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Pathogen Biology
Lethal yellowing is caused by a phytoplasma, a cell wall-less bacterium that belongs to the class Mollicutes. The phytoplasma has been classified as a member of group 16S rDNA RFLP group 16SrIV, subgroup A (16SrIV-A). The proposed name for the pathogen is ‘Candidatus Phytoplasma palmae.’
The phytoplasma, which is not culturable, is found only in the phloem of host plants. When observed in phloem sieve elements by electron microscopy, the shape of phytoplasma cells varies from bead-like to filamentous (Figure 13). Non-filamentous forms average 295 nm in diameter and filamentous forms average 142 nm in diameter and at least 16 µm in length. Each phytoplasma cell is enclosed by a trilaminar unit membrane and contains cytoplasm with DNA strands and ribosomes.
 Figure 13 |
Molecular studies have determined that the lethal yellowing phytoplasma exists as a group of nearly identical strains in the western Caribbean region. Collectively, these strains are phylogenetically distinct from phytoplasmas that infect coconut in Africa or southeast Asia. The lethal yellowing phytoplasma is most closely related to, but distinct from, phytoplasmas associated with decline-type diseases of the monocot Carludovica palmata (Cyclanthaceae) in Yucatán, Mexico, Phoenix canariensis (Canary Island date palm) in the Corpus Christi area of southern Texas (United States), and phytoplasmas causing a newly recognized coconut leaf yellowing syndrome in southwestern Mexico.
Copyright © 2008
by The American Phytopathological Society