Agrophages that threaten crops originate from sources of infection, which may be in the soil, on or in seeds, plant residues or living plants, either in or outside the field.
The best way to reduce the level of infection is to prevent or remove the sources, e.g. by avoiding the accumulation of weed seeds on the soil surface, avoiding monocultures (cause e.g. stem base diseases in cereals), too short a crop return time in the rotation (cause e.g. cabbage or rape syphilis, nematode infestation in potato crops), or pathogen-infected plant residues (e.g. rotten potatoes, fruit, rotting straw). Sources of infection can be eliminated by appropriate crop rotation, reducing weed infestation, preventing the development of pest populations by effectively eradicating the first small infections with available methods, sowing plants that reduce pest populations (e.g. mustard in the case of nematode pathogens of sugar beet).