The living plant surface represents a formidable barrier to most microorganisms and only a few of those that come in contact with an individual plant are capable of entering and subsequently establishing an infection which causes disease. The tools that the successful pathogen may use include: structures and enzymes which allow penetration of the plant surface (plant cuticle); toxins and enzymes that weaken or kill plant tissues; means of manipulating plant hormone levels to modify host morphology and defences; and effector proteins which interfere with the ability of the host to activate defences.