ABSTRACT
Plant diseases, particularly the smuts and rusts, have been described throughout history as serious problems in food and fiber production. However, it was not until the late 1900s that microorganisms were discovered as the causal agents of plant diseases. Microorganisms causing plant as well as human diseases include fungi, bacteria, and viruses, although the ranking in importance is different, with fungal organisms causing the vast majority of plant diseases. Although fungi comprise the most important group of plant pathogens, the number of infectious species is relatively small. Among the over 100,000 fungal organisms described, only a small fraction (approximately 200) is known to colonize living plants.
