Phytoplasma infection in Asclepias physocarpa

Asclepias physocarpa (sin. Gomphocarpus physocarpus; Asclepiadaceae) is a perennial ornamental mainly distributed in tropical and subtropical areas of the world, grown also for cut production and employed as flower in dry arrangements. In Italy the plant produced from seeds obtained in situ, is mainly cultivated in Liguria where it is maintained for three years. A. physocarpa is reported to be infected by a few viruses such as TSV and TSWV, but no reports of phytoplasma presence are available. In August 2002 in fields of plants of one and two years a severe stunting, associated with rosette-like symptoms were observed; in other plants symptoms of yellows and vein yellowing were also present. Mechanical inoculations on test plants as well as leaf dip preparations performed in order to verify virus presence, gave negative results. PAS-ELISA tests for INSV and TSWV also gave negative results. Molecular analyses (PCR/RFLP) were performed on nucleic acid extracted from 1 g of phloem tissue collected from symptomatic and asymptomatic plants in November and provided evidence for phytoplasma presence in symptomatic samples (stunting and yellows). RFLP on 16S ribosomal gene indicated that 16SrXII-A (stolbur) and 16SrI-B (aster yellows) phytoplasmas, sometimes in mixed infection, were present. The disease is seriously affecting A. physocarpa cultivations since high percentages of infection were observed in many fields (up to 30-40%). It is likely that the plants become infected during cultivation cycles since leafhopper presence was quite spread in all the fields examined. It was not possible to attribute a certain phytoplasma or mixture of phytoplasmas to the different symptoms observed; it is possible that the type of symptoms could be influenced by the plant stage at the moment of infection: i.e. plants infected while are young show stunting and rosetting while plants infected at older stages (2 years) only react with yellows to the presence of the same pathogen/s.