Oranges are to Seville what cherry blossom is to Kyoto, but the city is having to take preventive measures to protect its 48,000 orange trees from deadly bacteria that have already devastated citrus crops in Asia, Latin America and the US.
The EU’s Life for Citrus campaign, which includes Spain, Portugal, France and Italy, is developing strategies to stop the spread of huanglongbing (HLB), Mandarin for “citrus greening”, also known as yellow dragon disease. Caused by the bacteria Candidatus liberibacter, it is spread by insects and can completely destroy a citrus tree within five years.
The infestation has already reached epidemic levels in 48 Asian countries and 53 African states, as well as Brazil and the US. It was detected in China in 1943, in Africa in 1947 and by 2005 had begun to devastate Florida’s orange groves. It has yet to arrive in Europe but the insect that carries it has.
“One of the vectors, Trioza eryteae, has already been detected in the Canaries, Portugal and Galicia in north-west Spain, but not the bacteria,” says Francisco Arenas, head of Las Torres research institute in Andalucía, southern Spain. “The problem is that, once the vector arrives, sooner or later the disease comes too.”
The first sign that a tree has been infected is yellowing and distorted leaves. Then the tree produces less fruit and, within five years, is dead. There is, as yet, no treatment available; the only answer is to dig up and replant the trees or take preventive measures.
Seville city council and Life for Citrus have embarked on a pilot scheme to encourage the spread of insects and birds that prey on the pest that spreads the disease, as part of a trend of working with nature rather than against it.
In Seville’s Buhaira park, an orange plantation covering 1,800 sq metres will be planted with various species of ground cover, and bug hotels and nest boxes installed to encourage the ecosystem to thrive. The insect Tamarixia dryi, which preys on the larvae of Trioza, will then be introduced to the area, says Arenas.
Pedro Torrent Chocarro of the Seville parks and gardens department says that, while some native species prey on white fly and other species that damage the oranges, Trioza doesn’t have a native predator.
“The predator for Trioza is imported, initially from the Canaries, but now it is being bred in Galicia as well,” Torrent says. “The insects have been released in Galicia with good results and the plan is to release them in Portugal and Andalucía in due course.
“By optimising conditions for other predators, the orange trees are healthier and less susceptible to disease in general.”
The scheme will also involve the creation of an environment favourable for birds that prey on the insects, such as great tits, robins and blackbirds.
Since 2019, Seville’s parks and gardens staff have been trained to spot signs of Candidatus liberibacter infection. The city, which is investing €35,000 (£30,000) in the pilot, is also raising public awareness with screens containing QR codes providing information, as well as educational visits as part of the work to conserve “the city’s green heritage, culture and history”, says David Guevara, city councillor for the environment.
Seville’s orange trees, which were introduced by Arabs more than a millennium ago and produce the characteristic bitter fruit favoured by marmalade makers, account for about 25% of the city’s trees.
If the bacterium takes hold, it is estimated that destroying and replacing the affected trees would cost about €20m, while depriving Seville of its most magical moment, the sight and fragrance of orange blossom in early spring.
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