Pathogenic and Molecular Characterisation of Hyaloperonospora sp. Isolates Causing Downy Mildew on Wild Rocket

ABSTRACT

Downy mildew (DM) caused by the oomycete Hyaloperonospora diplotaxidis (= Peronospora diplotaxidis ) is a devastating foliar disease that affects wild rocket ( Diplotaxis tenuifolia ) worldwide. Understanding the diversity of pathogen populations and the existence of resistance host‐specific responses is crucial, enabling the selection of well‐adapted cultivars. This study aims to clarify the taxonomy, host range and genetic relationships of different H. diplotaxidis isolates, reinforcing its status as a distinct species from other Hyaloperonospora pathogens of the Brassicaceae family. The isolates obtained in three plant species ( D. tenuifolia , Raphanus sativus and Brassica oleracea ) and distinct locations were evaluated for virulence on 17 Brassicaceae genotypes at the seedling stage. Hyaloperonospora sp. isolates were separated into three groups according to the species of origin. H. diplotaxidis isolates were pathogenic only in the species of origin D. tenuifolia , and the same behaviour was observed in H. brassicae isolates collected in B. oleracea . In relation to H. brassicae isolates collected in R. sativus , some sporulation was observed in a broccoli ( B. oleracea var. italica ) accession. With the set of host genotypes evaluated, no differential responses were detected among H. diplotaxidis isolates. Four wild rocket genotypes were identified as potential sources of resistance. Molecular identification using ITS and cox2 barcoding regions confirmed the separation of Hyaloperonospora isolates into three distinct groups identified in pathogenicity trials. These findings contributed to the understanding of host specificity and genetic structure within Hyaloperonospora sp., supporting future breeding in wild rocket.