Mixed infection of dahlia plants in Poland with apple proliferation and aster yellows phytoplasmas

In summer 2004, five Dahlia cultorum plants of unknown cultivar, grown in garden plots in Skierniewice, Poland, exhibited bushy growth accompanied by shoot proliferation, narrowed leaf and flower bud deficiency. Research in Italy (Marzachi et al., 1999) had indicated that stunted growth and shoot proliferation symptoms in some dahlia plants was associated with aster yellows phytoplasma infection. To detect the possible presence of phytoplasmas in dahlias in Poland, three plants showing symptoms and two asymptomatic plants were assayed for the presence of phytoplasma 16S rDNA by PCR. Nucleic acids were extracted from leaves and roots using DNeasy Plant Mini Kit (Qiagen). A nested PCR was done using the universal phytoplasma primer pair P1/P7, followed by primers fA/rA or R16F2n/R16R2. To detect potential mixed infection in dahlias, the phytoplasma 16SrI or 16SrX group-specific primer pairs (R16(I)F1/R16(I)R1, fAT/rAS, fAT/rPRUS or fPD/rAT) were used for nested PCR. Phytoplasma identification was accompanied by restriction fragment length polymorphism (RFLP) analysis of R16F2n/R16R2-primed PCR product digested with AluI, MseI, HhaI, HpaII, SspI or RsaI endonucleases.

Products were generated by nested PCRs with universal and group-specific 16(I)F1/R16(I)R1 and fAT/rAS primer pairs exclusively from two diseased Dahlia plants (Fig. 1). No bands were amplified from one dahlia with disease symptoms or from healthy dahlia and Catharanthus roseus plants. RFLP profiles (Fig. 2) indicated that detected phytoplasmas belonged to the 16SrI group - subgroup B, now re-classified as the species ‘Candidatus Phytoplasma asteris’, and to the 16SrX group - subgroup A, now re-classified as the species ‘Candidatus Phytoplasma mali’. In Poland, 16SrI-B subgroup strains affect several ornamental crops including magnolia, rose, lily, tulip, freesia, and bleeding heart, while the 16SrX-A subgroup strain has been recorded less frequently, only in magnolia and rose plants (Kamińska & Śliwa, 2003; 2004). This report confirms infection of dahlia by aster yellows phytoplasma belonging to group 16Sr-B, and to our knowledge, records for the first time, infection by phytoplasma of group 16SrX-A.