Phylogeny of an Albugo sp. infecting Barbarea vulgaris in Denmark and its frequency of symptom development in natural populations of two evolutionary divergent plant types

• We analyzed the phylogeny of Albugo sp. infecting Barbarea vulgaris.
• We tested the incidence of white rust in natural populations of B. vulgaris.
• The isolate of Albugo sp. cluster with previously analysed samples from B. vulgaris.
• This supports that this Albugo sp. may be an independent specialised species.
• White rust was only found in some B. vulgaris populations.
• These belong to a chemo-ecotype found previous to be susceptible to Albugo sp.

The oomycete Albugo candida has long been considered a broad spectrum generalist pathogen, but recent studies suggest that it is diverged into several more specialized species in addition to the generalist Albugo candida sensu stricto. Whereas these species cause the disease white blister rust in many crucifer plants, asymptomatic endophytic infections may be important in the epidemiology of others. One of the plant species attacked by Albugo sp. is the wild crucifer Barbarea vulgaris ssp. arcuata, which is diverged into two phytochemically and genetically different types with different geographical distributions in Europe. These were previously shown to differ strongly in propensity to develop white rust upon controlled infections in the greenhouse. Here, we analyse the phylogenetic relatedness of this local Albugo sp. field isolate to other species and lines of Albugo spp., including others collected on B. vulgaris. We further ask whether the difference in incidence of white rust between the two types of B. vulgaris are also expressed in natural populations.Phylogenetically, the local Albugo sp. field isolate clustered tightly together with previously analysed samples from B. vulgaris, supporting that the Albugo sp. infecting B. vulgaris may indeed be an independent specialized species. White blister rust and Albugo DNA was only detected in two populations of the plant type that frequently develops symptoms upon controlled inoculations. The lack of white rust and Albugo sp. DNA in the other plant type may be due to either resistance, preventing infection, or asymptomatic infection of other tissues than leaves, which we analysed.