Morphological evidence supports the existence of multiple species in Pustula (Albuginaceae, Oomycota)

During the past five years, molecular phylogenetic studies revealed a high level of genetic diversity within Pustula on Asteraceae, which warrants its division into more than a dozen distinct species. An important step towards introducing the phylogenetic lineages as new species is to clarify, whether morphological characteristics could satisfactorily distinguish between them, including previously described but not widely accepted species. In the present study, white blister rust specimens of Pustula parasitic to Austroeupatorium, Cirsium, Tragopogon (Asteraceae), Centaurium, and Swertia (Gentianaceae) were compared, based on morphological examinations. The characteristics of sporangia and oospores allowed the separation into five distinct species, which correspond well with recently discovered phylogenetic lineages and show specific host ranges. From the five host genera, Cystopus brasiliensis, Pustula spinulosa, P. obtusata (illeg. syn. P. tragopogonis), P. centaurii, and Albugo swertiae were identified, respectively, among which C. brasiliensis and A. swertiae were combined into the genus Pustula in the present study. Our results demonstrate that the Pustula species on Asteraceae, which were previously considered to be a single species, are indeed not only phylogenetically, but also morphologically distinct entities and that both sporangia and oospores are potentially suitable for morphological delimitation of closely related species of the genus Pustula.