When a farmer grows a crop variety that is resistant to certain disease-causing pathogens, the invader is quickly recognised by the plant and specific defences are activated. These defences stop the pathogen in its tracks and no disease develops. If the crop variety is not resistant, but is susceptible to those pathogens, the invading pathogen manages to avoid recognition by the plant. The pathogen can then infect the plant, colonise it, and cause disease. The susceptible plant has all of the defences it needs to deal with invading pathogens, but because it fails to recognise the invader, the defences are not activated. However, it is possible to by-pass the recognition process and to directly activate the defences. This is known as induced resistance and it can be triggered in plants using compounds known as elicitors. These elicitors can work in two ways, they can activate defences directly, or they can activate defences only following pathogen attack. The latter is called priming and is potentially very useful, since it leads to the deployment of defences only when there is real danger.
There are many viruses, fungi and bacteria that cause plant diseases. Plant biologists are working to create plants with genetically-engineered resistance to these diseases.