Xylella fastidiosa sequence types and host specificity

The pathogen Xylella fastidiosa has been hitting the headlines in the UK recently, because there is growing concern over the risk of it reaching the UK shores and not being caught quickly. This bacteria ultimately causes a leaf scorch that, over time, progresses throughout the crown of its host, and will eventually cause tree mortality. Because it’s not native to the UK, its introduction would potentially be very significant, given no native tree species have any form of resistance to it. Furthermore, as it may attack a huge variety of trees and plants, understanding a little more about its biology is certainly something that should be pursued. With that in mind, I thought I’d look at a study from the District of Colombia, Washington, USA that sought to investigate the population structure of the bacteria in an urban streetscene, with specific focus upon how the bacteria associated with the constituent tree species.

Within the District’s street scape, infected trees of the species Morus alba, Platanus occidentalis, Quercus coccinea, Quercus palustris, Quercus phellos, Quercus rubra, and Ulmus americana had foliar samples taken from their structure, which were then taken into the laboratory and the constituent Xylella fastidiosa sequenced (DNA sequencing). Most samples were taken from trees displaying visible symptoms of infection, though some were also taken from trees, or locations upon an infected tree, where there were no symptoms (asymptomatic). The table below gives a breakdown of exactly what samples were taken from which trees.

The number of samples taken from each tree species.

Following DNA sequencing of all the samples, it was identified that there were five different ‘sequence types’ of the bacteria Xylella fastidiosa. These sequence types were almost exclusively unique to a specific tree genus, though one sequence type (ST-9), whilst found predominantly in oak, was also found in an elm sample (this specific sequence type has been found in elm before, and also sycamore). This near exclusivity of different sequence types occurred almost always, in spite of the fact that many study locations were comprised of infected trees of more than one of the species studied (for example, a site may have contained both mulberry and oak, or elm and sycamore, and so on). Curiously, the elm infected by a type found otherwise only on oak was not near to any oak tree.

Host specificity of different sequences.

This host specificity is important, because it means that a sequence type infected mulberry is highly unlikely to ever be pathogenic towards oak, for example. However, other studies have shown that ST-8, found here only on sycamore, could also be found on oak and elm. Therefore, certain sequence types may have the ability to infect more than one tree genus, though this is certainly not true for all sequence types.

In this sense, a few comments can be made: (1) Monocultures are certainly to be avoided, for those tree species that are susceptible to Xylella fastidiosa. They can swiftly become very extensive inoculum bases for the bacteria as a group, and if they succumb to the infection then an entire swathe of trees can readily be wiped-out. By planting and maintaining healthy populations of an array of tree species of different genera, the impact of Xylella fastidiosa can be reduced (though only in urban areas – outbreaks in woodland settings would be far more impactful, because of a reduced diversity of tree species).

(2) Management of the bacteria can be worked down to the sequence type level, and this may prove to be a double-edged sword in terms of management. In a positive light, because cross-contamination of a sequence type is likely to be quite infrequent, targeted management approaches can be created that are specific to the particular strain of the bacteria and specific to the area of infection. Of course, this is also concerning, because one control method for one sequence type may not necessarily work for another, and for those sequence types that do have more than one potential host genus, adaptations on the genetic level may enable it to branch-out to infect other tree genera as well, or become more adept as infecting peripheral species and genera in its current host range. This latter consideration may be one reason why limiting monocultures is important, as the build-up of the pathogen in a suitable inoculum base may potentially increase its pathogenicity.

Source: Harris, J. & Balci, Y. (2015) Population Structure of the Bacterial Pathogen Xylella fastidiosa among Street Trees in Washington DC. PLoS One. 10 (3). p1-11.