Biotrophic and necrotrophic parasites

The most obvious response of a host to a parasite is for the whole host to die. Indeed, we can draw a distinction between parasites that kill and then continue life on the dead host (necrotrophic parasites) and those for which the host must be alive (biotrophic parasites). Necrotrophic parasites blur the tidy distinctions between parasites, predators and saprotrophs (see Section 11.1). Insofar as host death is often inevitable and sometimes quite rapid, necrotrophic parasites are really predators, and once the host is dead they are saprotrophs. But for as long as the host is alive, necroparasites share many features with other types of parasite.

For a biotrophic parasite, the death of its host spells the end of its active life. Most parasites are biotrophic. Lucilia cuprina, the blowfly of sheep, however, is a necroparasite on an animal host. The fly lays eggs on the living host and the larvae (maggots) eat into its flesh and may kill it. The maggots continue to exploit the carcass after death but they are now detritivores rather than either parasites or predators. Necroparasites on plants include many that attack the vulnerable seedling stage and cause symptoms known as 'damping-off' of seedlings. Botrytis fabi is a typical fungal necroparasite of plants. It develops in the leaves of the bean Vicia faba, and the cells are killed, usually in advance of penetration. Spots and blotches of dead tissues form on the leaves and the pods. The fungus continues to develop as a decomposer, and spores are formed and then dispersed from the dead tissue, but not while the host tissue is still alive.

Most necroparasites can therefore be regarded as pioneer saprotrophs. They are one jump ahead of competitors because they can kill the host (or its parts) and so gain first access to the resources of its dead body. The response of the host to necroparasites is never very subtle. Amongst plant hosts, the most common response is to shed the infected leaves, or to form specialized barriers that isolate the infection. Potatoes, for example, form corky scabs on the tuber surface that isolate infections by Actinomyces scabies.

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