| Sumario: | A collection of 45 isolates of Sphaerotheca pannosa var. rosae was established from rose plants infected with powdery mildew, one of the diseases caused by this pathogen. After a preliminary molecular screening using RAPD, 16 isolates, collected in floricultural enterprises located in five municipalities of the savannas of Bogotá, Colombia, were seleced as representative of the variability present in S. pannosa. A practical methodology to conserve this obligated pathogen was developed using gelatin capsules, thus providing a permanent source of inoculum for pathological and molecular analyses. Pathogenicity studies conducted with rose cultivars Aalsmeer Gold, Charlotte, Classy, Konfetti, Livia, and Tineke showed that isolate Sp6 did not sporulate in cultivar Tineke, although this isolate affected all other cultivars. The other isolates sporulated on all cultivars, suggesting the presence of at least two pathotypes in the target population. A pathogen-host plant interaction therefore exists. The Internal Transcribed Space (ITS) of the 5.8S gene of ribosomal DNA was amplified using PCR, and a homogeneous and reproducible band, with a molecular weight of 295 bp, was obtained. The amplified product was digested with restriction enzymes that showed a similar band pattern for all isolates, indicating that the isolates belonged to the same species. Intraspecific polymorphisms were detected with 11 RAPD primers and a RAMS microsatellite primer. Three genetic groups, with 60% similarity (Dice index), were detected, indicating intraspecific genetic diversity. The high correlation (+0.89) between origin and genetic diversity, and between this and pathogenic variation (+0.63), suggests that differences exist in environment conditions and/or disease management among farms or in rose cultivars used.
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