PHYTOPLASMAS AND PHYTOPLASMA DISEASES

Recent progress concerning knowledge of ribosomal DNA (16S and intergenic spacer 16S-23S) has provided an increased understanding of the particularities of procaryotes previously named mycoplasma-like organisms (MLOs) and justified their new name 'phytoplasmas', which was recommended by the Subcommittee on the Taxonomy of Mollicutes of the International Organization for Mycoplasmology (IOM). The main characteristics of phytoplasmas supporting their classification among bacteria and viruses are described here. The PAUP (phylogenetic analysis using parcimony) method was applied to the 16S and intergenic 16S-23S fractions of the ribosomal DNA in order to establish the phylogenetic position of phytoplasmas among bacteria and viruses. Subsequently, attempts to classify different phytoplasmas were made, first by using the polymorphism of ribosomal DNA fractions amplified with universal phytoplasma primers. Thereafter, the knowledge of the ribosomal 16S and intergenic 16S-23S DNA sequences made it possible to establish a phylogenetic tree of phytoplasmas using the PAUP method. Thus, several phytoplasma groups were characterized Significant progress concerning the sensitivity and specificity of the diagnostic methods has been made on the basis of the studies of ribosomal and genomic DNA and antigens. A variety of techniques are available which remain to be adapted to particular purposes such as inventory and epidemiology or to original host species that tend to influence phytoplasma concentrations within infected plants such as herbaceous and woody species. The control of phytoplasma diseases depends on a better knowledge of the insect vectors, their life cycles and reservoirs. In the case of vegetatively propagated host plant species, especially woody species, sanitary improvement of the genitor plants through different methods is available. Finally, breeding for resistance is always the recommended method for the control of phytoplasma diseases. When this resistance is not present in natural conditions, transgenic plants resistant to insect vectors may be a promising way to control the diseases.