The Gramineae have the largest number of species associated with phytoplasma diseases worldwide, and are also the plant family where the majority of phytoplasma vector species (Delphacidae) have been found. Rice yellow dwarf
(RYD) and Rice orange leaf (ROL) are the two phytoplasma disease that have been reported to infect rice. RYD, a seriousproblemfor rice farmers, has only been detected to date in Asia,where it has been recorded from most rice-growing countries(Nakashima et al. 1993). Infected rice turns pale yellow andgradually starts to decay and produce numerous tillers. ROLhas also only been found in Asia to date and the symptoms are typified by orange-coloured leaves, which later roll inward anddesiccate. Despite the fact that infected plants die 2–3 weeks
after the symptoms appear, diseased plants are generally
distributed sporadically in the field so the disease does not as
yet cause serious yield losses (Hibino et al. 1987). Phytoplasma
have been associated with ROL in Thailand, Malaysia,
Indonesia and the Philippines based on electron microscopy
evidence (Hibino et al. 1987). Based on the 16S rDNA
sequence similarity between ROL from the Philippines and
Onion Yellows (OY) (99.9 %), along with other aster yellows
subgroup members (98.9–99.8 %), it is reasonable to classify
the ROL phytoplasma in the AY 16SrI group, thus
distinguishing it fromRYD, which belongs to the 16SrXI group
(Jung et al. 2002).