First report of phytoplasmas associated with Erysimum linifolium L. stunting

Erysimum linifolium L. (sin. Cheiranthus linifolium L.) (Brassicaceae), or Aegean wallflower, native to the Mediterranean region, is an evergreen perennial compact shrub that offers silvery-green foliage and beautiful spikes of lilac-mauve flowers through long periods, starting in mid-spring. In Liguria region (northern Italy) this species is mostly cultivated to be used in rock gardens or in mixed garden borders. In 2012, a phytoplasma-like disease was observed in a few pot-plants at an ornamental grower of Albenga area (Liguria region). Symptomatic E. linifolium showed reduced leaf size, rosetting and stunting; in some cases, shortening of internodes and growing reduction involved only a part of the plant. After first symptoms observation, an increasing percentage of symptomatic plants were found at flowering stage, when affected plants did not bloom. In order to verify phytoplasma presence and to determine their identity, samples from symptomatic and asymptomatic plants were collected and tested, after a chloroform/phenol nucleic acid extraction by direct PCR with primers P1A/P7A followed by nested-PCR with primers F1/B6 and R16F2n/R2. RFLP analyses performed with Tru1I and HhaI enzymes, allowed the identification, only in the symptomatic plants, of phytoplasmas belonging to subgroup 16SrI-B (‘Candidatus phytoplasma asteris’). Further confirmation of phytoplasmas identity was obtained after Tru1I RFLP analyses on tuf gene amplified with cocktail primers. ‘Candidatus Phytoplasma asteris” (16SrI) is associated with over 100 economically important plant diseases and represent on of the most diverse and widespread phytoplasma groups. Strains that belong to subgroups 16SrI-A, 16SrI-B and 16SrI-C are distributed worldwide and are associated with diseases in more than 80 plant herbaceous species transmitted by more than 30 species of insect vectors. The 16SrI-B represents the largest and most diverse strain cluster in the group in which at least 20 ribosomal subgroups were recognized. Some of the subgroups were only detected in woody hosts such as 16SrI-P identified in poplar in Eastern Europe, 16SrI-S and 16SrI-Q detected in cherry respectively in China and in Lithuania indicating the ability of some of its strains to infect all kind of plant species. The detection and identification of aster yellows phytoplasmas in E. linifolium in Italy represents however its first report in this species worldwide. Phytoplasmas belonging to 16SrII group (‘Ca. P. aurantifolia’) were detected in 2010 in E. cheiri (sin. C. cheiri) a different species cultivated in south-eastern Iran. In this case, infected plants showed witches’ broom and phyllody. Considering that E. linifolium is propagated by seed, it is very likely that leafhoppers are involved in 16SrI-B phytoplasma spreading in this species. Work on possible insect vector identification is in progress.