First report of 'Candidatus Liberibacter solanacearum' in common hogweed (Heracleum sphondylium) in Austria.
'Candidatus Liberibacter solanacearum' (CaLsol) causes important diseases in apiaceous and solanaceous crops (zebra chip disease) and has been reported from European countries during the last decade. After the first record of CaLsol on symptomatic carrots and celery from two fields in Austria in 2014, monitoring activities on plants and potential vectors were undertaken on 20 fields in the outbreak area around the villages of Arzl, Rum and Thaur (Inn valley near Innsbruck, Tyrol) between 2016 to 2019. Plants from species in the Apiaceae and Solanaceae were collected randomly in fields and field margins (n=84) and tested for infection with CaLsol using real time PCR. CaLsol was only detected in apiaceous plants (n=72), in which 43% of the samples were positive, but not in solanaceous plants (n=12; comprising Solanum dulcamara, S. lycopersicum and S. tuberosum). All positive CaLsol samples harboured the haplotype C, as confirmed by PCR and subsequent sequencing of the amplicons. The infected apiaceous species were carrot (Daucus carota; GenBank Accession No. MT753007.1), celery (Apium graveolens; MT753005), parsnip (Pastinaca sativa), parsley (Petroselinum crispum) and common hogweed (Heracleum sphondylium; MT753008.1). Of these species, CaLsol symptoms were only observed in carrot (leaf reddening and deformed roots with increased adventitious roots) and celery (leaf yellowing of the leaves and stunted petioles). CaLsol was not detected in other Apiaceae members including Aegopodium podagraria, Anethum graveolens, Anthriscus sylvestris and Foeniculum vulgare. One to three randomly selected positive samples from each species were sequenced each year with identities ranging from 99.4 to 100%, confirming all were haplotype C. This is thought to be the first known report of CaLsol infecting common hogweed. Futher study revealed a high infection rate of the carrot psyllids with CaLsol, especially females. In 2018, 41.85% of analysed Trioza apicalis individuals (110 ♂♂, 203 ♀♀) tested CaLsol-positive (♀♀: 49.85%; ♂♂: 27.27%). These 313 single psyllids were collected randomly from carrot plants in three different fields in 2018, in contrast to samples from 2016 and 2017, which were taken from selected symptomatic carrot plants and had an infection rate of 98%.