When a pathogen comes into contact with a potential host plant a complex of interacting factors including temperature, moisture, susceptibility of the plant tissue, effects of other microorganisms present, aggressiveness of the pathogen, and others determines whether infection occurs. Although some pathogens can attack almost all parts of the plant, most show some degree of specialization as to the tissues and organs invaded, as discussed later (p. 182). Broken or damaged tissue, unless it is toxic to the pathogen, is generally more liable to invasion by rather unspecialized parasites (necrotrophs), and wound parasites may enter the plant in this way only. More specialized parasites (biotrophs, obligate parasites) are likely to have difficulty in colonizing grossly damaged tissue as they require living host cells for their development, although they may be able to invade slightly damaged tissues. The processes of infection are conventionally grouped into prepenetration, penetration (p. 149) and postpenetration, although these stages are not completely distinct. The first two stages are described below, and the third, the further colonization of the infected plant, is discussed in the next chapter.
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