Not all Xylellas are born equal I

The first report about a disease caused by Xylella fastidiosa dates back to the end of the nineteenth century, when the so called “California vine disease” destroyed about 14,000 ha of grapes in the Los Angeles area. Newton Pierce (1856–1916), a bacteriologist, was assigned to study the epidemic and even though he was not able to identify the causal agent, he came to the conclusion that the disease was likely caused by a microscopic infectious agent. The disease was named Pierce’s disease in 1939 (PD), in a bulletin of the California Department of Agriculture, but for a long time the etiological agent was thought to be a virus, until it was recognized as a bacterium in 1973.
Since the first report in grape, Xylella fastidiosa was isolated and identified from an increasingly large number of plant hosts, with or without symptoms, and recognized to be the causal agent of different diseases. In many cases also wild plant species were found to carry this pathogen, but often in a latent stage only.
The distribution range of Xylella fastidiosa is usually limited to tropical and subtropical areas, being its optimal growing temperature 26–28°C. In some cases strains of Xylella fastidiosa have been found in much colder countries, such as Canada, even if usually this pathogen does not occur in areas with low winter temperatures, such as New York and the Pacific Northwest of USA, or at high altitudes. To date, most of the diseases caused by Xylella fastidiosa have been reported from North and South America. In Europe, Xylella fastidiosa was first recorded in Puglia region (southern Italy, province of Lecce), where it was recognized to be the causal agent of a dangerous disease of olive trees, the so-called olive quick decline syndrome (OQDS).
The initial belief that Xylella fastidiosa was a generalist pathogen capable of infecting a very large range of host plants has gradually changed with the discovery and characterization of genetically different strains of Xylella fastidiosa, each capable of infecting distinct hosts. Also the infection characteristics can vary considerably in different species. In some hosts the bacteria multiply locally but cannot move and eventually the infection can regress spontaneously, whereas in others the arrival of the bacteria leads earlier or later to an outbreak of a real catastrophy.
Xylella = Bacterium living in the xylem | © Shutterstock

Many insects feeding on the sap of the plant’s xylem, i.e. plant’s “aqueduct”, function as vectors (transmitters) of Xylella fastidiosa to host plants, especially sharpshooters and froghoppers or spittlebugs. After acquisition from the source plant, the bacterium is persistent in the vector and can multiply in its foregut.
The process of acquisition and transmission of Xylella fastidiosa by the vector is very complex and can be dependent on many variables such as host plant, vector species and bacterium subspecies in interaction environmental variables, first at all climate. Nevertheless, one important factor has been proven to influence the efficiency of acquisition, that is Xylella fastidiosa population size (number of live cells per gram of plant tissue). In fact, the feeding apparatus of vectors is not an easy environment to be colonized, due to the fast flow of sap, that was estimated to reach an average speed of 8 cm/s and the possible turbulence caused by the fast contractions of the muscles allowing the insect to pump sap from the plant, that are contracted and relaxed approximately once every second. Therefore, it is likely that only few cells, out of the thousands acquired by the vector when feeding from an infected plant, actually succeed to colonize the vector’s foregut.
Spread of Xylella disease | www.suertealta.es/la-xylella

Up to date, five to six different subspecies of Xylella fastidiosa, each colonizing different host plants, have been reported and classified: fastidiosa (earlier piercei), multiplex, pauca, sandyi, taschke and morus. A more detailed discussion of these subspecies, hosts attacked and diseases caused will be given in the second part of this blog.

[Partially based on https://doi.org/10.3389/fpls.2017.00944]

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