Recent advances of the causal agent of yellow leaf disease (YLD) on areca palm (<i>Areca catechu</i> L.)

Abstract

Areca palm (<italic>Areca catechu</italic> L.), commonly known as betel palm, is a versatile perennial evergreen, a valuable economic crop in Southeast Asia. Its cultivation is seriously threatened by a destructive disease known as yellow leaf disease (YLD). Since the 1960s, the causal agent of the YLD was proposed to be phytoplasma, a pleomorphic wall-less, <italic>in vitro</italic> unculturable bacteria, which was supported by many reports from India, Sri Lanka, and China, but convincing etiological evidence is still lacking. Recently, the YLD was reported to be associated with a newly emerged novel areca palm velarivirus 1 (APV1) that belongs to the genus <italic>Velarivirus</italic> of the family <italic>Closteroviridae</italic>. Furthermore, APV1 could be transmitted by both <italic>Pseudococcus cryptus</italic> and <italic>Ferrisia virgata</italic> mealybugs to areca palm seedlings and caused typical YLD symptoms. A growing body of evidence suggests that APV1 is the causal agent of YLD, although Koch’s postulate has not been fulfilled. The conundrum of the causal agent of YLD is not yet solved. This review aims to compile and critically review all the published data concerning YLD as per our knowledge.