Fungi, nematodes, bacteria, and viruses are probably the first things that come to mind when thinking of plant pathogens. These organisms certainly do cause damage to plants of economic importance, but it may surprise you to know that parasitic flowering plants are also important pathogens. The purpose of this chapter is to provide an overview of the life cycles and evolutionary relationships of these fascinating and unusual plants and also to focus upon those that negatively affect food and fiber crops.
Most plants are autotrophs and produce their own carbon sources through photosynthesis. Although some plants such as Indian pipe (Monotropa) (Figure 1) lack chlorophyll and appear to be parasitic, they are mycoheterotrophs (parasites of mycorrhizal fungi) and, hence, only indirectly parasitize the trees on which the mycorrhizal fungi are found. Here we define a parasitic plant as an angiosperm (flowering plant) that directly attaches to another plant via a haustorium. A haustorium is a modified root that forms a morphological and physiological link between the parasite and host (Figure 2) (Kuijt 1969). It is useful to make a distinction between the terms "parasite" and "pathogen." Parasite is from the Greek para (beside) and sitos (grain or food) which literally means "beside the food". If a plant also induces disease symptoms in a host, then it is a pathogen as well as parasite. A general term that refers to both parasites and mycotrophs that derive carbon from sources other than their own photosynthesis is heterotrophic, which simply means "different feeding.