First detection of Candidatus Phytoplasma australiense in Liquidambar styraciflua in Australia

Liquidambar styraciflua (sweet gum; Family Hamamelidaceae) is an ornamental tree native to North America, producing a hard wood suitable for the furniture industry. A liquidambar tree on the Waite Campus, Urrbrae, South Australia had symptoms of a chronic patchy chlorosis of the crown and dieback of apical and lateral branches (Fig. 1), when compared to five other trees each about 40 years old growing in the same roadside verge.

During summer (March), chlorotic shoots typically bore fewer and smaller leaves with tip necrosis and vein clearing (Fig. 2) which senesced earlier than healthy trees in autumn. The number of fruits on the tree was reduced.

Total nucleic acids were extracted and tested by nested PCR using primers P1/P7, followed by R16F2n/m23sr specific for the 16S rRNA gene of all known phytoplasmas. The PCR product (1600 bp) was cloned and sequenced (GenBank Accession No. DQ660363). It was compared with those of 8 other phytoplasmas using equal weight maximum parsimony analyses with bootstrap support for each of the clades with a branch and bound search within PAUP 4.0 using an anaeroplasma (Acc. No. M25050) as outgroup (Fig.3). Although the liquidambar and strawberry lethal yellows phytoplasma (Acc. No. AJ243045) were in the same clade, differences in the clade length indicated that that the two sequences were not identical. The strains within the clade belong to Ca. P. australiense, of the 16Sr XIIB group (Lee et al., 2000). Although the presence of Australian grapevine yellows (AGY) was suspected, PCR with strain specific primers for AGY (Davis et al., 1997) gave negative results (not shown). We propose the name liquidambar yellows for this disease.