The pathogenicity and ecology of some isolates representative of the four main Streptomyces species (S. scabies, S. europaeiscabiei, S. stelliscabiei and S. reticuliscabiei) identified as pathogenic to potato tubers were investigated. Three pathogenicity groups could be distinguished. Group 1 included all isolates of S. scabies, S. europaeiscabiei and S. stelliscabiei from common scab lesions of potato and other susceptible root crops. All these produced similar symptoms and were pathogenic to potato, carrot and radish. Group 2 included all isolates from S. reticuliscabiei netted scab lesions; they were pathogenic to both tubers and roots of only a few potato cultivars, and did not infect carrot or radish. Group 3 included three isolates of S. europaeiscabiei from netted scab lesions on cv. Bintje, which produced either common or netted scab symptoms depending on the potato cultivar or plant species. In an experiment on a few isolates from each of the three groups, held at various soil temperature regimes, the three from group 1 were most pathogenic at higher temperatures (20ЊC or 20/30ЊC), the two from group 2 were most pathogenic at a lower temperature (17ЊC). The group 3 isolate caused netted scab symptoms on susceptible cultivars at low temperatures (Յ 20ЊC) and deep-pitted lesions at higher temperatures. Since the groups identified differ in ecological requirements, it is important to adapt the control methods to the pathogenic species present in the soil.
The soilborne fungus Rhizoctonia solani is a pathogen of many plants and causes severe damage in crops around the world. Strains of R. solani from the anastomosis group (AG) 3 attack potatoes, leading to great yield losses and to the downgrading of production. The study of the genetic diversity of the strains of R. solani in France allows the structure of the populations to be determined and adapted control strategies against this pathogen to be established. The diversity of 73 French strains isolated from tubers grown in the main potato seed production areas and 31 strains isolated in nine other countries was assessed by phylogenetic analyses of (i) the internal transcribed spacer sequences (ITS1 and ITS2) of ribosomal RNA (rRNA), (ii) a part of the gene tef-1α and (iii) the total DNA fingerprints of each strain established by amplified fragment length polymorphism (AFLP). The determination of the AGs of R. solani based on the sequencing of the ITS region showed three different AGs among our collection (60 AG 3 PT, 8 AG 2-1 and 5 AG 5). Grouping of the strains belonging to the same AG was confirmed by sequencing of the gene tef-1α used for the first time to study the genetic diversity of R. solani. About 42% of ITS sequences and 72% of tef-1α sequences contained polymorphic sites, suggesting that the cells of R. solani strains contain several copies of ITS and the tef-1α gene within the same nucleus or between different nuclei. Phylogenetic trees showed a greater genetic diversity within AGs in tef-1α sequences than in ITS sequences. The AFLP analyses showed an even greater diversity among the strains demonstrating that the French strains of R. solani isolated from potatoes were not a clonal population. Moreover there was no relationship between the geographical origins of the strains or the variety from which they were isolated and their genetic diversity.
Three strains of Streptomyces reticuliscabiei and two strains of Streptomyces turgidiscabies were analysed, together with reference and type strains of other Streptomyces species, for phenotypic traits, DNA-DNA relatedness, comparison of 16S rRNA gene sequences and presence of necrotic protein gene (nec1) homologues in order to clarify their phylogenetic relationships. A numerical analysis of phenotypic characteristics showed that S. reticuliscabiei and S. turgidiscabies belong to the same cluster and share almost all morphological and biochemical traits that are important in the identification of Streptomyces species. DNA-DNA hybridization and phylogenetic comparisons of 16S rRNA gene sequences confirmed that the two species are genomically closely related. In contrast, pathological data showed that S. turgidiscabies and S. reticuliscabiei cause two distinct diseases. Gene homologues of nec1 were detected in S. turgidiscabies and other common scab species (Streptomyces scabiei, Streptomyces europaeiscabiei and Streptomyces stelliscabiei), but not in S. reticuliscabiei. To avoid confusion between agents causing separate diseases, it is proposed that the existing distinct species names are retained: S. turgidiscabies involved in common scab and S. reticuliscabiei involved in netted scab.More than 450 Streptomyces species have been described since 1964 (Shirling & Gottlieb, 1968a, b, 1969 as microorganisms dwelling mostly in the soil, but also in other environments (seawater, sediments, plant surfaces, etc). Almost all are saprophytic and only a few are known to be pathogens. The most important and widespread plant diseases caused by Streptomyces species are potato scabs which appear as superficial or deep lesions of the tuber skin. Two main types of scabs, common scab and netted scab, have been identified and characterized from diseased potato tubers in Europe. Recent data showed that common and netted scab are two separate diseases, differing in the type of symptoms produced, the causal Streptomyces species (Bouchek-Mechiche et al., 2000a), the range of host species affected, the response of potato cultivars to infection and the optimum soil temperature required for disease expression (Bouchek-Mechiche et al., 2000b).Streptomyces scabiei is the main species isolated from common scab lesions (Lambert & Loria, 1989a). It is prevalent in most potato-growing countries around the world. However, other Streptomyces species, including Streptomyces acidiscabies (Bonde & McIntyre, 1968;Lambert & Loria, 1989b), Streptomyces turgidiscabies (Miyajima et al., 1998), Streptomyces europaeiscabiei, Streptomyces stelliscabiei (Bouchek-Mechiche et al., 2000a), Streptomyces luridiscabiei, Streptomyces puniciscabiei and Streptomyces niveiscabiei (Park et al., 2003) have also been recently described as able to cause common scab. The similarity in symptoms and host range of these species suggests a common mechanism of pathogenicity, even though they are morphologically and genetically distinct. S. scabiei, S. acidiscabies and S. tur...
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