Taxonomic revision identifies nine new Peronospora species and host shifts on Veronica

A multi-locus taxonomic revision of downy mildews on Veronica identified nine new Peronospora species and recorded two host shifts involving ornamental plants formerly classified in the subgenus Hebe. The study examined 105 Peronospora specimens, including one from Paederota, and combined detailed morphological comparisons with phylogenetic analyses of two nuclear and five mitochondrial loci.

Before the revision, nine Peronospora species had been validly described from Veronica, although about 500 Veronica species had been reported as hosts of downy mildew. Many earlier identifications assigned a single pathogen to numerous hosts within the plant family. The authors note that recent studies have instead shown that most Peronospora species are highly host-specific, making re-examination of host–pathogen relationships within Veronica necessary.

Material came from Eurasia, Oceania and South America, including Argentina, Australia, Austria, the Czech Republic, Germany, Greece, Palestine, South Korea, Spain, Sweden and Switzerland. The investigators examined conidiophores, conidia and, where available, oospores from dried herbarium material. They also sequenced the nuclear ITS and nrLSU regions and the mitochondrial COX1, COX2, COX2-1 spacer, NAD1 and RPS10 regions. The concatenated phylogenetic dataset comprised 104 sequences spanning 5,767 aligned sites.

Minimum Evolution, Maximum Likelihood and Bayesian analyses produced no significant topological conflicts. The reconstruction identified 17 Peronospora lineages on Veronica, comprising seven previously described species, one newly combined species and 10 previously undescribed species. One lineage was represented only by DNA sequences and lacked specimens for morphological analysis, so the researchers formally described nine rather than 10 new species. Based on phylogenetic results and morphological differences, they accepted 16 species: seven previously described taxa and nine new species.

The new taxa were Peronospora brevibrachia, P. chionodendron, P. fidelia, P. gracilis, P. microspora, P. obscura, P. petricosa, P. sagittaria and P. seminaria. Each was associated with a particular Veronica host in the specimens examined, including V. chamaedrys for P. brevibrachia, V. verna for P. chionodendron, V. triphyllos for P. fidelia and V. praecox for P. gracilis. The revision also showed that several pathogen–host records had previously been combined too broadly. P. agrestis was restricted in the examined material to V. polita and the closely related V. filiformis and V. persica, while the pathogen on V. arvensis was genetically distinct. P. arvensis was likewise found principally on its type host, V. hederifolia, whereas the pathogen on V. triphyllos was distinct.

Morphology and host identification were important because some Veronica species are difficult to distinguish, and several specimens previously assigned to different hosts or pathogens represented other taxa. DNA barcoding of the host plants helped resolve these ambiguities. For example, specimens labelled V. sublobata differed from V. hederifolia by only two ITS nucleotides and were identical to V. hederifolia in the matK barcode. In another case, a sequence assigned to P. aquatica from V. anagallis-aquatica clustered with the distinct pathogen P. sagittaria from V. catenata. Host ITS and matK sequences supported the conclusion that the original host identification was mistaken.

The study therefore combined pathogen and host molecular identification with oomycete morphology rather than relying on either plant names or a single barcode locus alone. Several newly recognised species were close relatives whose nuclear sequences alone did not provide sufficient resolution. P. fidelia and P. gracilis differed by two gaps in ITS and one base in LSU D1–3, with no variation in LSU D6–8, but substantial differences occurred in four mitochondrial loci. The pathogen on V. triloba and P. veronicae-cymbalariae showed a comparable pattern, supporting the use of multiple loci for taxonomic separation of closely related downy mildews.

The study also recorded what the authors interpreted as host shifts to ornamental plants. Peronospora palustris, previously reported only from V. scutellata, was found on Veronica armstrongii, formerly Hebe armstrongii, and V. diosmifolia, formerly H. diosmifolia. P. petricosa, found on V. arvensis in Europe, Asia, South America and Australia, was also detected on V. odora, formerly H. buxifolia, in Australia. Because the natural ranges of the Hebe species do not overlap with those of the pathogens’ principal hosts, and no native Peronospora species are confirmed from Oceania, the authors considered these associations evidence of recent host shifts rather than long-standing relationships.

The findings are bounded by the material available to the revision. One of the 17 detected lineages could not be described morphologically because no specimens were available, and the European record base was substantially larger than that from other regions. The study established genetic and morphological associations among available specimens; it did not experimentally test the host shifts or determine their effects on plant health, trade or disease spread. Even with those limitations, the revision shows that downy mildew diversity on Veronica is considerably greater than the previous count of nine recognised species and that many apparent broad host ranges reflected misidentification or lumping of distinct lineages.

Subject of Research: Taxonomic revision of Peronospora downy mildews infecting Veronica

Article Title: A revision of Peronospora species on Veronica unravels a species-rich group of downy mildew pathogens with host shifts to economically important ornamental plants

Article References: Mu, M., Choi, Y.-J., Brand, T., Ploch, S., Voglmayr, H., & Thines, M. (2026). A revision of Peronospora species on Veronica unravels a species-rich group of downy mildew pathogens with host shifts to economically important ornamental plants. IMA Fungus, 17, Article e186696. https://doi.org/10.3897/imafungus.17.186696

Image Credits: AI Generated

DOI: 10.3897/imafungus.17.186696

Keywords: Peronospora, Veronica, downy mildew, oomycetes, plant pathology, taxonomy, phylogenetics, host specificity, host shifts, ornamental plants, revision, species