X. fastidiosa is a fastidious Gram-negative, xylem-limited bacterium, rod-shaped with rippled cell walls. It is strictly aerobic (microaerophilic), non-flagellate, does not form spores and measures 0.1-0.5 x 1-5 µm. The peach strain was given by Nyland et al. (1973) as 0.35 x 2.3 µm. See also Bradbury (1991). Thread-like strands (fimbriae) attached to the polar ends of bacterial cells can be observed in electron microscopy (Mircetich et al., 1976) and scanning electron microscopy (Feil et al., 2003b). These probably function in bacterial attachment and 'twitching' movement (Meng et al., 2005).
Symptoms are not reliable for detection of infected plants in transit.
X. fastidiosa can be detected microscopically (light or electron) in vessels in cross-sections of petioles (French et al., 1977) or by examining xylem sap squeezed from symptomatic stems or petioles or flushed from stems or petioles onto microscope slides (De Lima et al., 1998). Flushing of xylem sap from shoots with a pressure chamber allows the testing of larger sample sizes and avoids inhibitors for PCR (Bextine and Miller, 2004). Methods such as grafting to susceptible indicator plants or vector tests (Hutchins et al., 1953) are still available, and may have their place in certification schemes in which woody indicators are routinely used. X. fastidiosa can also be isolated onto suitable selective media (Davis et al., 1978, 1983;Raju et al., 1982;Wells et al., 1983). The identity of cultured bacteria can be confirmed by SDS-PAGE (Bazzi et al., 1994). Serological methods are less sensitive (10- to 100-fold) than culture but are the easiest means of detecting and identifying the bacterium, by ELISA or use of fluorescent antibodies (French et al., 1978;Walter, 1987;Hopkins and Adlerz, 1988;Sherald and Lei, 1991). Strains differ in quantitative reaction to antisera and in ease and efficiency of culture. DNA hybridization probes and PCR primers specific to X. fastidiosa have been developed (Firrao and Bazzi, 1994;Minsavage et al., 1994). X. fastidiosa can also be detected in its insect vectors (Yonce and Chang, 1987). The characterization and identification of strains chiefly employs molecular genetic methods (e.g., Chen et al., 1992;Hendson et al., 2001;Coletta-Filho et al., 2003), and can be expected to remain indefinitely in a state of change.
Different diagnostic methods used or developed for the detection and identification of X. fastidiosa are detailed in Janse (2009). Recent advances in detection include on-site molecular detection using real-time loop-mediated isothermal amplification (Yaseen et al., 2015).
Related invasive species
- Xylella fastidiosa
Related Farm Practice
- Insecticides
- Recovery
- Light
- Orchards
- Damage
- Thinning
- Production
- Movement
- Control
- Vectors
On grapevines
The most characteristic symptom of primary infection is leaf scorch. An early sign is sudden drying of part of a green leaf, which then turns brown while adjacent tissues turn yellow or red. The desiccation spreads and the whole leaf may shrivel and drop, leaving only the petiole attached. Diseased stems often mature irregularly, with patches of brown and green tissue. In later years, infected plants develop late and produce stunted chlorotic shoots. Chronically infected plants may have small, distorted leaves with interveinal chlorosis and shoots with shortened internodes. Highly susceptible cultivars rarely survive more than 2-3 years, despite any signs of recovery early in the second growing season. Young vines succumb more quickly than do older vines. More tolerant cultivars may survive chronic infection for more than 5 years (Hewitt et al., 1942;Goodwin and Purcell, 1992).
On peaches
Young shoots are stunted and bear greener, denser foliage (due to shorter internodes) than healthy trees. Lateral branches grow horizontally or droop, so that the tree seems uniform, compact and rounded. Leaves and flowers appear early, and leaves remain on the tree longer than on healthy trees. Affected trees yield increasingly fewer and smaller fruits until, after 3-5 years, they become economically worthless (Hutchins, 1933).
On citrus
Trees can start showing the symptoms of variegated chlorosis from nursery size up to more than 10 years of age. Younger trees (1-3 years) become systemically colonized by X. fastidiosa much faster than do older trees. Trees more than 8-10 years old are not usually totally affected, but rather have symptoms on the extremities of branches. Affected trees show foliar chlorosis resembling zinc deficiency with interveinal chlorosis. The chlorosis appears on young leaves as they mature and may also occur on older leaves. Newly affected trees show sectoring of symptoms, whereas trees which have been affected for a period of time show the variegated chlorosis throughout the canopy. As the leaves mature, small, light-brown, slightly raised gummy lesions (becoming dark-brown or even necrotic) appear on the underside, directly opposite the yellow chlorotic areas on the upper side.
Fruit size is greatly reduced;it may take 550 affected fruits to fill a field box, compared with 250 normal fruits. The sugar content of affected fruit is higher than in non-affected fruit, and the fruit has a hard rind, causing damage to juicing machines. Blossom and fruit set occur at the same time on healthy and affected trees, but normal fruit thinning does not occur on affected trees and the fruits remain small but open earlier. Since more fruits remain, total production is not greatly reduced. On affected trees of cv. Pera and other orange cultivars, fruits often occur in clusters of 4-10, resembling grape clusters. Affected trees show stunting and slow growth rate;twigs and branches die back and the canopy thins, but affected trees do not die (Chang et al., 1993a,b;Lee et al., 1991, 1993).
Control has been achieved by removing inoculum in established orange groves and using sanitary measures to prevent infection of nurseries and new groves. All symptomatic branches from trees older than 3 years are cut off up to 1 m below the most basal symptoms. Symptomatic trees less than 4 years old are removed. To prevent the infection of nursery trees, nurseries are located away from citrus plantings, sharpshooters are controlled prophylactically by insecticides, and buds are taken from trees tested free of X. fastidiosa and grown vectors in screen houses or glass houses to exclude vectors. The effectiveness of these measures (Rodas, 1994) indicates that most spread of variegated chlorosis is from tree to tree within citrus orchards (Laranjeira, 1997).
On olives
On olives, quick decline syndrome is characterised by the development of leaf scorch symptoms and desiccation of small twigs and branches. Symptoms generally initiate in the upper part of the canopy on one or two branches, and then extend to the remainder of the crown. Severely affected plants are often pruned heavily, favouring spindly new growth which also succumbs to scorch symptoms. The tree may send out suckers from the base of the plant which subsequently die back, until the root system dies entirely (Martelli, 2016a). Grafting experiments have demonstrated that it takes at least 7 months for leaf scorch symptoms to appear on the grafted plant part (European Food Safety Authority, 2015).
Symptoms are found on all known varieties of olive. Older varieties, such as Ogliarola Salentina, Cellina di Nardò and common varieties Frantoio and Coratina, appear susceptible. It is suggested that the variety Leccino seems less susceptible, although records are based on field observations and are yet to be experimentally confirmed. Apparent variation in olive varietal susceptibility may be the result of differences in disease vector pressures in the areas where the disease is present (European Food Safety Authority, 2015).
Vectors
Vector feeding causes no visible damage. Xylem feeders are prodigious feeders, consuming hundreds of times their body volumes per day in xylem sap. Most non-xylem-feeding leafhoppers produce a sugary or particulate excrement, but that of xylem feeders is watery, drying to a fine whitish powder (brochosomes) where abundant (Rakitov, 2004). The excrement of froghopper nymphs takes the form of persistent bubbles or 'froth;that surrounds the body of the insect, presumably to provide protection from natural enemies.
No grapevine (Vitis spp.) species are known to be immune to Pierce’s disease strains of X. fastidiosa, but American species used as rootstocks (V. aestivalis, V. berlandieri, V. candicans, V. rupestris) and hybrids derived from them are tolerant and some may be resistant, as is V. rotundifolia (Goheen and Hopkins, 1988). Almonds and lucerne can be hosts of the grapevine strains, but the diseases caused by X. fastidiosa in these three crop species are independent within California, USA, suggesting as yet unidentified biological differences (Purcell, 1980b). A very high percentage (75% of those tested) of crop, wild plant and weed species can carry Pierce’s disease strains of the bacterium without symptoms (e.g. wild grasses, sedges, lilies, various bushes and trees) (Raju et al., 1983;Hopkins and Adlerz, 1988;Hill and Purcell, 1995b). It is likely that in most symptomless host species, X. fastidiosa multiplies to lower populations and moves systemically less often than in pathological hosts. For example, blackberry (Rubus spp.) can be a systemic host, but the bacterium multiplies in mugwort (Artemisia douglasiana) without systemic movement (Hill and Purcell, 1995b). Hosts can be classified as propagative or non-propagative, systemic or non-systemic, and symptomatic or non-symptomatic (Purcell and Saunders, 1999b). Propagative, systemic hosts are the best hosts for efficient vector acquisition of bacteria, but vectors can acquire the bacterium from non-systemic hosts. Acquisition efficiency is proportional to the populations of live bacterial cells within plant tissues (Hill and Purcell, 1997).
Peach (Prunus persica) strains of X. fastidiosa cause peach phony disease (Wells et al., 1983), which also attacks Prunus salicina (causing leaf scald). All cultivars, forms and hybrids of peach are attacked, whether on their own roots or other rootstocks. Plums (Prunus domestica), almonds (P. dulcis), apricots (P. armeniaca) and the wild P. angustifolia were reported susceptible to phony disease before the association with X. fastidiosa was established. This reported range partly overlaps that of the grapevine-infecting strains. Various perennial weeds of orchards, such as Sorghum halepense, may act as reservoirs for the peach-infecting strain (Yonce, 1983;Yonce and Chang, 1987), but the plant host range of Prunus strains from the south-eastern USA has not been investigated extensively. Pierce's disease strains also cause almond leaf scorch disease (Davis et al., 1980), but the almond strains infect grape in low populations and without causing disease (Almeida and Purcell, 2003).
X. fastidiosa in the wide sense also causes leaf scorch in Acer rubrum (Sherald et al., 1987), Morus rubra (Kostka et al., 1986), Platanus occidentalis (Sherald, 1993a,b) (wilt and leaf scorch), Quercus rubra (Chang and Walker, 1988), Ulmus americana and Vinca minor (stunt). Strains from Ulmus and from P. occidentalis are not reciprocally infectious (Sherald, 1993a). The bacteria involved are not known to be transmissible to grapevine. Diseases of numerous woody ornamental plants in southern California, USA, including olive, date palm and rosemary, have been associated with X. fastidiosa but a causal relationship is still unproven (Wong and Cooksey, 2004). Until their relationships and pest significance have been clarified, they can all be regarded as potentially dangerous for Europe and the Mediterranean region.
X. fastidiosa causes citrus variegated chlorosis in Brazil (Lee et al., 1991;Chang et al., 1993;Hartung et al., 1994) and Argentina (Brlanksy et al., 1993). The disease affects mostly sweet oranges (Citrus sinensis);it has been observed especially on cultivars Pera, Hamlin, Natal and Valencia. It occurs on trees propagated on all commonly used rootstocks in Brazil: C. limonia, C. reshni and C. volkameriana. The disease has not been observed on C. latifolia or mandarins (C. reticulata), even when the trees were planted in severely affected orange groves (Li et al., 2000). The effectiveness of removing diseased citrus trees to prevent further spread of variegated chlorosis in citrus (Rodas, 1994) strongly suggests that most spread of this disease is from tree to tree within the crop. Control measures require the production of disease-free nursery trees in protected environments.
Citrus blight in Florida, USA, has been associated with X. fastidiosa (Adlerz et al., 1989;Hopkins et al., 1996);however the preponderance of evidence suggests that it is not the cause of blight (Derrick and Timmer, 2000).
Plum leaf scald is an important crop-limiting disease caused by X. fastidiosa from Brazil through Argentina. The South American plum leaf scald strains appear to differ from those in North America, as there are no reports of phony disease of peach in South America. The plum leaf scald strains in Brazil may have wide plant host ranges (Leite et al., 1997). A leaf scorching disease of coffee (De Lima et al., 1998) is caused by strains of X. fastidiosa that appear to be closely related to the citrus variegated chlorosis strains (Rosato et al., 1998), but its ability to cause disease in citrus (Li et al., 2001) is controversial.
In Europe and the Mediterranean region, grapevine and citrus are clearly the most significant potential crop hosts, although peach and plum are also important. Strains that cause leaf scorch diseases in oak, elm, sycamore (plane) (Hearon et al., 1980), mulberry (Kostka et al., 1986) and other tree species are also potentially damaging. Many other hosts could carry the bacterium, without necessarily being significantly affected.
X. fastidiosa has been implicated as the causal agent of olive quick decline syndrome in Europe. In 2013, X. fastidiosa subsp. pauca was associated with quick decline syndrome on olive, almond and oleander in Europe (southern Italy, Apulia region) (European Food Safety Authority, 2015). Symptomatic olive trees were often affected by multiple pests, including X. fastidiosa, several fungal species, and Zeuzera pyrina (leopard moth) (Nigro et al., 2013). Recent experimental evidence (Saponari et al., 2016) has confirmed X. fastidiosa as the causal agent of olive quick decline syndrome in Italy (European Food Safety Authority, 2016). In the USA a study evaluating olive as a host for X. fastidiosa concluded that subsp. multiplex was present but was not the cause of the leaf scorch and dieback symptoms observed on olive trees in California (Krugner et al., 2014). However, X. fastidiosa subsp. pauca has been implicated as a causal agent of olive plant dieback and leaf desiccation in Argentina (Haelterman et al., 2015). More recently, leaf scorch symptoms on olive trees in Brazil have been associated with X. fastidiosa subsp. pauca (Coletta-Filho et al., 2016).
The host range of X. fastidiosa based on the available peer-reviewed literature is presented in European Food Safety Authority (2015).
According to the European Food Safety Authority (2016), the current list of host plant species for X. fastidiosa consists of 359 plant species (including hybrids) from 204 genera and 75 different botanical families.