Journal of Advances in Microbiology

Yellow leaf disease (YLD) is a chronic, yield-limiting disorder of areca palm (Areca catechu L.) reported across India, China and Sri Lanka, characterised by progressive foliar chlorosis, root degeneration and crown decline. For more than four decades the disorder has been associated with phytoplasma infection, yet independent surveys have repeatedly reported different 16Sr taxonomic groups in different regions, and a substantial proportion of symptomatic material has tested negative for phytoplasma altogether. Over the past six years this single-pathogen framing has been complicated further by the discovery of areca palm velarivirus 1, by long-standing evidence implicating nutrient imbalance and waterlogging, and, most recently, by direct demonstration of phytoplasma-velarivirus co-infection in symptomatic palms. In parallel, a smaller but growing body of amplicon and shotgun metagenomic studies has begun to characterise the bacterial and fungal communities of the areca phyllosphere, endosphere and rhizosphere, revealing consistent reductions in microbial richness and shifts in dominant taxa in diseased tissue relative to healthy tissue. This review synthesises, for the first time, the phytoplasma, viral, nutritional and metagenomic literatures on areca YLD within a single microbiome-centred framework, evaluating the strength, consistency and methodological quality of the evidence for each proposed explanatory model. The available evidence indicates that no single causal agent adequately accounts for the geographic and temporal variability of YLD symptomatology, and that microbial community composition, host nutrition and at least two distinct classes of pathogen may interact to produce a shared clinical phenotype. Methodological heterogeneity, small and geographically restricted sample sizes, near-exclusive reliance on amplicon rather than shotgun sequencing, and the absence of longitudinal or manipulative experiments currently prevent firm causal attribution. Priority research needs include shotgun metagenomic and metatranscriptomic surveys spanning the phytoplasma-positive, velarivirus-positive, co-infected and pathogen-negative disease classes, standardised nutrient and microbiome sampling across the major producing regions, and controlled inoculation studies capable of testing Koch-type postulates for each candidate agent. A microbiome-integrated diagnostic and research framework is proposed as the most defensible route toward resolving the persistent aetiological uncertainty surrounding this economically important disease.