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 HLB in Africa and Europe

Abstract

Citrus surveys were conducted at high (>700 m), medium (300-600 m) and low (<200 m) altitudes in Tanzania in 2014/15. Adults and nymphs of Trioza erytreae (del Guercio) were abundant in the highlands and less abundant at medium altitudes. Unexpectedly, adults and nymphs of Diaphorina citri Kuwayama, the Asian citrus psyllid, were found and collected at medium altitudes, around Morogoro. No psyllids were observed at low altitudes. Severe huanglongbing symptoms and tree decline were evident at high altitudes, while mild and few symptoms were observed at intermediate and low altitudes, respectively. DNA was extracted from leaf and psyllid samples and subjected to conventional PCR (cPCR) with seven different primer sets and RT qPCR with two primer sets. cPCR bands were sequenced and subjected to phylogenetic analysis. Candidatus Liberibacter africanus (Laf) was detected in highland leaf and T. erytreae samples from high and medium altitudes by all methods. Sequences from leaves and psyllids were similar to those from South Africa. Candidatus Liberibacter asiaticus (Las) was detected by qPCR in medium altitude leaf samples, but cross-reaction with Laf was likely because presence of Las was not confirmed by cPCR and sequencing. Neither Laf nor Las were detected in D. citri samples. This is the first reported occurrence of D. citri in Africa. Predictions were made of the potential distribution of D. citri and Las in Africa and along the Mediterranean coast using the correlative models MAXENT and Multi-Model Framework. Additional surveys at medium and low altitudes and quarantine measures are recommended.