Phylogenetic analyses reveal only Albugo rorippae causing white rust on horseradish in Brazil

Armoracia rusticana, commonly known as horseradish, is a Brassicaceae condiment common in southern Brazilian cuisine. In 2020, plants with leaf lesions and white pustules were observed in a cultivation area in the metropolitan region of Curitiba, state of Paraná, Brazil. Based on phylogenetic studies, Albugo sensu lato was classified into the genera Albugo, Pustula, and Wilsoniana. As there is still no molecular characterization of the specimens that cause white rust in Brazil, this study aimed to identify the causal agent of white rust in A. rusticana. Symptomatic plants were collected to identify the oomycete and placed near asymptomatic plants for the pathogenicity test. Plants exposed to the diseased plants showed symptoms 14 days after exposure, while plants without exposure to diseased plants remained asymptomatic. Comparison of symptoms and morphology matched the characteristics of Albugo sensu lato. After morphometry and sequence comparison of the partial region of the cytochrome c oxidase subunit II (cox2) mtDNA gene, the oomycete was identified as Albugo rorippae. To date, this species is reported on A. rusticana, Rorippa amphibia, Rorippa palustris, and Rorippa sylvestris in Argentina, Australia, Germany, Czech Republic, Poland, and Romania. According to the literature, the Albugo specimens that colonize Armoracia sp. have low efficiency in colonizing Rorippa sp., and there are some differences in the oospores of the specimens isolated from these two hosts. Therefore, a comprehensive sampling of Albugo sp. in Armoracia and Rorippa species is essential to determine whether there is host genus specialization. This is the first report of A. rorippae in Armoracia rusticana in Brazil.