First report of Diaphorina citri in Tanzania
In Tanzania, intensive surveys on huanglongbing (associated with ‘Candidatus Liberibacter africanus’ – EPPO A1 List) and its vector Trioza erytreae (Hemiptera: Triozidae – EPPO A2 List) were conducted in 2014-2015 in 4 regions (Tanga, Coast, Morogoro and Kagera). These surveys were conducted in citrus orchards and on backyard citrus trees at various altitudes: high (>700 m), medium (300-600 m) and low (<200m). Trees were inspected for the presence of huanglongbing symptoms and leaf galls caused by T. erytreae. Symptomatic leaf samples and psyllid specimens were also collected for further testing. Results showed that T. erytreae adults and nymphs were abundant in the highlands and less abundant at medium altitudes. Unexpectedly, adults and nymphs of the other known psyllid vector of huanglongbing, Diaphorina citri (Hemiptera: Liviidae – EPPO A1 List), were found at medium altitudes around Morogoro. No psyllid vectors of huanglongbing were observed at low altitudes. The presence of D. citri in Tanzania is also a first record for mainland Africa. Severe huanglongbing symptoms, including tree decline, were observed at high altitudes, while only occasional and mild symptoms were observed at medium and low altitudes. Leaf and psyllid samples were collected and tested (different PCR methods with several primer sets, sequencing and phylogenetic analysis) for the presence of ‘Candidatus Liberibacter spp.’ ‘Ca. L. africanus’ was detected in leaf and T. erytreae samples collected from high and medium altitudes. The presence of ‘Ca. L. asiaticus’ could not be confirmed by all methods. Preliminary positive results were obtained by qPCR on 4 adult specimens of T. erytreae but were not confirmed by other methods and are therefore suspected to be cross-reactions with ‘Ca. L. africanus’. It is also recalled that during another study, the presence of ‘Ca. L. asiaticus’ had been reported once in Ethiopia (where D. citri has not been found). Finally, all D. citri specimens tested negative for the presence of ‘Ca. L. asiaticus and ‘Ca. L. africanus’. In addition to these surveys, the potential distribution of D. citri and ‘Ca. L. asiaticus’ in Africa and Europe has been studied using two predictive models (Maxent and Multi-Model-Framework). Both models predicted that most of the citrus-growing areas in Africa and some in Europe are suitable for the establishment of D. citri. Some areas were predicted as being unsuitable such as the Balkans, Egypt, Greek islands, Northern Italy, Central Spain, central parts of South Africa, and Turkey (except for the coast). In addition, some areas within Algeria, Morocco and Tunisia were also predicted as being unsuitable for D. citri establishement.
It is concluded that more tests should be conducted on psyllids in Tanzania for the presence of ‘Ca. L. asiaticus’, as for the moment it cannot be concluded whether it is present or not in Tanzania. It is also noted that the Morogoro area where D. citri has been found is also an area where citrus nurseries are located. The arrival of D. citri in mainland Africa represents a serious threat to African citriculture, and possibly to other continents and Europe in particular. It is stressed that phytosanitary measures should be undertaken to prevent movement of D. citri within and outside Tanzania.
Sources
Shimwela MM, Narouei-Khandan, HA, Halbert SE, Keremane ML, Minsavage GV, Timilsina S, Massawe DP, Jones JB, van Bruggen AHC (2016) First occurrence of Diaphorina citri in East Africa, characterization of the Ca. Liberibacter species causing huanglongbing (HLB) in Tanzania, and potential further spread of D. citri and HBL in Africa and Europe. European Journal of Plant Pathology doi: 10.1007/s10658-016-0921-y