Universal, Rapid, and Visual Detection Methods for Phytoplasmas Associated with Coconut Lethal Yellowing Diseases Targeting 16S rRNA Gene Sequences.

Coconut lethal yellowing (LY) diseases caused by phytoplasmas are devastating diseases for coconut cultivation and seriously threaten the coconut industry around world. The phytoplasmas associated with the LY diseases belonged to six 16Sr groups containing 16SrI, 16SrIV, 16SrXI, 16SrXIV, 16SrXXII, and 16SrXXXII with comparatively higher variable levels. Conserved regions of the 16S rRNA genes of LY phytoplasmas belonging to the six 16Sr groups were obtained in the study. Based on the conserved region sequences of 16S rRNA genes, two sets of LAMP primers, Co-4 and Co-6, were designed and screened, and the rapid and visual detection methods universal for different groups LY phytoplasmas were established. The entire detection reactions of the universal detection methods could be completed with only 30 to 40 min of constant temperature amplification at 64°C, and the detection results were judged by the color changes of the reaction systems, which are convenient and quick. For the six groups of phytoplasmas, the estimated minimum detection limit range of the universal detection primers Co-4 and Co-6 were identical: 4.8 × 101 to 4.8 × 107 copies per 200 µl. The universal detection methods for the LY phytoplasmas established in the study are of great significance for the rapid diagnosis and identification and the efficient monitoring and early warning as well as the port inspection and quarantine of the LY phytoplasmas and their related diseases.

Multilocus Sequence Analysis Reveals Three Distinct Populations of "Candidatus Phytoplasma palmicola" with a Specific Geographical Distribution on the African Continent.

To sustain epidemiological studies on coconut lethal yellowing disease (CLYD), a devastating disease in Africa caused by a phytoplasma, we developed a multilocus sequence typing (MLST) scheme for "Candidatus Phytoplasma palmicola" based on eight housekeeping genes. At the continental level, eight different sequence types were identified among 132 "Candidatus Phytoplasma palmicola"-infected coconuts collected in Ghana, Nigeria, and Mozambique, where CLYD epidemics are still very active. "Candidatus Phytoplasma palmicola" appeared to be a bacterium that is subject to strong bottlenecks, reducing the fixation of positively selected beneficial mutations into the bacterial population. This phenomenon, as well as a limited plant host range, might explain the observed country-specific distribution of the eight haplotypes. As an alternative means to increase fitness, bacteria can also undergo genetic exchange; however, no evidence for such recombination events was found for "Candidatus Phytoplasma palmicola." The implications for CLYD epidemiology and prophylactic control are discussed. The usefulness of seven housekeeping genes to investigate the genetic diversity in the genus "Candidatus Phytoplasma" is underlined.IMPORTANCE Coconut is an important crop for both industry and small stakeholders in many intertropical countries. Phytoplasma-associated lethal yellowing-like diseases have become one of the major pests that limit coconut cultivation as they have emerged in different parts of the world. We developed a multilocus sequence typing scheme (MLST) for tracking epidemics of "Ca Phytoplasma palmicola," which is responsible for coconut lethal yellowing disease (CLYD) on the African continent. MLST analysis applied to diseased coconut samples collected in western and eastern African countries also showed the existence of three distinct populations of "Ca Phytoplasma palmicola" with low intrapopulation diversity. The reasons for the observed strong geographic patterns remain to be established but could result from the lethality of CLYD and the dominance of short-distance insect-mediated transmission.

Molecular and biological characterization of phytoplasmas from coconut palms affected by the lethal yellowing disease in Africa.

Côte d'Ivoire lethal yellowing (CILY) is a devastating disease associated with phytoplasmas and has recently rapidly spread to several coconut-growing areas in the Country. Phytoplasmas are phloem-restricted bacteria that affect plant species worldwide. These bacteria are transmitted by plant sap-feeding insects, and their cultivation was recently achieved in complex artificial media. In this study, phytoplasmas were isolated for the first time from coconut palm trunk borings in both solid and liquid media from CILY symptom-bearing and symptomless coconut palms. The colony morphology, PCR and sequencing analyses indicated the presence of phytoplasmas from different ribosomal groups. This study reports the first biochemical characterization of two of these phytoplasma isolates. Moreover, a disc-diffusion antibiotic susceptibility assay revealed that these bacteria exhibit tobramycin susceptibility and cephalexin hydrate and rifampicin resistance. Urea and arginine hydrolysis, and glucose fermentation tests that were performed on colonies of phytoplasmas and Acholeplasma laidlawii indicated that both phytoplasmas tested were negative for urea and positive for glucose and arginine, whereas A. laidlawii was positive for glucose and negative for urea and arginine. The growth of coconut phytoplasmas in both solid and liquid artificial media and the biological characterization of these isolates are novel and important advancements in the field of disease management and containment measures for the CILY disease. The characterization of isolated phytoplasmas will allow for more efficient management strategies in both the prevention of a coconut phytoplasma epidemics and the reduction of the economic impact of the disease in the affected areas.