First report of phytoplasmas infecting swan plants (Gomphocarpus physocarpus) in Liguria, Italy

Swan plants [Gomphocarpus physocarpus E. Mey. (=Asclepias physocarpa L.); Asclepiadaceae are deciduous subshrubs grown in the open in the Albenga area of the Italian Riviera. During summer 2001, several plants in a local field (field A) showed witches’-broom, dwarfing, yellowing and decline symptoms with total loss of normal flower production on one plant (Fig. 1). Witches’-brooms, floral virescence and phyllody were all symptoms observed on 60-70% of plants (Fig. 2) in a second field (field B). No virus particles were observed by transmission electron microscopy (TEM) in negatively stained preparations from symptomatic plants nor were viruses isolated from several herbaceous hosts after mechanical inoculation of these hosts with preparations from diseased swan plants. Total nucleic acids were extracted from tissues (Marzachì et al., 2000) of both symptomatic (five plants from field A and four field B) and asymptomatic plants (one from each field). Resulting extracts were analysed by a polymerase chain reaction (PCR) assay employing universal rRNA primer pair P1/P7 (Schneider et al., 1995). A 1.8 kbp rDNA product was generated exclusively from all symptomatic plants. Restriction fragment length polymorphism (RFLP) patterns after digestion of P1/P7-primed products with endonuclease MseI (Fig. 3) indicated that all five symptomatic plants analysed from field A contained a phytoplasma indistinguishable from stolbur (Stol) phytoplasma, an established 16SrDNA RFLP group (16Sr) subgroup A (16SrXII-A) strain. By comparison, four plants from field B all contained phytoplasmas indistinguishable from European aster yellows, a phytoplasma previously classified as a 16SrI-B subgroup strain. Using these same assay methods, Stol phytoplasma was also detected in symptomatic Alstroemeria sp. and Solanum nigrum L. plants collected near field A. Phytoplasmas have been previously observed by TEM in phloem sieve elements of Gomphocarpus sp. with yellowing symptoms in the Brazilian State of São Paolo (Kitajima and Costa, 1979) whereas in the north-eastern USA, dwarfed and chlorotic Asclepias syriaca L. (=A. fruticosa L., =Gomphocarpus fruticosus R. Br.) reportedly contained 16SrIII group phytoplasmas (Griffiths et al.1994). This is the first record of phytoplasmas infecting members of the Asclepiadaceae family in the Old World.