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 (that is an area equivalent to almost 20,000 football (soccer) pitches – just slightly smaller than the entire country of Liechtenstein!). 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 a number of different diseases. In many cases, wild plant species were also found to carry this pathogen, although often only in a latent stage.
The distribution range of Xylella fastidiosa is usually limited to tropical and subtropical areas, with 26–28°C being its optimal growing temperature range. 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 in 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. Depending on the strain of bacteria and the variety of the host plant, the symptoms of infection can vary considerably in different species. In some hosts, the bacteria multiply locally but cannot move around the host plant and, eventually, the infection can regress spontaneously; whereas in others the arrival of the bacteria inevitably leads, sooner or later, to the outbreak of a real catastrophe.
Many insects feeding on the sap of the plant’s xylem, i.e. the 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 the host plant, vector species and bacterium subspecies, in interaction with 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 for the bacteria to colonize, due to the fast flow of sap, that has been 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.
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 next parts of this blog.
