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Plant pathogens fall into two broad categories: necrotrophs (those that kill plant cells before parasitising them), and biotrophs (those that obtain nutrients from living cells). Failure of pathogens to invade suitable host cells (dead in the case of necrotrophs; alive for biotrophs) will prevent them from infecting the host and the plant will be resistant. Additionally, the establishment of a parasitic relationship is dependent on the responses of the plant under attack.

Resistant hosts prevent or slow the development and reproduction of the majority of pathogen propagules that they come into contact with. Resistance can be expressed at many stages in the infection process, from inhibition of propagule germination and penetration, to the restriction of colony development after the pathogen has become established. The defence barriers erected by plants are a co-ordinated system of molecular, cellular and tissue-based responses to pathogen attack.

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