P e s t s , D i s e a s e s , AN D D i s o r d e r s
Parasitic plants Some plants are unable to produce all or some of their nutrients themselves and are parasitic, or partially parasitic on other plants. More than 4,000 species of parasitic flowering plants are known to exist; they may be obligate—unable to complete their life cycle without a host—or facultative. Obligate parasites often have no chlorophyll at all, so lack the green coloring typical of most plants; facultative parasites do
Epifagus virginiana, beech drops
have chlorophyll and can complete their life cycle independent of the host.
Some parasites are generalists, such as dodder (Cuscuta) and red bartsia (Odontites vernus), being able to parasitize many plant species. Others are specialists that parasitize just a few or even one
Striga coccinea, witchweed
species, such as beech drops (Epifagus virginiana) on American beech (Fagus grandifolia). Parasitic plants attach to either the stems or the roots of the host plant. They have modified roots, called haustoria (singular: haustorium), which enter the host plant and then penetrate the phloem, xylem, or both to extract nutrients. Broomrape (Orobanche), dodder and witchweed (Striga) cause huge economic losses in a variety of crops. Mistletoes (Viscum) cause economic damage to forest and ornamental trees.
Broomrape Broomrape (Orobanche) is a genus of more than 200 species of obligate parasites that completely lack chlorophyll and attach to the roots of their hosts. They have yellow or straw-colored stems, leaves reduced to triangular scales, and bear yellow, white, or blue flowers that look like antirrhinums. Only the flowerheads are visible above the soil surface. The seedlings put out a root-like growth, which attaches to the roots of nearby hosts. 207