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This document discusses plant pathogens such as bacteria and viruses that affect agriculture. It notes that agriculture and livestock production can significantly fragment ecosystems by replacing many species with just a few crop species. Some cultivated species are exotic to the areas where they are grown. This environmental degradation occurs in ecosystems where plants, animals, and microbes previously cohabited and coevolved. The document also mentions that some beneficial bacteria like Pseudomonas, which control fungal diseases, can become pathogens of exotic crops. It introduces the "disease triangle" concept involving a susceptible host, virulent pathogen, and favorable environment.Read less