Early detection of 'olive tree leprosy' with drones

Using high-tech cameras that detect heat and the electromagnetic spectrum from X-ray to radio waves, researchers were able to spot diseased trees that, on the ground, still seemed healthy, they reported in the journal Nature Plants.

"After infection, it takes four to 14 months before visual symptoms are observable by plant pathologists in the field," lead author Pablo Zarco-Tejada, an agricultural engineer at the European Commission's Joint Research Centre in Ispra, Italy, told AFP.

"The problem is that during this entire time the tree remains a potential source of infection."

Once Xylella fastidiosa bacteria -- carried by tiny sap-sucking insects known as leafhoppers -- take hold, there is no cure and the plant is doomed.

The only way to fight the spread of what is known as "olive tree leprosy" is to destroy diseased trees.

"Early detection is critical for the eradication of the bacteria," Zarco-Tejada said.

Since it hit the Apulia region in 2013, the microscopic pathogen has killed more than a million olive trees in Italy, and 10 million more are currently affected.

The bug has also attacked orchards in Spain and France, and both Greece and Portugal are bracing for its likely arrival.

Some 350 plants are vulnerable, including grape vines, citrus and almond trees.

Known in the United States as Pierce's disease, it devastated California vineyards in the late 19th century.

To test their approach, Zarco-Tejada and international team of researchers fitted thermal and hyperspectral cameras on a small plane, and then analysed images of orchards.

At the same time, they tested olive trees on the ground for infection.

They found that the bacterial infection can be remotely detected three to six months before visible symptoms appear.

"We extracted the spectral signature and the temperature from each single tree crown," Zarco-Tejada said.

The data were then fed into a model built with machine-learning methods.

The cameras can be easily installed on planes or drones, similar to ones used for aerial photography and surveillance. The cost would depend in part on the size of the area covered.

The insect-borne pathogen has likely spread more quickly as global warming takes hold, according to a study published earlier this year in the journal Biorxiv.

But a more immediate factor in its expansion is probably global trade, the study said.

The authors are currently measuring for Xylella fastidiosa in almond orchards in central Spain.

Italy and Spain together account for nearly 70 percent of global olive oil output, according to the International Olive Council (IOC).