Abstract
Huanglongbing (HLB), one of the most devastating citrus diseases in the world, was detected in Mexico in 2009. Currently, HLB is associated with the bacteria Candidatus Liberibacter spp., although several phytoplasmas have been found from trees showing HLB-like symptoms in Brazil and China. The aim of this study was thus to determine if, in addition to ‘Ca. L. asiaticus’ (CLas), phytoplasma species are also associated with HLB-like symptoms in citrus groves of Mexico. Citrus plants exhibiting symptoms such as diffuse chlorosis, blotchy mottle and vein yellowing were collected in the Mexican States of Nayarit, Colima and Sinaloa between August 2011 and September 2012. Samples were then evaluated for phytoplasmas and CLas by PCR, using primers that respectively target the genes for the 16S ribosomal RNA and 50S ribosomal protein of the β operon (rplA-rplJ). Out of 86 HLB-symptomatic citrus plants, 54 were positive for CLas, 20 were positive for phytoplasmas, 7 were found in mixed infections with both pathogens and 19 samples were negative for CLas and phytoplasmas. Actual and virtual RFLP analyses of the 16S rDNA sequences enabled us to classify two HLB phytoplasma strains as members of the aster yellows group (16SrI) ‘Candidatus Phytoplasma asteris’, which was confirmed by phylogenetic analysis. The HLB phytoplasma strain identified from Nayarit (HLBpc-Nay-IB) belongs to subgroup B (16SrI-B), and the strains identified from Colima (HLBpc-Col-IS) and Sinaloa (HLBpc-Sin-IS) belong to subgroup S (16SrI-S). The partial ‘Ca. L. asiaticus’ rplA-rplJ gene sequences were 100% identical to the ‘Ca. L. asiaticus’ strains isolated from several countries affected by HLB. These results confirm the association of ‘Candidatus Phytoplasma asteris’ with HLB-like symptoms in citrus groves in Mexico. Nonetheless, further studies are required to fully describe the ‘Ca. L. asiaticus’ and ‘Ca. P. asteris’ interactions in citrus, which will greatly assist the design of efficient management strategies.
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