Epidemiology of sage downy mildew, Peronospora salviae-officinalis
Downy mildew of common sage (Salvia officinalis), caused by Peronospora salviae-officinalis, has become a serious problem in sage production worldwide. The effect of temperature was determined for conidia germination and disease development. In vitro, conidial germination rate was highest at temperatures between 10 °C and 20 °C and was strongly reduced at temperatures above 25, but conidia were also able to germinate at 2 °C. Temperatures between 15 and 20 °C were most favourable for infection and disease progress in infection experiments in climate chambers, with highest sporulating leaf area observed at 15 and 20 °C, and highest symptomatic leaf area at 20 °C. P. salviae-officinalis is still able to infect sage plants at 5 °C, but sporulation was only observed at higher temperatures. Oospores developed 14 days after inoculation at 15 °C, and 8 days after inoculation at 20 and 25 °C. The infection trials also showed that dark incubation is not a prerequisite for successful infection of sage. Furthermore, P. salviae-officinalis needs two events of leaf wetness or high humidity to complete its asexual life cycle. First, a leaf wetness event of at least three hours is needed for conidial germination and penetration of the host. Second, high humidity of at least 90% is needed at the end of the infection cycle for sporulation. Seed washing revealed the presence of P. salviae-officinalis oospores in seed lots of sage 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.