Types of plant pathogens and their antagonists

The term disease, in plant pathology, is defined as the malfunctioning of plant cells and tissues, resulting from continuous irritation and leading to development of symptoms. Plant diseases can be caused by several types of microorganism but the most frequently encountered and diverse are the fungi, bacteria, and viruses. Plant pathogens vary in the intensity of their relations with host plants from parasites, requiring the host to live, to saprophytes, which feed on dead organic matter. They range in their use of host tissues from being biotrophs, which feed on living cells, to necrotrophs, which kill plant tissues before ingesting their contents. Some plant pathogens are obligate parasites and can only grow and multiply in nature when living on or in living organisms (so these are all biotrophs). These include viruses, powdery and downy mildews, rust fungi, and some bacteria, among others. Other plant pathogens, including many fungi and bacteria, can live on dead as well as living plant material and these are therefore facultative parasites.

Both fungal and bacterial plant pathogens exist in the environment inside or outside of plants or in or on dead plant material. Many pathogens have active epiphytic (occurring on the plant surface without causing infection) and saprophytic phases in their life cycles. Some species have special persistent stages that live many years. Dispersal of bacteria and fungi is by wind and rain and sometimes they are carried by higher organisms.