Yellow leaf disease (YLD) is an economically important disease affecting betel palm in several countries, the cause of which remains unclear despite associations with putative agents, including phytoplasmas. In this study, we screened the potential casual agents associated with YLD in Hainan, China using next-generation sequencing and revealed the association of areca palm velarivirus 1 (APV1) with the YLD-affected palm. The complete genome of the APV1-WNY isolate was determined to be 17,546 nucleotides in length, approximately 1.5 kb longer than the previously reported APV1_HN genome. Transmission electron microscopy showed that APV1 particles are flexuous and filamentous, a typical morphology of species in the Closteroviridae family. Comparison of symptomatic and symptomless tree populations showed a strong association between APV1 and YLD. APV1 was detected in Pseudococcus sp. mealybugs sampled from YLD-affected trees in many locations, suggesting that mealybugs are a potential transmission vector for APV1. Although further studies are needed to confirm a causal relationship, these results provide timely information for the prevention and management of YLD associated with APV1.

Yellow leaf disease (YLD) is the most destructive disease of betel palm (Areca catechu). A strong association between YLD and areca palm velarivirus 1 (APV1) has been observed. However, the causal relationship between APV1 and disease, and the transmission mode, require further investigation. This work showed that APV1 was transmitted by both Ferrisia virgata and Pseudococcus cryptus mealybugs, and caused YLD symptoms in betel palm seedlings; therefore, we demonstrate that APV1 is a causal agent of YLD. APV1 was detected in the stylets, foreguts, midguts, and hindguts of the vectors via both immunocapture RT-PCR and immunofluorescence assays. APV1 was not transmitted transovarially from viruliferous female F. virgata to their progeny. In summary, the transmission of APV1 by F. virgata may occur in a non-circulative, semi-persistent manner. This study fills important gaps in our knowledge of velarivirus transmission, which is critical for developing YLD management practices.

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Yellow leaf disease (YLD) has been a major limiting factor threatening areca palm commonly known as betel palm (Areca catechu L.) plantations in Hainan, China. The YLD disease is closely associated with areca palm velarivirus 1 (APV1), which belongs to the family Closteroviridae. YLD-affected betel palms show more serious yellowing symptoms in winter than in summer based on anecdotal observations. In the present work, the underlying mechanism was investigated. We first observed that the severity of YLD symptoms was closely related with the APV1 viral titer determined by qRT-PCR and ELISA under natural conditions. To further investigate whether temperature plays a key role in APV1 accumulation, the areca palm seedlings were artificially inoculated with APV1-positive mealybugs (Ferrisia virgata) and then cultivated under controlled conditions. According to our results, the YLD symptoms severity in inoculated seedlings were closely associated with temperature, e.g., severest symptoms at low temperature (16/22 ± 2°C, night/day), severer symptoms at room temperature (24/26 ± 2°C, night/day), while moderate symptoms at high temperature (27/34 ± 2°C, night/day). The qRT-PCR and ELISA results showed that APV1 titer accumulates significantly abundant at low temperature as compared to high and room temperatures. In conclusion, this is the first report about the temperature effects on the symptoms severity of YLD and APV1 titer, which may have important implications for the epidemiology of YLD.

Background Areca palm (Areca catechu L.) is an important commercial crop in southeast Asia, but its cultivation is threatened by yellowing leaf disease (YLD). Areca palm velarivirus 1 (APV1) was recently associated with YLD, but little is known regarding its population and genetic diversity. To assess the diversity of YLD, the APV1 genome was sequenced in YLD samples collected from different sites in Hainan. Results Twenty new and complete APV1 genomes were identified. The APV1 isolates had highly conserved sequences in seven open reading frames (ORFs; > 95% nucleotide [nt] identity) at the 3′ terminal, but there was diversity (81–87% nt identity) in three ORFs at the 5′ terminal. Phylogenetic analysis divided the APV1 isolates into three phylogroups, with 16 isolates (> 70%) in phylogroup A. Mixed infections with different genotypes in the same tree were identified; this was closely correlated with higher levels of genetic recombination. Conclusions Phylogroup A is the most prevalent APV1 genotype in areca palm plantations in Hainan, China. Mixed infection with different genotypes can lead to genomic recombination of APV1. Our data provide a foundation for accurate diagnostics, characterization of etiology, and elucidation of the evolutionary relationships of APV1 populations.