Non-crop sources of Rapeseed Phyllody phytoplasma ('Candidatus Phytoplasma asteris': 16SrI-B and 16SrI-(B/L)L), and closely related strains

In Poland, a phytoplasma belonging to the subgroup 16SrI-B of 'Ca. Phytoplasma asteris' species causes disease in winter oilseed rape (Brassica napus), named rapeseed phyllody (RP). Affected plants exhibit malformations of flowers - phyllody, leading to losses in seed production. However, the potential sources of RP phytoplasma (RPp) outside oilseed rape are poorly understood. A screening survey of non-crop wild plants associated with winter oilseed rape was performed in order to identify alternative plant hosts of RPp. Nested polymerase chain reaction (PCR) and sequencing of PCR-amplified 16S rDNA fragments were employed for the detection and identification of phytoplasma strains affecting oilseed rape crops and adjacent wild plants. Phytoplasmas belonging to subgroups 16SrI-B, 16SrI-L, 16SrI-(B/L)L and 16SrI-C, as well as subgroup 16SrXII-H, were detected in non-crop plants. All investigated oilseed rapes manifesting RP disease were positive to 'Ca. Phytoplasma asteris' ribosomal subgroup 16SrI-B or heterogenic variant 16SrI-(B/L)L. Wild plants species were predominantly infected with the same phytoplasma subgroups as oilseed rapes. However, comparative sequence analysis revealed single nucleotide polymorphisms that indicate divergence of 16SrI-B and 16SrI-(B/L)L phytoplasma strains in non-crop species. Surprisingly, most of the weeds which tested positive for phytoplasma did not exhibit any characteristic symptoms of infection. In this study we identified for the first time perennial (Elymus rep ens, Lolium perenne) and annual (Echinochloa crus-galli, Polygonum aviculare, Viola arvensis, Matricaria perforate, Stelaria media) plants as asymptomatic reservoirs of RPp related strains in the winter oilseed rape agroecosystem. We also confirmed RPp related infections in diseased Apera spica-venti, Sisymbrium officinale, Matricaria discoidea, and Papaver rhoeas. These plants potentially play a significant role in RPp evolution and disease aetiology and epidemiology.