STUDIES ON SWEET-POTATO LITTLE-LEAF PHYTOPLASMA DETECTED IN SWEET-POTATO AND OTHER PLANT-SPECIES GROWING IN NORTHERN AUSTRALIA

Symptoms of vein clearing, small leaves (''little leaf''), stunting of plants, proliferation of shoots, and small tuberous roots were observed in sweet potato growing in the Darwin region of the Northern Territory, Australia (12 30' S latitude) in December 1990. This was the first report of such symptoms in sweet potato but there was anecdotal evidence of similar symptoms in other crop, ornamental, and weed host species growing in this region. Plant pathogenic mycoplasmalike organisms, or phytoplasmas, were observed in sieve tube elements of diseased sweet potato using fluorescent and electron microscopy. To screen a range of local plant host species for phytoplasmas, a region of the 16S rRNA phytoplasma gene was amplified in the polymerase chain reaction (PCR). The relatedness of phytoplasmas detected in different host species was determined by amplifying a larger 16S rRNA fragment and subjecting it to restriction fragment length polymorphism (RFLP) analysis using four restriction enzymes. No differences were detected between phytoplasmas associated with the local plant host species, and the RFLP pattern of the phytoplasmas from this region appeared to be different from those published for other phytoplasmas. This suggests that the phytoplasmas from this region of northern Australia compose a new subgroup. Phytoplasma chromosomal DNA was extracted from diseased sweet potato, and the chromosome was linearized by gamma-irradiation prior to separation by pulsed-field gel electrophoresis. The size of the full-length sweet potato little-leaf phytoplasma chromosome was 600 kb, which is one of the smallest phytoplasma genome sizes reported so far.