This is the second in our blog series about Xylella, one of the most dangerous plant bacteria in the world. Xylella causes a variety of diseases, which have huge economic, societal, and cultural impacts and affect agriculture, rural landscapes, gardens, and the environment. The bacteria live in the plant xylem tissue (part of the system which transports water and nutrients around a plant) and are spread by insect vectors feeding on sap from the plant xylem (though the bacteria can also be spread mechanically (i.e. by humans when grafting one plant onto another)). Symptoms associated with the presence of Xylella fastidiosa in plants vary broadly: from non-expressed infections, to swift decline and plant death within a short period of time (one to four growing seasons).
As stated in part one of this blog series, up to date, six different subspecies of Xylella fastidiosa, each with a preference towards colonizing different host plants, have been reported and classified: fastidiosa, multiplex, pauca, sandyi, taschke and morus.
The following graphic illustrates the known impacts of various subspecies of Xylella fastidiosa on our major fruit trees as well as the diseases caused and major countries affected:
Grapes
The destruction of around 14,000 ha of grapevines in the Los Angeles area at the end of the nineteen century by then called “California vine disease” appears the be first documented case of major attack of Xylella fastidiosa on economically significant agricultural crops. In the southern San Joaquin Valley of California, epidemics of what is now called Pierce’s disease of grapevine are associated with Xylella fastidiosa subspecies fastidiosa and the glassy-winged sharpshooter, Homalodisca vitripennis, as the vector.
Pierce’s disease causes symptoms on grapevines that are very similar to those caused by water stress, as the bacteria produces gummy films that block the xylem of the host plant and cause it to shrivel and, ultimately, die. Symptoms can also appear similar to those caused by salt toxicity. Fruit production is severely reduced, with leaf scorching and wilting leading to shrivelled, unsaleable fruit. The disease can be so severe that it can kill commercial grapevines within 1–4 years. As countermeasures, vineyards in this area are typically treated with a systemic neonicotinoid in spring of each year to reduce the population of the insect vector.
Is seems that activities of Xylella fastidiosa against grapevines are mostly limited to Western Californian vineyards where the bacteria flourish thanks to a combination of favourable climatic conditions for them and their main vector.
A promising innovative solution has been reported recently by Texas A&M AgriLife Center for Phage Technology. The research work led by Dr. Carlos Gonzalez has led to the discovery of the first curative and preventive bacteriophage treatment against Xylella fastidiosa. A bacteriophage therapy is a precision treatment of bacterial infections that use viruses that only infect and kill the bacterium. Bacteriophages are considered a promising alternative to antibiotics for treating infections in humans, animals and plants.
The treatment with the commercial name XylPhi-PD has been approved by the U.S. Environmental Protection Agency, EPA, is registered with the California Department of Pesticide Regulation, CDPR, and is approved for use in organic production by the Organic Materials Review Institute, OMRI.
XylPhi-PD is an injectable treatment that targets the disease from within the plant’s vascular system, helping to cure the infected grapevine and stopping the spread to surrounding vines. After two seasons of use in high-disease-pressure vineyards, it’s been demonstrated that XylPhi-PD treatment of diverse wine varietals consistently prompted reductions of Pierce’s disease by 60%, across differing conditions and locales.
Next up: the discussion of further food crops susceptible to Xylella fastidiosa will continue in the third part of this blog series.
