When was Xylella fastidiosa first detected in Apulia, Italy?
The first report of X. fastidiosa occurred in October 2013 by CNR researchers in Bari. Several independent research groups have now detected this bacterium in the region. Because it was found so consistently from olive trees with a severe newly recognized disease and not in healthy olive trees, it was considered the main suspect as causing the olive disease. It is not known in what year the new olive disease first appeared or when X. fastidiosa was introduced, but given the size of the area affected we speculate that the introduction occurred as many as ten or more years ago.
What plant species is this bacterium associated with in Italy?
The main plant species currently associated with X. fastidiosa is olive. In addition the following plants have been reported as symptomatic hosts harboring X. fastidiosa: almond, oleander, cherry, and several other perennial ornamentals. Previously, researchers in Florida, California, and Brazil have found that most plant species tested as hosts support some multiplication of the bacterium without showing symptoms.
Where did this strain of X. fastidiosa, dubbed CoDiRO, come from?
There has been only one genotype of X. fastidiosa detected in Apulia, the strain was named CoDiRO (Complesso del Disseccamento Rapido dell’Olivo) due to its association with an olive disease emerging in the region. Molecular testing by different Italian research groups has yielded identical results, demonstrating that the CoDiRO strain matches a strain previously found in Costa Rica.
How did the CoDiRO strain arrive in Apulia?
There have been several recent interceptions of X. fastidiosa in infected ornamental coffee plants in Italy, France, Holland, and Germany. In all but one case these plants originated from Costa Rica. In Corsica and Nice, France in 2015, scientists verified the occurrence of a different subspecies of X. fastidiosa (subspecies multiplex) in numerous plant species, some with disease symptoms, many without. Corsican olive trees tested negative. Together with the genetic data, the most logical interpretation of available information to date is that the CoDiRO strain arrived from Central America via the introduction of infected plant material.
Is X. fastidiosa causing disease in olive trees in Italy?
Xylella fastidiosa has been detected and isolated from symptomatic olive trees. In plant pathology, conclusive evidence that a pathogen causes a specific disease requires fulfillment of Koch’s postulates, a series of experiments that lead to proof of pathogenicity. The researches to fulfill Koch’s postulates for strain CoDiRO and olive proved that this strain of X. fastidiosa cause the olive quick decline syndrome (OQDS).
How is the bacterium spreading in the region?
Movement of X. fastidiosa from one plant to another occurs exclusively via insect vectors and use of infected grafting material. Researchers in Apulia have already identified one vector species, a common and widespread spittlebug, Philaenus spumarius. In experiments, P. spumarius transmitted X. fastidiosa experimentally from olive to olive This insect emerges from eggs in late winter-early spring. As weeds in orchards and other nearby locations dry or die in late spring, the adult spittlebugs move into trees to feed. Olive appears to be a favored host at that time. By mid-summer, testing of spittlebugs from olive trees showed that a majority of the insects harbored X. fastidiosa.
A USDA-led study in California indicated that X. fastidiosa does not cause disease in olive, correct?
Yes, that is correct. The study was led by one of us (R. Krugner) and it failed to demonstrate that X. fastidiosa causes disease in olive in California. However, the strains of X. fastidiosa used in that study belonged to two subspecies: fastidiosa and multiplex. The CoDiRO strain belongs to a different subspecies, named pauca, which is not known to occur in the USA. Because of differences in plant host range among X. fastidiosa subspecies and genotypes within subspecies, the results obtained in California are not directly transferable to the case in Apulia. The role of the CoDiRO strain on the etiology of the olive disease in Apulia remains to be proven by scientists.
But all these subspecies are considered to be within the species X. fastidiosa. Are there any meaningful differences?
Yes. By definition bacterial species group large amounts of genetic and phenotypic diversity. As an analogy, this might be the equivalent of calling all primates one species. In the case of X. fastidiosa, specifically, it is known that two genotypes within the same subspecies that occur at the same site and share insect vectors can have different plant host ranges. Subspecies of X. fastidiosa are categories used to classify genetic groupings of strains that have similar plant host ranges.
Are efforts to manage the problem going to work?
Unfortunately this is an emerging disease and a new introduction of a pathogen in the region. We suggest that first control attempts must rely on past experiences that show what works best in other diseases caused by X. fastidiosa, such as in grape (Pierce’s disease in North America) or citrus (citrus variegated chlorosis in Brazil). Management methods evolve from a combination of basic and applied field research and the results from testing the most promising control methods. Because property lines do nothing to stop flying vector insects from spreading X. fastidiosa, most effective control measures require regional education and coordination. Without near universal cooperation and coordination of control measures, disease management will fail. Effective disease management may also need to vary from region to region because of regional differences in insect vectors, climate, and other factors. Unfortunately, we cannot speed up nature to answer these questions.
—
Alexander Purcell, Rodrigo Krugner, and Rodrigo Almeida