Search

Aim: To quantify current and predict future distribution of the citrus greening pathogens "Candidatus Liberibacter asiaticus" (Las) in Africa and "Candidatus Liberibacter africanus" (Laf) globally. Location: Africa. Methods: Three species distribution models (MaxEnt, BIOCLIM and Boosted Regression Trees) were used to predict the current and future potential distribution of Las in Africa, and the potential global distribution of Laf, using long‐term bioclimatic variables. Two climate change scenarios (moderate and extreme) were employed to determine how future climate alterations may affect the potential distribution of Las in Africa. Presence data from global reports of Las, as well as the new positional points obtained in this survey, were used to predict the habitat suitability of the pathogen in Africa, while the presence data points of Laf were used to predict the global habitat suitability. Testing data comprised 25% of the presence only points. Results: Consensus of the three models predicted a potential distribution of Las in large areas of Western, Eastern and sub‐Saharan Africa. North Africa was mostly unsuitable for Las, except for the northern fringes. The potential distribution of Laf included South and Central America, Asia and Australia. In Europe, the United Kingdom and the Iberian Peninsula showed marginal suitability for Laf. The projections under the future climate change scenarios showed an increase in the Laf habitat suitability hotspots under the extreme scenario. Main conclusions: This study highlights the potential establishment and distribution in Africa of Las‐associated Huanglongbing and globally for Laf‐associated with African citrus greening disease. The ensemble modelling approach for the distribution of plant pathogens is a valuable tool for the development of strategies for crop protection. These results constitute an early alert for citrus‐producing regions that should inform strategies for surveillance and preventive management against the invasion and spread of this destructive disease. [ABSTRACT FROM AUTHOR]

Citrus greening (Huanglongbing, HLB) is a widespread and economically important citrus disease all over the world. The disease is caused by a phloem-limited fastidious gram negative bacterium, ' Candidatus Liberibacter spp.' which belongs to the alpha-proteobacteria group classified on the basis of its 16SrDNA sequence. Although the pathogen has been classified under three distinct groups, viz. Asian, African and American isolates, nothing is known about the status and the molecular variabilities among the Indian HLB isolates collected from different citrus cultivars grown in India. Five different HLB isolates showing variable symptoms based on their severity of infection on different citrus, viz. Mosambi, Rangpur lime, Cleopatra mandarin, acid lime and rough lemon, were studied by PCR amplification, sequence and evolutionary analysis of their 16S and 16S/23S rDNA intergenic regions. Analysis of the 16S/23S rDNA intergenic region separated all five Indian isolates from existing African isolates but failed to differentiate among Asian, American and Indian isolates. However, further analysis of complete 16S rDNA clearly indicated that Indian isolates fall within the Asian HLB group. Overall, our results suggest that all the five Indian HLB isolates taken for the current analysis belong to the Candidatus Liberibacter asiaticus strain, which showed distinct sequence variabilities and produced noticeable symptoms on the citrus trees. These results provide a robust framework for understanding how differences in pathogenicity among various HLB isolates may be related to evolutionary history. [ABSTRACT FROM AUTHOR]

African citrus psyllid (Trioza erytreae (Del Guercio)) is a vector insect of the bacterium Candidatus Liberibacter africanus, the putative causal agent of Huanglongbing, the most devastating citrus disease in the world. The insect was found on the island of Madeira in 1994 and in mainland Portugal in 2015. Present in the north and center of the country, it is a threat to Algarve, the main citrus-producing region. Trioza erytreae eggs and first instar nymphs are sensitive to the combination of high temperatures and low relative humidity. Daily maximum air temperature and minimum relative humidity data from 18 weather stations were used to calculate the water vapor pressure deficit (vpd) from 2004 to 2018 at various locations. Based on the mean vpd and the number of unfavorable days (vpd < 34.5 and vpd < 56 mbar) of two time periods (February to May and June to September), less favorable zones for T. erytreae were identified. The zones with thermal and water conditions like those observed in the Castelo Branco and Portalegre (Center), Beja (Alentejo), Alte, and Norinha (Algarve) stations showed climatic restrictions to the development of eggs and first instar nymphs of African citrus psyllid. Effective control measures, such as the introduction and mass release of Tamarixia dryi (Waterson), a specific parasitoid, and chemical control are necessary in favorable periods for T. erytreae development, such as in spring and in areas with limited or no climate restrictions. [ABSTRACT FROM AUTHOR]