Xylella Bacterium Found in Puglia’s Dying Olive Trees

Italian scientists have found DNA of Xylella fastidiosa, a quarantine bacterium never before recorded in the European Union, in dying olive trees in the Salento area of Puglia, on the Ionian coast around Gallipoli. Italy’s plant protection authorities reported it this month, and the movement of nursery plants out of the affected area has been banned. Here is what has been found so far, what is still unknown, and why growers across the Mediterranean are paying attention.

What happened

For some time, olive growers in the Salento, the heel of Italy’s boot, have been reporting trees that scorch and dry out, branch by branch, and spread from grove to grove. The Regional Plant Protection Service of Puglia started investigating with experts from the University of Bari and the Institute of Plant Virology of the National Research Council (CNR). Many of the sick trees were a century old or more.

In mid-October the European and Mediterranean Plant Protection Organization (EPPO) was alerted to a possible outbreak, and Italy’s national plant protection organization then reported that Xylella fastidiosa appears to be involved. Almost at the same time, the research team from Bari published its first results in the Journal of Plant Pathology. Samples from the olive trees tested positive for the bacterium, and so did almond and oleander plants growing nearby that showed the same leaf scorch. Serological tests confirmed the result.

Xylella fastidiosa is on EPPO’s A1 list, meaning it is a quarantine pest considered absent from the region. This is the first time it has been detected in the European Union.

What is still unknown

The picture is not yet complete, and the scientists say so. According to EPPO’s report, the investigators also found fungi of the genus Phaeoacremonium in the wood of the sick trees, and the disease “does not seem to be exclusively due to” the bacterium. Several questions remain open:

• The bacterium has not yet been isolated in pure culture, which is needed for final identification and to prove it causes the disease.
• The exact strain is not known. In California, a strain found in olive trees attacks oleander and almond but not grapevines.
• The local insect that carries it from tree to tree has not been identified.
• Surveys are under way to find out how large the infected area is.

Why it matters

Xylella lives in the water-conducting vessels of a plant and blocks them, which is why the leaves scorch as if from drought. It has a very wide host range, more than 100 plant species, and in the Americas it is best known for Pierce’s disease of grapevines. There is no cure for an infected plant.

The Salento is not just olive country, it is ancient olive country. Its groves of Ogliarola Salentina and Cellina di Nardò trees, many of them huge and centuries old, are part of the landscape and the local economy. The first measure, the ban on moving nursery material out of the area, is aimed at the most obvious way a plant disease travels long distances: in the back of a truck, on a young plant.

What it means for buyers

There is no food safety issue. The bacterium attacks the plant, not the people who eat olives or use the oil, and oil from Puglia is as safe as ever. For growers and nurseries, though, this is a serious warning, and the coming months of surveys will show whether it is a local problem or something bigger.

Sources

• EPPO Reporting Service 2013/184: First report of Xylella fastidiosa in Italy
• EPPO: Xylella fastidiosa in the EPPO region (first reports)
• Saponari et al., Scientific Reports: Isolation and pathogenicity of Xylella fastidiosa in southern Italy (background on the autumn 2013 detection)
• National Geographic: background on where the first withered trees appeared, near Gallipoli