The economic impact of plant diseases probably first became apparent when plants were grown together as crops. The majority of plant pathogens have a limited host range and many are species-specific. A mixed flora, where hosts of plant diseases are intermingled with other species, acts as a buffering system with respect to pathogens. The chances of pathogens finding fresh hosts in a species-rich habitat are much lower than in a crop consisting of a single plant species. Pathogens evolved over millions of years in species-rich habitats, and to succeed in finding new hosts they developed highly efficient mechanisms of spread.

With the aggregation of plants together as crops, almost ideal conditions were created for the establishment and spread of pathogens. The continuous processes of selection and propagation of crop species have resulted in the situation today where the planting of large areas of land with crops of near-identical genotype is common. It is therefore not surprising that microorganisms that have an efficient dispersal mechanism, and that can infect crops, have on occasion caused devastating losses in yield.

The historical importance of plant diseases and their economic and social effects have been well documented. Currently, within the agricultural systems of developed countries, plant diseases are unlikely to cause crop losses to the extent of those associated with, for example, late blight of potato (Plate 1) in Eire in the 1840s and in Germany in 1917–18. The development and use of crop protection chemicals from the late nineteenth century onwards, and the establishment of scientifically based plant breeding programmes in the early twentieth century, have led to reductions in losses due to disease. Despite increases in the yield and quality of crops over the past century, it is estimated that about 20–30% of potential crop yield is lost annually through attack by plant pathogens and pests, and competition from weeds. Losses to this extent or even greater may occur in less developed countries, but loss of potential yield due to attack by pathogens may be much less in crops of countries within the European Union. The EU is now self sufficient in many foodstuffs, and yields in excess of national and EU requirements occur with some crops, notably cereals, for which a considerable export trade has developed in some countries such as the UK.