nutritional needs.