Abstract:

The aim of this work was to identify the pathogen causing downy mildew in Angelica sinensis in Yuzhong County， Lanzhou City， to provide a theoretical basis for field diagnosis and control of the disease. The pathogen was preliminarily identified by recording disease symptoms and by observing its morphology under a microscope. Hyphae were collected from diseased plants and the DNA was extracted. The 28S rDNA D1/D2 region and the mitochondrial DNA （mtDNA） gene cytochrome c oxidase subunit II （cox2） were sequenced， and phylogenetic trees were constructed based on the sequencing results. The disease primarily affected the leaves. At the onset of the disease， irregular chlorotic spots appeared on the front side of the leaves， followed by the development of a frosty white mildew layer. Eventually， the chlorotic spots became brown dead spots， and in severe cases， the entire plant died. The morphology of the sporangiophores and sporangia were consistent with those of Plasmopara angelicae. The 28S rDNA D1/D2 region displayed 99.87% sequence similarity to that of Plasmopara pimpinellae HV 634， while the cox2 gene exhibited 100% sequence similarity to that of P. pimpinellae TCM-155. Phylogenetic analysis revealed that the Plasmopara species infecting Apiaceae plants were clustered together and were more distantly related to Plasmopara species infecting host plants from different families. Molecular identification based on the 28S rDNA D1/D2 region sequence and cox2 gene sequence corroborated the close relationship between the pathogen and P. pimpinellae that parasitizes members of the Apiaceae. Additionally， morphological identification confirmed the downy mildew on A. sinensis as P. angelicae， consistent with the membership of A. sinensis in the Apiaceae family. This study is the first comprehensive report on the pathogen responsible for downy mildew in A. sinensis in China.

Key words: Angelica sinensis, downy mildew, morphological characteristic, phylogenetic relationship, Plasmopara angelicae