Plants are the first step in every food chain, and also provide habitats for many organisms. Even if they are not crop plants providing food directly, they provide many ecosystem services. This means that humans have a responsibility to both wild and crop plants.
The plants and crops in any place have evolved with the local pathogens so are able to survive. Those same pathogens can be deadly to plants in other parts of the world. This is why the movement of foods, seeds, timber, whole plants and soil is regulated. Many countries including the UK inspect these when they enter the country, and infected plant material is safely disposed of.
Many crop plants are planted close together in a monoculture - the aim of this is to provide enough food and make them easier to harvest. This makes them a bigger target for pathogens, which can then spread easily as plants of the same species are so close together. In polyculture different plant species are planted together. This makes it harder for the pathogen to spread.
Some crops are clones. In an ordinary plant population there is a lot of variation, and so at least some individuals can survive infection. Some plants - such as banana plants and potato plants (even across different varieties) - are genetically identical. If a disease can kill one, it may kill them all. An example of this is when potato blight caused the Irish potato famine.
Bananas are grown as a monoculture. As they are clones, they are all very susceptible to a particular fungus disease. Some crops are sprayed with a chemical fungicide up to 40 times a year to reduce the risk of infection. This is an example of chemical control.
In biological control a new species is introduced into an ecosystem. Pests that spread diseases can be controlled if the new species is a predator of the pest.
Pathogens or their spores may remain in the soil after a crop is harvested and infect the new crop the following year. Crop rotation can help prevent this because different crops tend to be affected by different pathogens. The more different a crop is, the less likely it is to be affected by pathogens from the previous year.