A plant pathogen is a disease-causing organism which attacks plants. Plant pathogens are of interest for a number of reasons, ranging from concerns about fragile ecosystems to the desire to protect the food supply. There are numerous types of pathogens which can target plants, including viruses, bacteria, protozoans, parasites, worms, and archaea. People who study plant pathogens are known as phytopathologists or plant pathologists.
Plant pathogens can attack in a number of different ways. Some colonize the tissue in the plant, others settle on the surface of the plant, and others may go for specific areas such as the roots, stems, and leaves. Pathogens commonly cause problems like tissue death, browning, a decrease in fruiting, problems with setting flowers, and so forth. In extreme cases, they can kill the host plant.
Like pathogens which attack humans, plant pathogens are very diverse. A plant pathogen may have evolved to attack a specific genus or species of plants, or to more broadly infect most plants. Some plant pathogens take advantage of specific biological processes which can occur within host species, while others settle for using plants as a home because they have valuable resources such as nutrients.
In agriculture, plant pathogens cause serious damage each year. Once a plant has been attacked by a plant pathogen, its products usually cannot be sold. The investment in the plant is rendered useless by the pathogen, and usually great expense is involved in eradicating pathogens from the field and addressing other fallout from the infestation. In some cases, a field becomes so diseased that it must be allowed to lie fallow for several years to recover before attempts are made to grow crops there again.
A plant pathogen can also be of concern when it is accidentally imported or deliberately introduced to a vulnerable ecosystem. Some island nations are especially concerned with this, as they have unique flora which could be decimated by an introduced pathogen from the mainland. Pathogens can also be a problem when they spread between districts; for example, a pathogen which attacks grapes and is limited to Europe can pose a major threat if it reaches wineries in Australia, which may not be prepared to manage the pathogen.
Ecologists are interested in the study of pathogens which can infect plants, looking at their effects on various species and ecosystems. When plants experience dieoffs and other problems, a plant pathogen is often the cause, and a pathologist may be called in to find out which pathogen is responsible, and develop a plan for managing it.