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Disease Cycle and Epidemiology

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Xylella fastidiosa is spread primarily by leafhopper insects (subfamily Cicadellidae) known as sharpshooters, and to a lesser extent, spittlebugs (family Cercopidae). These insects have piercing-sucking mouthparts and subsist on xylem fluid. Both adult and immature (nymph) stages acquire the bacterium when feeding on succulent tissues of infected hosts. As xylem fluid is drawn into the insect, bacterial cells attach to the cibarial pump and the lining of the esophagus (collectively known as the foregut of the insect). There, the bacterium multiplies and forms a biofilm, where it becomes encased and protected in a bacterial “glycocalyx” (made of polysaccharide and protein), extracting nutrients from xylem fluid as it is pumped through the insect.

Once an insect acquires the bacterium, transmission to a new host can begin within 1 to 2 hours. In the early stages of feeding, bacterial cells become dislodged and are pumped directly into the xylem where systemic movement within the host occurs. An adult can transmit the bacterium for the remainder of its life, whereas nymphs, which shed the foregut during molting, can do so only until the next molt. In grapes with Pierce’s disease, a threshold population of bacteria (10⁴ to 10⁵ colony forming units per gram) is needed before transmission by insect vectors becomes likely. It is not known whether a similar threshold is needed for the pathogens that cause BLS in shade trees.

Xylem-feeding insects, particularly leafhoppers, can be polyphagous (i.e., they feed on many different hosts within a single season). Many of the alternative (non economically important) hosts of X. fastidiosa mentioned earlier serve as a food source for potential leafhopper vectors, and many leafhoppers overwinter as adults on these alternative plants. Alternative hosts may be the source of a substantial amount of inoculum that is transmitted to economically important crops, such as grape and peach, by vectors that feed on both types of hosts. The insects that vector some economically important diseases (Table 5), such as Pierce’s disease of grapevine and phony peach disease, are known (e.g., species of the leafhoppers Graphocephala (Figure 18), Homalodisca, and Oncometopia), and their role in the disease process is well characterized. Insects that vector BLS in shade trees, however, have yet to be identified, nor yet is known the role that alternative hosts play in the disease process for shade tree hosts. Current research on BLS in oak and other shade tree hosts indicates that several known vectors of other diseases caused by X. fastidiosa are present in shade trees during the growing season (e.g., Graphocephala and Oncometopia species). Their role in the transmission of BLS has yet to be confirmed.

Figure 18

In some hosts, X. fastidiosa is known to pass through root grafts (e.g., almond, citrus, grape, peach) and to seed (e.g., citrus). The importance of these methods of transmission in shade trees, however, is not known. Since disease development within populations of trees is most often random, direct tree-to-tree spread is unlikely. It is not known, however, how long urban forest trees remain asymptomatic following infection, thus other methods of transmission may, indeed, occur.

Mechanism of disease development

X. fastidiosa lives and multiples within the tracheary elements, tracheids, vessels, and intercellular spaces of xylem tissue (Figure 19). Compared to phloem, xylem fluid is nutritionally poor but does consist of amino, organic, and inorganic acids. These compounds, especially the amino acids glutamine and asparagine, are used by both the bacterium and the insect vector for growth. The quality and composition of xylem fluid varies both between hosts and within a host by season, time of day, and with the health and age of the host plant. This affects the feeding behavior of the vector; insects move from plant to plant to obtain the right combinations of nutrients needed for different stages of growth. Those insects that harbor X. fastidiosa transmit the pathogen to new hosts as they search for suitable sources of nutrients.


Figure 19a	Figure 19b

Cells of X. fastidiosa attach to xylem vessel walls as well as to the foregut of insect vectors by producing biofilms (Figure 20). Bacterial cells aggregate together and are encased in a self-produced matrix of polysaccharide. This protects the cells and is thought to enhance pathogenicity. Terminal fimbriae (also called type IV pili) are important for biofilm formation. In addition, although the bacterium lacks flagella for motility, terminal fimbriae aid in a type of incremental movement called “twitching motility,” which enables bacterial cells to move against the xylem stream.


Figure 20a	Figure 20b

Symptoms of water stress (evident as leaf scorch) result from high populations of bacterial cells in xylem tissue as well as overproduction of defense compounds, such as pectins, and tyloses, produced by the host plant in response to infection. Embolisms (or air bubbles) eventually form and help to plug affected xylem vessels, leading to reduced xylem function and water stress. When prolonged, tissue available for photosynthesis is reduced and starch reserves are depleted, resulting in leaf scorching and premature senescence.

Populations of bacterial cells within xylem tissue fluctuate seasonally. In diseases where stunting is a primary symptom, such as phony peach disease and alfalfa dwarf, bacteria congregate in the roots. In hosts where leaf scorch is a primary symptom, such as BLS of shade trees, the pathogen resides above ground, and populations are greatest in the veins and petioles of symptomatic leaves. Most strains of X. fastidiosa are sensitive to cold, and in shade trees, the pathogen is presumed to overwinter in more protected parts of the tree, such as the trunk or roots, and as the growing season progresses, populations increase and the pathogen moves acropetally to distal portions of the canopy. Indeed, most Xylella-associated diseases occur in regions of North America where the winters are mild, such as the southeastern U.S. For example, the threat of Pierce’s disease is low where the average January temperature is less than 36 F. Strains that cause bacterial leaf scorch of urban forest trees, however, are more cold hardy and have been detected as far north New York and southern Ontario. These reports, however, are rare; the pathogen that affects oak has the most northern range (the disease is epidemic in certain regions of New Jersey) and may be more tolerant of cold than those than affect sycamore and elm, which are more prevalent in the mid-Atlantic states and south. Although cold hardiness of the pathogen may explain the geographic distribution of Xylella-associated diseases, vector movement may also play a role. For bacterial leaf scorch of shade trees, these questions have yet to be answered.

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