xylella fastidiosa xylella fastidiosa xylella fastidiosa is an aerobic, gram-negative bacterium of the genus xylella. it is a plant pathogen, that grows in the water transport tissues of plants' (xylem vessels) and is transmitted exclusively by xylem sap-feeding insects such as sharpshooters and spittlebugs. many plant diseases are due to 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), olive quick decline syndrome (oqds), and citrus variegated chlorosis (cvc). while the largest outbreaks of x. fastidiosa–related diseases have occurred in the americas and europe, this pathogen has also been found in taiwan, israel, and a few other countries worldwide. xylella fastidiosa scientific classification domain: bacteria phylum: pseudomonadota class: gammaproteobacteria order: xanthomonadales family: xanthomonadaceae genus: xylella species: x. fastidiosa binomial name xylella fastidiosawells et al., 1987 xylella fastidiosa can infect an extremely wide range of plants, many of which do not show any symptoms of disease. disease occurs in plant species that are susceptible due to blockage of water flow in the xylem vessels caused by several factors: bacterial obstruction, overreaction of the plant immune response (tylose formation), and formation of air embolisms. a strain of x. fastidiosa responsible for citrus variegated chlorosis was the first bacterial plant pathogen to have its genome sequenced, in part because of its importance in agriculture. due to the significant impacts of this pathogen on agricultural crops around the world there is substantial investment in scientific research related to x. fastidiosa and the diseases it causes. 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, but instead is transmitted through "xylem feed-ing, suctorial homopteran insects such as sharpshooter leafhoppers and spittle bugs" 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. once established in a new region, x. fastidiosa spread is dependent on the obligate transmission by xylem-sap feeding insect. 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. strains eb92-1 is a nonpathogenic strain of x. f. which is used as a biocontrol of its relatives. (really it is dramatically less pathogenic. it does colonize grape vines but rarely and less severely.) zhang et al., 2011 finds very little genomic distance between pathogenic and eb92-1 strains. 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, zinc, and iron deficiencies, manifesting as leaf scorching and stunting in leaves turning them yellowish-brown, gummy substance around leaves, fruit reduction in size and quality, 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. there usually is a slight amount of vascular function that keeps the plant alive, but makes its fruit or branches die, making the specific plant economically nonproductive. this can cause a massive drop on supply of quality fruit. smaller colonies usually occur throughout a high proportion of xylem vessels of a symptomatic plant. x. fastidiosa 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. x. fastidiosa can be found in about 600 different plant species. withering and desiccation of branches leaf chlorosis dwarfing or lack of growth of the plant drooping appearance and shorter internodes shriveled fruits on infected plants premature fruit abscission gum-like substance on leave hardening and size reduction of fruit 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. some isolates cause almond leaf scorch, in california that includes cfbp8071 and m23. coffee leaf scorch (cls) is a disease caused by the causal agent xylella fastidiosa that is economically significant in brazil. citrus variegated chlorosis (cvc), another significant disease in this region caused by a strain of x. fastidiosa has been shown to infect coffee plants with cls. the disease has also been found in costa rica's central valley where it is referred to as ‘crespera’ disease by coffee growers. symptoms of the bacterial infection in coffee plants feature curling leaf margins, chlorosis and irregularly shaped leaves, stunting and reduced plant growth, and branch atrophy. the disease reduced coffee production by up to 30% in plantations across brazil. x. fastidiosa was discovered in apulia, italy in 2013 for the first time as a destructive disease agent of olive trees and likely came from strains present in asymptomatic plant material imported from costa rica. 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. list of subspecies fastidiosa susceptible plants (recreated from efsa panel) family genus adoxaceae sambucus amaranthaceae alternanthera, chenopodium anacardiaceae rhus, toxicodendron apiaceae conium, datura, daucus, oenanthe apocynaceae nerium, vinca araliaceae hedera asteraceae ambrosia, artemisia, baccharis, callistephus, conyza, franseria, helianthus, lactuca, solidago, sonchus, xanthium betulaceae alnus boraginaceae amsinckia brassicaceae brassica cannaceae canna caprifoliaceae lonicera, symphoricarpos convolvulaceae convolvulus, ipomoea cyperaceae cyperus fabaceae acacia, chamaecrista, cytisus, genista, lathyrus, lupinus, medicago, melilotus, spartium, trifolium, vicia fagaceae quercus geraniaceae erodium, pelargonium juglandaceae juglans lamiaceae callicarpa, origanum, melissa, mentha, rosmarinus, salvia lauraceae persea, umbellularia magnoliaceae magnolia malvaceae malva myrtaceae eucalyptus, eugenia, metrosideros oleaceae fraxinus, syringa onagraceae epilobium, fuchsia, clarkia, oenothera pittosporaceae pittosporum platanaceae platanus poaceae avena, bromus, cynodon, digitaria, echinochloa, eragrostis, eriochola, festuca, holcus, hordeum, lolium, paspalum, pennisetum, phalaris, phleum, poa, setaria, sorghum polygonaceae persicaria, polygonum, rheum, rumex portulacaceae montia, portulaca resedaceae reseda rhamnaceae rhamnus rosaceae cotoneaster, fragaria, photinia, prunus, rosa, rubus rubiaceae coffea, coprosma rutaceae citrus salicaceae populus, salix sapindaceae acer, aesculus scrophulariaceae veronica simmondsiaceae simmondsia solanaceae datura, lycopersicon, nicotiana, solanum urticaceae urtica verbenaceae duranta vitaceae ampelopsis, parthenocissus, vitis 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