Xylella fastidiosa
Xylella fastidiosa is an aerobic, Gram-negative bacterium of the monotypic genus "Xylella". It is a plant pathogen, and is transmitted exclusively by xylem fluid-feeding sap insects. Many plant diseases are due to symptomatic infections of "X. fastidiosa", including bacterial leaf scorch, oleander leaf scorch, coffee leaf scorch (CLS), alfalfa dwarf, phony peach disease, and the economically important Pierce's disease of grapes (PD) and citrus variegated chlorosis (CVC). In Europe, it has attacked olive trees in the Salento area of Southern Italy, causing olive quick decline syndrome (OQDS). While distributions of "X. fastidiosa"–related diseases are mostly  limited to the Americas, outbreaks have occurred in Taiwan, Slovakia, and other countries worldwide. Many plants are asymptomatic carriers of the bacteria, which can contribute to their spread. Pathogenicity of this species occurs only when a large proportion of xylem vessels in a plant are colonized; often, the colonies of bacteria themselves are not large enough to completely block the vessels, and the mechanism of pathogenesis is largely unknown. A subspecies of "X. fastidiosa" responsible for citrus variegated chlorosis was the first nonviral plant pathogen to have its genome sequenced, in part because of its potential to devastate affected crops . Pathogen anatomy and disease cycle. "Xylella fastidiosa" is rod-shaped, and at least one subspecies has two types of pili on only one pole; longer, type IV pili are used for locomotion, while shorter, type I pili assist in biofilm formation inside their hosts. As demonstrated using a PD-related strain, the bacterium has a characteristic twitching motion that enables groups of bacteria to travel upstream against heavy flow, such as that found in xylem vessels. It is obligately insect-vector transmitted from xylem-feeding insects directly into xylem, but infected plant material for vegetative propagation (e.g. grafting) can produce  mature plants that also have an "X. fastidiosa" disease. In the wild, infections tend to occur during warmer seasons, when insect vector populations peak. The bacterium is not seed transmitted and has been historically difficult to culture (fastidious), as its specific epithet, "fastidiosa", reflects. "X. fastidiosa" can be divided into four subspecies that affect different plants and have separate origins. "X. fastidiosa" subsp. "fastidiosa" is the most studied subspecies, as it is the causal agent of PD; it is thought to have originated in southern Central America, and also affects other species of plants. "X. f. multiplex" affects many trees , including stone-fruit ones such as peaches and plums, and is thought to originate in temperate and southern North America. "X. f. pauca" is believed to have originated in South America. It is the causal agent of citrus variegated chlorosis (CVC) in Brazil and also affects South American coffee crops, causing coffee leaf scorch. "X. f. sandyi" is thought to have originated in the southern part of the United States, and is notable for causing oleander leaf scorch. "X. fastidiosa" has a two-part lifecycle, which occurs inside an insect vector and inside a susceptible plant. While the bacterium has  been found across the globe, only once the bacterium reaches systemic levels do symptoms present themselves. Within susceptible plant hosts, "X. fastidiosa" forms a biofilm-like layer within xylem cells and tracheary elements that can completely block the water transport in affected vessels. Symptoms. Significant variation in symptoms is seen between diseases, though some symptoms are expressed across species. On a macroscopic scale, plants infected with a "X. fastidiosa"-related disease exhibit symptoms of water deficiencies, manifesting as leaf scorching and stunting in leaves, fruit, and overall plant height. As the bacterium progressively colonizes xylem tissues, affected plants often block off  their xylem tissue, which can limit the spread of this pathogen; blocking can occur in the form of polysaccharide-rich gels, tyloses, or both. These plant defenses do not seem to hinder the movement of "X. fastidiosa". Occlusion of vascular tissue, while a normal plant response to infection, makes symptoms significantly worse; as the bacterium itself also reduces vascular function, a 90% reduction of vascular hydraulic function was seen in susceptible "Vitis vinifera". This bacterium rarely completely blocks vascular tissue. Smaller colonies usually occur throughout a high proportion of xylem vessels of a symptomatic plant. Xylella is a Gram-negative , xylem-limited illness that is spread by insects. It can damage a variety of broadleaved tree species that are commonly grown in the United States. Xylella Fastidiosa can be found in about 600 different plant species. Pierce's disease. Severe PD symptoms include shriveled fruit, leaf scorching, and premature abscission of leaves, with bare petioles remaining on stems. Citrus variegated chlorosis. This disease is named after the characteristic spotty chlorosis on upper sides of citrus leaves. Fruits of infected plants are small and hard. Leaf scorches. In coffee, premature abscission of leaves and fruits is of bigger concern than scorching . Environment. "X. fastidiosa" occurs worldwide, though its diseases are most prominent in riparian habitats including the southeastern United States, California, and South America. Symptoms of "X. fastidiosa" diseases worsen during hot, dry periods in the summer; lack of water and maximum demand from a full canopy of leaves, combined with symptoms due to disease, stress infected plants to a breaking point. Cold winters can limit the spread of the disease, as it occurs in California, but not in regions with milder winters such as Brazil. Additionally, dry summers seem to delay symptom development of PD in California. Any conditions that  increase vector populations can increase disease incidence, such as seasonal rainfall and forests or tree cover adjacent to crops, which serve as alternate food sources and overwintering locations for leafhoppers. Alexander Purcell, an expert on "X. fastidiosa", hypothesized that plants foreign to "X. fastidiosa"s area of origin, the neotropical regions, are more susceptible to symptom development. Thus, plants from warmer climates are more resistant to "X. fastidiosa" disease development, while plants from areas with harsher winters, such as grapes, are more severely affected by this disease. Host species. "X. fastidiosa" has a very wide host range; as of 2020, its  known host range was 595 plant species, with 343 species confirmed by two different detection methods, in 85 botanical families. Most "X. fastidiosa" host plants are dicots, but it has also been reported in monocots and ginkgo, a gymnosperm. However, the vast majority of host plants remain asymptomatic, making them reservoirs for infection. Due to the temperate climates of South America and the southeastern and west coast of the United States, "X. fastidiosa" can be a limiting factor in fruit crop production, particularly for stone fruits in northern Florida and grapes in California. In South America, "X. fastidiosa" can cause  significant losses in the citrus and coffee industries; a third of today's citrus crops in Brazil has CVC symptoms. "X. fastidiosa" also colonizes the foreguts of insect vectors, which can be any xylem-feeding insects, often sharpshooters in the Cicadellidae subfamily Cicadellinae. After an insect acquires "X. fastidiosa", it has a short latent period around 2 hours, then the bacterium is transmissible for a period of a few months or as long as the insect is alive. The bacterium multiplies within its vectors, forming a "bacterial carpet" within the foregut of its host. If the host sheds its foregut  during molting, the vector is no longer infected, but can reacquire the pathogen. At present, no evidence shows that the bacterium has any detrimental effect on its insect hosts. Oleander. Oleander leaf scorch is a disease of landscape oleanders ("Nerium oleander") caused by a "X. fastidiosa" strain that has become prevalent in California and Arizona, starting in the mid-1990s. This disease is transmitted by a type of leafhopper (insect) called the glassy-winged sharpshooter ("Homalodisca coagulata"). Oleander is commonly used in decorative landscaping in California, so the plants serve as widely distributed reservoirs for "Xylella". Both almond and oleander plants  in the Italian region of Apulia have also tested positive for the pathogen. Grape vines. Pierce's disease (PD) was discovered in 1892 by Newton B. Pierce (1856–1916; California's first professional plant pathologist) on grapes in California near Anaheim, where it was known as "Anaheim disease". The disease is endemic in Northern California, being spread by the blue-green sharpshooter, which attacks only grapevines adjacent to riparian habitats. It became a real threat to California's wine industry when the glassy-winged sharpshooter, native to the Southeast United States, was discovered in the Temecula Valley in California in 1996 ; it spreads PD much more extensively than other vectors. Symptoms of infection on grape vines. When a grape vine becomes infected, the bacterium causes a gel to form in the xylem tissue of the vine, preventing water from being drawn through the vine. Leaves on vines with Pierce's disease turn yellow and brown, and eventually drop off the vine. Shoots also die. After one to five years, the vine itself dies. The proximity of vineyards to citrus groves compounds the threat, because citrus is not only a host of sharpshooter eggs, but also is a popular overwintering site for  this insect. Collaborative efforts for solutions. In a unique effort, growers, administrators, policy makers, and researchers are working on a solution for this immense "X. fastidiosa" threat. No cure has been found, but the understanding of "X. fastidiosa" and glassy-winged sharpshooter biology has markedly increased since 2000, when the California Department of Food and Agriculture, in collaboration with different universities, such as University of California, Davis; University of California, Berkeley; University of California, Riverside, and University of Houston–Downtown started to focus their research on this pest. The research explores the different aspects of the disease propagation from the  vector to the host plant and within the host plant, to the impact of the disease on California's economy. All researchers working on Pierce's disease meet annually in San Diego in mid-December to discuss the progress in their field. All proceedings from this symposium can be found on the Pierce's disease website, developed and managed by the Public Intellectual Property Resource for Agriculture (PIPRA). Few resistant "Vitis vinifera" varieties are known, and Chardonnay and Pinot noir are especially susceptible, but muscadine grapes ("V. rotundifolia") have a natural resistance. Pierce's disease is found in the Southeastern  United States and Mexico. Also, it was reported by Luis G. Jiménez-Arias in Costa Rica, and Venezuela, and possibly in other parts of Central and South America. There are isolated hot spots of the disease near creeks in Napa and Sonoma in Northern California. Work is underway at UC Davis to breed PD resistance from "V. rotundifolia" into "V. vinifera". The first generation was 50% high-quality "V. vinifera" genes, the next 75%, the third 87% and the fourth 94%. In the spring of 2007, seedlings that are 94% "V. vinifera "were planted. A resistant variety, 'Victoria Red', was  released for use especially in Coastal Texas. Olive trees. In October 2013, the bacterium was found infecting olive trees in the region of Apulia in southern Italy. The disease caused rapid decline in olive grove yields, and by April 2015, was affecting the whole Province of Lecce and other zones of Apulia, though it had not previously been confirmed in Europe. The subspecies involved in Italy is "X. f. pauca", which shows a marked preference for olive trees and warm conditions and is thought to be unlikely to spread to Northern Europe. The cycle in olives has been called olive  quick decline syndrome (in ). The disease causes withering and desiccation of terminal shoots, distributed randomly at first but then expanding to the rest of the canopy resulting in the collapse and death of trees. In affected groves, all plants normally show symptoms. The most severely affected olives are the century-old trees of local cultivars Cellina di Nardò and Ogliarola salentina. By 2015, the disease had infected up to a million olive trees in Apulia and "Xylella fastidiosa" had reached Corsica, By October 2015, it had reached Mainland France, near Nice, in Provence-Alpes-Côte d'Azur, affecting the non-native  myrtle-leaf milkwort ("Polygala myrtifolia"). This is the subspecies "X. fastidiosa" subsp. "multiplex" which is considered to be a different genetic variant of the bacterium to that found in Italy. On 18 August 2016 in Corsica, 279 foci of the infection have been detected, concentrated mostly in the south and the west of the island. In August 2016, the bacterium was detected in Germany in an oleander plant. In January 2017 it was detected in Mallorca and Ibiza. Notably, in 2016, olive leaf scorch was first detected in "X. fastidiosa"s native range, in Brazil. In June 2017, it was detected  in the Iberian peninsula, specifically in Guadalest, Alicante. In 2018, it was detected in Spain and Portugal, and in Israel in 2019. Genome sequencing. The genome of "X. fastidiosa" was sequenced by a pool of over 30 research laboratories in the state of São Paulo, Brazil, funded by the São Paulo Research Foundation. In popular culture. Two episodes of the Canadian television show "ReGenesis" investigated "X. fastidiosa", the cause of a massive loss in Florida orange groves.