Morphological and molecular characterization of downy mildew on sweet basil (Ocimum basilicum) caused by Peronospora belbahrii in Turkiye

Background Sweet basil (Ocimum basilicum) is one of the most significant aromatic plants in Turkiye. Recently, a new pathogen induced symptoms were discovered and identified as basil downy mildew caused by Peronospora belbahrii Thines. The pathogen has been introduced into the country and it has quickly become the most damaging disease in basil cultivation. The purpose of this study was to investigate the molecular and morphological properties of the causal organism of downy mildew observed on sweet basil and determine the disease incidence and prevalence in Antalya province. Methods and results According to morphological characteristics (conidia, conidiophores) disease was determined as downy mildew caused by P. belbahrii. Pathogenicity tests were performed by spraying with a sporangial suspension of P. belbahrii (1 x 10(5) sporangia/mL). After 1 week, all inoculated plants exhibited characteristic downy mildew symptoms on their leaves, whereas non-inoculated control plants remained disease-free. All molecular analyses involving the internal transcribed spacer region were amplified using Nested PCR with primer pairs ITS4 and ITS6 for the first round and ITS4 and DC6 for the second round. Resulting sequences of all the nested PCR products had 99% similarity with P. belbahrii isolates. Disease incidence was 22.4-70.2% of sweet basil cultivation area in Antalya province. Conclusions Based on the molecular analysis, morphological characteristics and pathogenicity tests the pathogen was identified as P. belbahrii. To our knowledge, this is the first report of downy mildew caused by P. belbahrii on sweet basil in Turkiye.