Open Access

Mascha Hoffmeister

• Downy mildew of common sage (Salvia officinalis), caused by Peronospora salviae-officinalis, has become a serious problem in sage production worldwide. The causal agent of the disease belongs to the Pe. belbahrii species complex and was described as a species of its own in 2009. Nevertheless, very little is known about its infection biology and epidemiology. The aims of the current study were therefore to unravel the life cycle of this downy mildew and gain deeper insights into the epidemiology of the disease, as well as to clarify the species boundaries in the Pe. belbahrii species complex. Infection studies showed that temperatures between 15 and 20 °C were most favourable for infection and disease progress. At 5 °C Pe. salviae-officinalis is still able to infect sage plants, but sporulation was only observed at higher temperatures. Furthermore, Pe. salviae-officinalis needs two events of leaf wetness or high humidity, a first one of at least three hours for conidial germination and penetration of the host, and a second one for sporulation. Additionally, contamination of sage seeds by Pe. salviae-officinalis was proven by seed washing and by PCR and DNA sequence comparisons, suggesting that infested seeds might play a major role in the fast spread of sage downy mildew, which is an important finding for phytosanitary or quarantine measures. A protocol for fluorescence staining and confocal laser scanning microscopy was established and the whole life cycle of Pe. salviae-officinalis was tracked including oospore formation. The method was also used to examine samples of Pe. lamii on Lamium purpureum and Pe. belbahrii on Ocimum basilicum demonstrating the usefulness of this method for studying the infection process of downy mildews in general. Peronospora species parasitizing S. sclarea, S. pratensis, O. basilicum, and Plectranthus scutellarioides were studied using light microscopy and molecular phylogenetic analyses based on six loci (ITS rDNA, cox1, cox2, ef1a, hsp90 and β-tubulin). The downy mildew on S. pratensis was shown to be distinct from Pe. salviae-officinalis and closely related to Pe. glechomae, and is herein described as a new taxon, Peronospora salviae-pratensis. The downy mildew on S. sclarea was found to be caused by Peronospora salviae-officinalis. The multi-gene phylogeny revealed that the causal agent of downy mildew on coleus is distinct from Pe. belbahrii on basil, and is herein described as a new taxon, Pe. choii.