In vivo antibiogram to test the action of synthetic and natural compounds on phytoplasmas

With the aim of finding new strategies to control phytoplasmal diseases, a series of natural and synthetic compounds were tested to study their activity in Catharanthus roseus L. healthy and graft-infected with two phytoplasmas (Candidatus Phytoplasma asteris and Candidatus Phytoplasma ulmi). A preliminary investigation was carried out by testing the responses of plants to treatment with tetracycline. The compound was applied in several ways (spraying, root absorption and scion dipping), and at different concentrations and number and timing of treatments. The results obtained showed that scion dipping was the most suitable method in our experimental conditions, as it needed the lowest amount of compound and permitted the quantification of the compound absorbed. Thus, this procedure was adopted to test some natural substances never assayed before in this host-pathogen system: (i) Cercosporin, Spirolaxin and Cladosporol, organic compounds of fungal origin with alleged antibiotic activity against phytopathogenic bacteria; (ii) Carvone and Pulegone, two active terpenoids of plant origin; (iii) the mineral salt Potassium Alum; (iv) the synthetic basic Dienes stain and (v) a Methion-in-Riboflavin mixture. The activity of these compounds was evaluated on the basis of phytotoxicity, symptom evolution and microscope observations. This testing method is likely to provide a sustainable and rational procedure for investigating the triangle host-phytoplasma-compound. It will be adopted in further research to estimate the anti-phytoplasmal activity of other known and novel chemicals, both synthetic and natural.