Palmageddon in Australia? A lethal disease of palms associated with a new phytoplasma in Cairns

Abstract

A phytoplasma associated with a rapid lethal wilt of ornamental palms in Cairns was detected by nested and real time PCR. Leaf death progressed from older to younger leaves, culminating in meristem rot. With a maximum 16SrRNA gene sequence similarity of 96%, this phytoplasma is most closely related to, but not a member of, the novel taxon 'Candidatus Phytoplasma noviguineense'. The next most closely related sequences all belonged to phytoplasmas in the 16SrIV and 16SrXXII groups. 'Ca. P. noviguineense' and members of these groups are implicated in devastating diseases of coconut overseas and have never been recorded before in Australia. A key symptom in Cairns is shared with these exotic coconut diseases: dead leaves hang down, skirt like, before eventually falling. Phylogenetic trees comparing 16SrRNA and three ribosomal protein gene sequences against those of known candidatus phytoplasma species, plus other variants, confirmed the uniqueness of this phytoplasma. It was first detected in late 2017 in a Dypsis poivreana within the Cairns Botanic Gardens, (CBG). In 2018, it was also found in a Euterpe precatoria just outside the CBG and in a coconut palm (Cocos nucifera) and an Alexandrea palm (Archontophoenix alexandrae), about 2 km south east of the CBG. Later investigations up to March 2019 detected the phytoplasma in more dying palms. These were a Verschaffeltia splendida and a Brassiophoenix drymophloeodes in the CBG, a Euterpe sp. about 600m west of the CBG, a Phoenix sp. 1.5 km west of the CBG, and a Carpentaria palm (Carpentaria acuminata) about 3 km south of the CBG. Detections at six separate locations suggests a capable insect vector is also present. During these investigations, a total of 33 other palms tested negative for phytoplasma, strongly suggesting this phytoplasma could be a causal agent of serious disease, rather than just an occasional phloem inhabitant.