Witch’s broom phytoplasma infecting Echinacea pallida in Australia

Purple coneflower (<i>Echinacea pallida</i>), is commercially cultivated in Australia for its medicinal properties. In 2005, coneflower fields in Tasmania, Australia began exhibiting symptoms typical of phytoplasma infection, including virescence, phyllody and chlorosis. Surveys of fields in 2011 indicated the presence of symptoms within one field only. Disease distribution in this field was assessed by hierarchical sampling, incorporating 10 spatially referenced transects. Along each transect, 20 individual plants were assessed at 1 m intervals for the proportion of symptomatic flower heads. Overall, incidence of infected plants was estimated at 32%, while the mean percentage of symptomatic flower heads was 12%. Spatial analysis indicated a random distribution of symptoms across the field. Phytoplasma infection was confirmed by DNA sequencing of a 1.2 kb region of the 16S RNA gene, obtained by amplifying total DNA extractions from symptomatic coneflower tissue. Individual sequences shared greater than 99% homology with ‘Candidatus Phytoplasma australasiae’. Comparison of virtual restriction fragment profiles from the same genetic region confirmed that the pathogen belonged to the Witch’s Broom (16SrII-D) group of phytoplasmas. All previous reports of phytoplasma infection of coneflower have indicated Aster Yellows (16SrI-C) group phytoplasmas as the causal agents.