Coconut lethal yellowing.
Lethal yellowing (LY) is a devastating disease that affects at least 35 species of palms throughout the Caribbean region. Over the last 50 years, its effects have been most conspicuous on coconut (Cocos nucifera) palm due to its abundance and importance of this species to the rural economies in the region. The etiological agent of LY is an unculturable phytopathogenic mollicute belonging to the genus Phytoplasma. This small, cell wall-less bacterium parasitizes the phloem of host palms and is transmitted to palms in a circulative-propagative manner by the vector planthopper Myndus crudus. Confirmation of preliminary LY diagnoses based on symptoms has traditionally relied upon observation of phytoplasma cells within phloem sieve elements by electron microscopy (EM). Improvements in extraction methods coupled with development of novel DNA-based assays have enabled sensitive detection and specific identification of LY phytoplasma in host tissues, facilitated studies on disease epidemiology and phytoplasma ecology, and provided a means to differentiate and classify these organisms for taxonomic purposes. Evidence provided by DNA hybridization and polymerase chain reaction (PCR) assays indicates that LY phytoplasma exists as a group of near identical strains within the Caribbean region. Collectively, these strains are phylogenetically distinct from phytoplasmas associated with lethal yellowing-like diseases of coconut that occur in Africa or southeast Asia.