PHYTOPLASMAS AND PHYTOPLASMA DISEASES

Recent progress concerning knowledge of ribosomal DNA (16S and interge nic spacer 16S-23S) has provided an increased understanding of the par ticularities of procaryotes previously named mycoplasma-like organisms (MLOs) and justified their new name 'phytoplasmas', which was recomme nded by the Subcommittee on the Taxonomy of Mollicutes of the Internat ional Organization for Mycoplasmology (IOM). The main characteristics of phytoplasmas supporting their classification among bacteria and vir uses are described here. The PAUP (phylogenetic analysis using parcimo ny) method was applied to the 16S and intergenic 16S-23S fractions of the ribosomal DNA in order to establish the phylogenetic position of p hytoplasmas among bacteria and viruses. Subsequently, attempts to clas sify different phytoplasmas were made, first by using the polymorphism of ribosomal DNA fractions amplified with universal phytoplasma prime rs. 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 t he studies of ribosomal and genomic DNA and antigens. A variety of tec hniques are available which remain to be adapted to particular purpose s such as inventory and epidemiology or to original host species that tend to influence phytoplasma concentrations within infected plants su ch as herbaceous and woody species. The control of phytoplasma disease s depends on a better knowledge of the insect vectors, their life cycl es and reservoirs. In the case of vegetatively propagated host plant s pecies, especially woody species, sanitary improvement of the genitor plants through different methods is available. Finally, breeding for r esistance is always the recommended method for the control of phytopla sma diseases. When this resistance is not present in natural condition s, transgenic plants resistant to insect vectors may be a promising wa y to control the diseases.