Photography by © A. Franck, CIRAD.

A study by the French research centre CIRAD demonstrates, for the first time, the ability of the African psyllid Trioza erytreae to effectively transmit CLas, the bacteria that causes the most serious form of Huanglongbing (HLB), which is the main disease affecting citrus worldwide. Given that this transmitting vector is already present in the south of Portugal and threatens to reach the citrus plantations of Huelva, the partners of the Pre-HLB project, dedicated to the prevention and control of HLB, are calling on the European Union to take all measures to prevent any introduction of the CLas bacterium.

The coordinator of the project, Leandro Peña, warns that “the conclusions of this research are very worrying because the entry of plants infected by CLas into the Iberian Peninsula could therefore be disastrous for the region. We have the precedents of powers such as Brazil, China and the United States, where HLB has decimated citrus, drastically reducing production yields and income. To date only two regions of the world have been spared: Australia/New Zealand and the Mediterranean. And the risk of the disease becoming established here is high, even more so in the wake of this report”.

Several species of the bacterium Candidatus Liberibacter spp. cause HLB in citrus. “The one of Asian origin (CLas) is the most aggressive, inducing severe symptoms that lead to the rapid death of the tree,” explains Bernard Reynaud, first author of the study and director of the PVBMT research unit at CIRAD/Université de la Réunion: “This Asian bacteria is naturally transmitted by another species of psyllid (Diaphorina citri), also of Asian origin. But the consortium has found that the African psyllid present in Europe (Trioza erytreae) is also capable of effectively transmitting the CLas bacterium to citrus fruit”.

Hélène Delatte, entomologist at CIRAD and co-author of the study, adds that “tests were carried out for six months in Réunion, the only region in the world where the two psyllid species cohabit with the CLas bacterium”. The result concludes that Trioza erytreae carries even greater amounts of bacteria than the Asian species. “The originality of our study was to be able to compare transmission by the two psyllid species under the same experimental conditions. We will now continue our work to better characterise the transmission cycle and interactions with CLas in the African psyllid,” says Bernard Reynaud.

Recently, the Asian bacterium CLas was introduced on the African continent, in Ethiopia and Kenya. Bernard Reynaud warns that “it is also important to reinforce surveillance in Africa, as this vector transmitter is widespread there, which increases the risk areas”.