Huanglongbing (HLB) is the most serious disease affecting citrus crops worldwide, but the Mediterranean has so far been spared. For the first time, a team coordinated by CIRAD has clearly demonstrated the ability of an insect, the African psyllid Trioza erytreae, to transmit the bacterium responsible for the most serious form of HLB. The psyllid is already found in the Iberian Peninsula, hence the threat of a major epidemic if the bacterium were to be introduced.

Since it began to spread rapidly in the 2000s, Huanglongbing disease (HLB) has decimated citrus crops, substantially reducing yields and revenues for producing countries (Brazil, China, USA). To date, just two world regions have escaped: Australia/New Zealand and the Mediterranean. According to a 2019 report by the French ANSES agency, there is a high risk of the disease becoming established in the Mediterranean. The reason is the presence in Spain and Portugal of the psyllid Trioza erytreae, the vector insect of the bacterium responsible for the moderate form of HLB in Africa.

Trioza erytreae is capable of transmitting the bacterium responsible for the most serious form of HLB © A. Franck, CIRADTrioza erytreae is capable of transmitting the bacterium responsible for the most serious form of HLB © A. Franck, CIRAD

Could the psyllid cause a major HLB epidemic in the Mediterranean? Yes, if a bacterium that originated in Asia , CLas, which causes the most serious form of the disease, were to be introduced into the region. A recent study, the results of which have been published in Frontiers in Plant Science, demonstrated for the first time that T. erytreae is capable of transmitting the bacterium. "Any introduction of plants infected with CLas into the Iberian Peninsula could therefore have disastrous consequences for the region", says Leandro Pena, coordinator of the H2020 PreHLB project under which the study was conducted.

Several species of the bacterium Candidatus Liberibacter spp. cause HLB in citrus. "The Asian species (CLas) is the most aggressive, causing severe symptoms and rapidly killing affected trees", Bernard Reynaud, lead author of the study and Head of the CIRAD/University of Réunion PVBMT joint research unit. "The spread of the bacterium CLas has caused major economic losses worldwide." The Asian bacterium is naturally transmitted by another psyllid species, Diaphorina citri, which also originated in Asia. However, the project consortium has now shown for the first time that the African psyllid found in Europe (Trioza erytreae) can also effectively pass on the bacterium CLas to citrus crops.

"We did six months of tests in Réunion, the only world region where the two psyllid species are found along with the bacterium CLas", says CIRAD entomologist Hélène Delatte, a co-author of the study. The tests involved confining psyllid larvae in test tubes, on young shoots from trees infected with the bacterium CLas. The adult insects were subsequently transferred to new tubes containing healthy young seedlings. The results showed that both psyllid species were capable of transmitting the bacterium CLas. Trioza erytreae, the species found in Europe, even carries greater quantities of bacteria. "Our study was original in that we compared transmission by the two psyllid species under identical experimental conditions", Bernard Reynaud adds. "This was possible because both species are found in Réunion." The conditions reproduced in the laboratory were similar to those in Mediterranean citrus orchards.

Over and above the major health and economic implications, these results are interesting from a scientific point of view. "These two psyllid species originated in very distant regions, and we are surprised that they both proved capable of propagating and transmitting the bacterium CLas", Bernard Reynaud observes. "We are going to continue our work, to characterize the transmission cycle and interactions with CLas in the African psyllid."

This study demonstrated the importance of preventing any introduction of the bacterium CLas into Europe, which could trigger a major HLB epidemic.

This work was supported by the European Union, the Réunion Regional Council and the European Agricultural Fund for Rural Development.

A heightened risk for Africa too
The psyllid Trioza erytreae is an insect that originated in Africa and lives in medium altitude regions. The Asian bacterium CLas was also recently introduced into Africa, in Ethiopia and Kenya. As Bernard Reynaud warns, "Now that we know Trioza erytreae is capable of effectively transmitting the bacterium CLas, it is important to step up monitoring in Africa, because the psyllid is widespread there, which means more zones are at risk, particularly in highland regions."