Diversity of phytoplasmas in northern Australian sugarcane and other grasses

Plant pathogenic phytoplasmas found in grasses in northern Australia have t he potential to adversely affect sugarcane production. To begin assessment of this threat, the number of grass species with phytoplasmas, the identity of those phytoplasmas, and their relationship with disease symptoms were d etermined. Sugarcane with and without symptoms of yellow leaf syndrome was included in the surveys. Molecular methods were used to detect and characte rize phytoplasmas in grass species exhibiting symptoms typical of phytoplas ma disease. Sugarcane samples were from the Ord River Irrigation Area, West ern Australia, and Samford, Queensland. Samples of other grasses were from Wyndham, Kununurra and Broome, Western Australia, and Darwin, Northern Terr itory. Our survey identified four new phytoplasma host species and confirme d four known previously. Counting phytoplasmas, phytoplasma variants, and m ixtures of phytoplasmas and variants, these eight host species had 33 diffe rent infections. Two phytoplasmas were new, cenchrus bunchy shoot which is related to Candidatus phytoplasma australiense, and sorghum bunchy shoot wh ich is not closely related to any described phytoplasma. Twenty-five phytop lasma isolates were detected in sugarcane. Of these, tomato big bud phytopl asma was the most common. In most cases no clear association between phytop lasmas and symptoms could be determined. None of the phytoplasmas in Austra lian sugarcane, but two in other grasses, were closely related to phytoplas mas associated with white leaf and grassy shoot diseases in Asian sugarcane . This study demonstrates that diversity of phytoplasmas and grass host spe cies in northern Australia is greater than previously thought, and that sym ptoms alone are not always reliable indicators of phytoplasma presence or a bsence. It provides the groundwork to improve future field surveys, and for initiation of transmission trials to determine whether insect vectors capa ble of transmitting phytoplasmas from native grasses to sugarcane are prese nt in the region.