Preliminary study on the effects of two sar inducers and prohexadione calcium on the development of phytoplasmas in vinca

The use of natural or synthetic inducers of plant-defence responses in crop protection represents a valuable strategy to avoid environmental risks and the development of resistant pathogen strains. We studied the effects of two plant resistance inducers (aluminium tris and acibenzolar-S-methyl) and a growth regulator (prohexadione calcium) using the plant pathogen system Catharanthus roseum-chrysanthemum yellows phytoplasma (CY). Aluminium tris (O-ethyl phosphonate) was one of the first chemicals capable of inducing systemic acquired resistance (SAR) towards pathogenic fungi and bacteria. Acibenzolar-S-methyl (BTH) is a functional analogue of salicylic acid and a particularly effective SAR inducer. Prohexadione calcium, a plant growth regulator, was chosen for its efficacy in controlling apple proliferation, a phytoplasma disease of apple. Fifty-day-old C. roseum seedlings were sprayed with the chemicals on trial and after four days were grafted with CY-infected scions of the same species. A second treatment was made 21 days after grafting. Plants were visually observed weekly and tissues, collected below the grafting union, were analysed with a light microscope. CY phytoplasma were stained using either DAPI or Diene's stain. For two month after treatment, all chemicals significantly reduced the appearance of symptoms (yellows and virescence). After three months, aluminium tristreated plants showed only mild symptoms of yellows while no differences were observed between plants treated with the remaining chemicals and the infected non-treated control plants. The efficacy of aluminium tris to impair phytoplasma development was also assessed by microscopic observation of sampled tissues.