Plant pathology (also phytopathology) is the scientific study of plant diseases caused by pathogens (infectious diseases) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. Not included are ectoparasites like insects, mites, vertebrate or other pests that affect plant health by consumption of plant tissues. Plant pathology also involves the study of pathogen identification, disease etiology, disease cycles, economic impact, plant disease epidemiology, plant disease resistance, how plant diseases affect humans and animals, pathosystem genetics, and management of plant diseases.
Plant pathogens
Fungi
The majority of phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes.
The fungi reproduce both sexually and asexually via the production of spores. These spores may be spread long distances by air or water, or they may be soil borne. Many soil borne spores, normally zoospores, are capable of living saprotrophically, carrying out the first part of their lifecycle in the soil.
Fungal diseases can be controlled through the use of fungicides in agriculture, however new races of fungi often evolve that are resistant to various fungicides.
Biotrophic fungal pathogens colonize living plant tissue and obtain nutrients from living host cells. Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells. See Powdery Mildew and Rice Blast images below.
Significant fungal plant pathogens include:
Ascomycetes
Fusarium spp. (causal agents of Fusarium wilt disease)
Thielaviopsis spp. (causal agents of: canker rot, black root rot, Thielaviopsis root rot)
Verticillium spp.
Magnaporthe grisea (causal agent of blast of rice and gray leaf spot in turfgrasses)
Basidiomycetes
Rhizoctonia spp.
Phakospora pachyrhizi (causal agent of soybean rust)
Puccinia spp. (causal agents of severe rusts of virtually all cereal grains and cultivated grasses)
Oomycetes
The oomycetes are not true fungi but are fungal-like organisms. They include some of the most destructive plant pathogens including the genus Phytophthora which includes the causal agents of potato late blight and sudden oak death.
Despite not being closely related to the fungi, the oomycetes have developed very similar infection strategies and so many plant pathologists group them with fungal pathogens.
Significant oomycete plant pathogens
Pythium spp.
Phytophthora spp.; including the causal agent of the Great Irish Famine (1845–1849
