Plant pathology or phytopathology is the scientific study of plant diseases caused by pathogens and environmental conditions. Plant pathology 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.
Phytophthora infestans is an oomycete or water mold, a fungus-like microorganism that causes the serious potato and tomato disease known as late blight or potato blight. Early blight, caused by Alternaria solani, is also often called "potato blight". Late blight was a major culprit in the 1840s European, the 1845–1852 Irish, and the 1846 Highland potato famines. The organism can also infect some other members of the Solanaceae. The pathogen is favored by moist, cool environments: sporulation is optimal at 12–18 °C (54–64 °F) in water-saturated or nearly saturated environments, and zoospore production is favored at temperatures below 15 °C (59 °F). Lesion growth rates are typically optimal at a slightly warmer temperature range of 20 to 24 °C.
Dickeya dadantii is a gram-negative bacillus that belongs to the family Pectobacteriaceae. It was formerly known as Erwinia chrysanthemi but was reassigned as Dickeya dadantii in 2005. Members of this family are facultative anaerobes, able to ferment sugars to lactic acid, have nitrate reductase, but lack oxidases. Even though many clinical pathogens are part of the order Enterobacterales, most members of this family are plant pathogens. D. dadantii is a motile, nonsporing, straight rod-shaped cell with rounded ends, much like the other members of the genus, Dickeya. Cells range in size from 0.8 to 3.2 μm by 0.5 to 0.8 μm and are surrounded by numerous flagella (peritrichous).
Xanthomonas campestris is a gram-negative, obligate aerobic bacterium that is a member of the Xanthomonas genus, which is a group of bacteria that are commonly known for their association with plant disease. This species includes Xanthomonas campestris pv. campestris, the cause of black rot in brassicas, one of the most important diseases of brassicas worldwide.
Rhizoctonia solani is a species of fungus in the order Cantharellales. Basidiocarps are thin, effused, and web-like, but the fungus is more typically encountered in its anamorphic state, as hyphae and sclerotia. The name Rhizoctonia solani is currently applied to a complex of related species that await further research. In its wide sense, Rhizoctonia solani is a facultative plant pathogen with a wide host range and worldwide distribution. It causes various plant diseases such as root rot, damping off, and wire stem. It can also form mycorrhizal associations with orchids.
Ralstonia solanacearum is an aerobic non-spore-forming, Gram-negative, plant pathogenic bacterium. R. solanacearum is soil-borne and motile with a polar flagellar tuft. It colonises the xylem, causing bacterial wilt in a very wide range of potential host plants. It is known as Granville wilt when it occurs in tobacco. Bacterial wilts of tomato, pepper, eggplant, and Irish potato caused by R. solanacearum were among the first diseases that Erwin Frink Smith proved to be caused by a bacterial pathogen. Because of its devastating lethality, R. solanacearum is now one of the more intensively studied phytopathogenic bacteria, and bacterial wilt of tomato is a model system for investigating mechanisms of pathogenesis. Ralstonia was until recently classified as Pseudomonas, with similarity in most aspects, except that it does not produce fluorescent pigment like Pseudomonas. The genomes from different strains vary from 5.5 Mb up to 6 Mb, roughly being 3.5 Mb of a chromosome and 2 Mb of a megaplasmid. While the strain GMI1000 was one of the first phytopathogenic bacteria to have its genome completed, the strain UY031 was the first R. solanacearum to have its methylome reported. Within the R. solanacearum species complex, the four major monophyletic clusters of strains are termed phylotypes, that are geographically distinct: phylotypes I-IV are found in Asia, the Americas, Africa, and Oceania, respectively.
Phytophthora erythroseptica—also known as pink rot along with several other species of Phytophthora—is a plant pathogen. It infects potatoes causing their tubers to turn pink and damages leaves. It also infects tulips (Tulipa) damaging their leaves and shoots.
Stemphylium solani is a plant pathogen fungus in the phylum Ascomycota. It is the causal pathogen for grey leaf spot in tomatoes and leaf blight in alliums and cotton, though a wide range of additional species can serve as hosts. Symptoms include white spots on leaves and stems that progress to sunken red or purple lesions and finally leaf necrosis. S. solani reproduces and spreads through the formation of conidia on conidiophores. The teleomorph name of Stemphyllium is Pleospora though there are no naturally known occurrences of sexual reproduction. Resistant varieties of tomato and cotton are common, though the pathogen remains an important disease in Chinese garlic cultivation.
Pectobacterium carotovorum is a bacterium of the family Pectobacteriaceae; it used to be a member of the genus Erwinia.
Alternaria solani is a fungal pathogen that produces a disease in tomato and potato plants called early blight. The pathogen produces distinctive "bullseye" patterned leaf spots and can also cause stem lesions and fruit rot on tomato and tuber blight on potato. Despite the name "early," foliar symptoms usually occur on older leaves. If uncontrolled, early blight can cause significant yield reductions. Primary methods of controlling this disease include preventing long periods of wetness on leaf surfaces and applying fungicides. Early blight can also be caused by Alternaria tomatophila, which is more virulent on stems and leaves of tomato plants than Alternaria solani.
Helminthosporium solani is a fungal plant pathogen responsible for the plant disease known as silver scurf. Silver scurf is a blemish disease, meaning the effect it has on tubers is mostly cosmetic and affects "fresh market, processing and seed tuber potatoes." There are some reports of it affecting development, meaning growth and tuber yield. This is caused by light brown lesions, which in turn change the permeability of tuber skin and then it causes tuber shrinkage and water loss, which finally causes weight loss. The disease has become economically important because silver scurf affected potatoes for processing and direct consumption have been rejected by the industry. The disease cycle can be divided into two stages: field and storage. It is mainly a seed borne disease and the primary source of inoculum is mainly infected potato seed tubers. Symptoms develop and worsen in storage because the conditions are conducive to sporulation. The ideal conditions for the spread of this disease are high temperatures and high humidity. There are also many cultural practices that favor spread and development. There are multiple ways to help control the disease.
Plant disease resistance protects plants from pathogens in two ways: by pre-formed structures and chemicals, and by infection-induced responses of the immune system. Relative to a susceptible plant, disease resistance is the reduction of pathogen growth on or in the plant, while the term disease tolerance describes plants that exhibit little disease damage despite substantial pathogen levels. Disease outcome is determined by the three-way interaction of the pathogen, the plant, and the environmental conditions.
Chaetomium cupreum is a fungus in the family Chaetomiaceae. It is able to decay in manufactured cellulosic materials, and is known to antagonize a wide range of soil microorganisms. This species is component of the biocontrol agent, Ketomium, a commercial biofungicide. It has also been investigated for use in the production of natural dyes. Chaetomium cupreum is mesophilic and known to occur in harsh environments and can rapidly colonize organic substrates in soil. Laboratory cultures of C. cupreum can be propagated on a range of common growth media including potato dextrose at ambient or higher than ambient temperature producing cottony white colonies with a reddish reverse.
Bacterial soft rots are caused by several types of bacteria, but most commonly by species of gram-negative bacteria, Erwinia, Pectobacterium, and Pseudomonas. It is a destructive disease of fruits, vegetables, and ornamentals found worldwide, and affects genera from nearly all the plant families. The bacteria mainly attack the fleshy storage organs of their hosts, but they also affect succulent buds, stems, and petiole tissues. With the aid of special enzymes, the plant is turned into a liquid mush in order for the bacteria to consume the plant cell's nutrients. Disease spread can be caused by simple physical interaction between infected and healthy tissues during storage or transit. The disease can also be spread by insects. Control of the disease is not always very effective, but sanitary practices in production, storing, and processing are something that can be done in order to slow the spread of the disease and protect yields.
Streptomyces scabiei is a streptomycete bacterium species found in soils around the world. Unlike most of the 500 or so Streptomyces species it is a plant pathogen causing corky lesions to form on tuber and root crops as well as decreasing the growth of seedlings. Along with other closely related species it causes the potato disease common scab, which is an economically important disease in many potato growing areas. It was first described in 1892, being classified as a fungus, before being renamed in 1914 and again in 1948. Several other species of Streptomyces cause similar diseases to S. scabiei but other, more closely related species, do not.
Pectobacterium atrosepticum is a species of bacterium. It is a plant pathogen causing blackleg of potato. Its type strain is CFBP 1526T. Its genome has been sequenced.
Beet vascular necrosis and rot is a soft rot disease caused by the bacterium Pectobacterium carotovorum subsp. betavasculorum, which has also been known as Pectobacterium betavasculorum and Erwinia carotovora subsp. betavasculorum. It was classified in the genus Erwinia until genetic evidence suggested that it belongs to its own group; however, the name Erwinia is still in use. As such, the disease is sometimes called Erwinia rot today. It is a very destructive disease that has been reported across the United States as well as in Egypt. Symptoms include wilting and black streaks on the leaves and petioles. It is usually not fatal to the plant, but in severe cases the beets will become hollowed and unmarketable. The bacteria is a generalist species which rots beets and other plants by secreting digestive enzymes that break down the cell wall and parenchyma tissues. The bacteria thrive in warm and wet conditions, but cannot survive long in fallow soil. However, it is able to persist for long periods of time in the rhizosphere of weeds and non-host crops. While it is difficult to eradicate, there are cultural practices that can be used to control the spread of the disease, such as avoiding injury to the plants and reducing or eliminating application of nitrogen fertilizer.
Blackleg is a plant disease of potato caused by pectolytic bacteria that can result in stunting, wilting, chlorosis of leaves, necrosis of several tissues, a decline in yield, and at times the death of the potato plant. The term "blackleg" originates from the typical blackening and decay of the lower stem portion, or "leg", of the plant.
Rice-sheath blight is a disease caused by Rhizoctonia solani, a basidiomycete, that causes major limitations on rice production in India and other countries of Asia. It is also a problem in the southern US, where rice is also produced. It can decrease yield up to 50%, and reduce its quality. It causes lesions on the rice plant, and can also cause pre- and post-emergence seedling blight, banded leaf blight, panicle infection and spotted seed.
Amy Olymbia Charkowski is an American plant pathologist and Professor of Plant Pathology at Colorado State University. She was elected Fellow of the American Association for the Advancement of Science in 2020.
