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 the 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.
Wells et al., 1987
Many plants are asymptomatic carriers of the bacteria, which can contribute to its spread. Pathogenicity of the bacterium 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 non-viral 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 assists in biofilm formation inside its 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 a 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.
Xylella 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 kinds of plants. X. fastidiosa subsp. multiplex affects many trees, including stone-fruit ones such as peaches and plums and is thought to originate in temperate and southern North America. Xylella fastidiosa subsp. pauca is believed to have originated in South America. It is the causal agent of CVC in Brazil and also affects South American coffee crops, causing coffee leaf scorch. Xylella fastidiosa subsp. sandyi is thought to have originated in the southern part of the United States of America, and is notable for causing oleander leaf scorch.
X. fastidiosa has a two-part life cycle 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.
There is significant variation in symptoms 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 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.
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.
In coffee, premature abscission of leaves and fruits is of bigger concern than scorching.
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 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 Xylella 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.
X. fastidiosa has a very wide host range: as of 2015, its host range was currently in excess of 560 plant species, with 312 species confirmed via two different detection methods, in 82 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 western 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; 1/3 of today's citrus crops in Brazil have CVC symptoms.
Xylella 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 acquiresX. fastidiosa, it has a short latent period of about 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, there is no evidence that the bacterium has any detrimental effect on its insect hosts.
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 (GWSS), native to the southeast United States, was discovered in the Temecula Valley in California in 1996. The GWSS spreads PD much more extensively than other vectors.
Symptoms of infection
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 will turn yellow and brown, and eventually drop off the vine. Shoots will also die. After one to five years, the vine itself will die. The proximity of vineyards to citrus orchards compounds the threat, because citrus is not only a host of sharpshooter eggs, but it is also a popular overwintering site of this insect. Likewise, oleander, a common landscaping plant in California, serves as a reservoir for Xylella.
Collaborative efforts for solutions
A unique effort from 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 Xylella 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 (UC Davis); University of California, Berkeley; University of California, Riverside, and the 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).
There are no resistant Vitis vinifera varieties, and Chardonnay and Pinot noir are especially sensitive, but muscadine grapes (Vitis 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 Vitis rotundifolia into Vitis vinifera. The first generation was 50% high-quality vinifera genes, the next 75%, the third 87% and the fourth 94%. In the spring of 2007, seedlings that are 94% vinifera were planted.
Oleander leaf scorch is a disease of landscape oleanders (Nerium oleander) caused by a X. fastidiosa strain which has become prevalent in California and Arizona, USA starting in the mid 1990s. This disease is transmitted by a type of leafhopper (insect) called the Glassy-winged sharpshooter (Homalodisca coagulata).
In October 2013, the bacterium was found infecting olive trees in the region of Apulia in southern Italy. The disease was causing a rapid decline in olive plantations and by April 2015, it was affecting the whole Province of Lecce and other zones of Apulia. The bacterium had never previously been confirmed in Europe. Almond and oleander plants in the region have also tested positive for the pathogen. The subspecies involved in Italy is X. fastidiosa subsp. pauca. This shows a marked preference for olive trees and warm conditions and is thought unlikely to spread into Northern Europe.
The disease has been called olive quick decline syndrome (OQDS; in Italian: complesso del disseccamento rapido dell'olivo). The disease causes withering and desiccation of terminal shoots, distributed randomly at first but which then expands to the rest of the canopy. This results in the collapse and death of the trees. In the affected groves, all of the plants show symptoms. The most severely affected olives are the century-old trees of local cultivars Cellina di Nardò and Ogliarola salentina.
Spread of syndrome
By the beginning of 2015, this disease had infected up to a million trees in the southern region of Apulia. By July 2015, Xylella fastidiosa had reached Corsica, by October 2015, it had reached Mainland France, near Nice, in Provence-Alpes-Côte d'Azur, affecting the myrtle-leaf milkwort (Polygala myrtifolia). This is the subspecies X. fastidiosa subsp. multiplex which is considered to be a new genetic variant of the bacterium, different 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. In June 2017, it was detected in the Iberian peninsula, specifically in Guadalest, Alicante. In April 2018 it was detected in Madrid.
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