(pl. parasites)
Terms discussed: biotroph (pl. biotrophs), facultative, flecking, host (pl. hosts), necrotroph (pl. nectrotrophs), obligate, parasitism
A parasite is an organism that feeds on another organism (known as the host) without killing it (without killing it quickly, anyway). To be considered a parasite, an organism usually lives (as permanently as it can manage) on or in the host. A more thorough discussion of why we consider certain organisms to be parasites can be found under the entry on Fungal Lifestyles. This entry discusses technical terms associated with parasitism.
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A fungus that is restricted to a parasitic lifestyle is called an obligate parasite. If the fungus is a parasite sometimes, but is also able to feed in other ways (usually as a saprobe) in the absence of a host, it is called a facultative parasite.
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Similarly, a fungus that kills the cells of its host before ingesting their contents is called a necrotroph. A fungus that needs to feed on cells that are still alive is called a biotroph. Some plants exploit the biotrophic requirements of most smuts by a resistance response called flecking: as soon as the fungus mycelium touches a cell of the plant, the cell commits suicide, and the fungus is unable to feed on it. In the case of a mycelium from a germinating spore, this means the the mycelium quickly exhausts its food reserves and dies. The tiny circles of dead leaf cells surrounding each germinated spore give the phenomenon its name.
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