Downy mildew of cinquefoil

Systematic position and nature of the pathogen: Peronospora potentillae belongs to the kingdom Chromista, within the genus Peronospora. It is an obligate biotrophic pathogen known for causing downy mildew. As an oomycete, it functions similarly to fungi, but it is distinct in its physiological requirements and life cycle, specifically targeting plants of the Rosaceae family, primarily the Potentilla genus.

Diseases and host plants: The primary symptom of infection is the development of angular or diffuse chlorotic lesions on the upper leaf surfaces. Under high humidity, a white to greyish-purple downy mold appears on the underside of these lesions, containing sporangiophores. This disease significantly impacts the health and visual appearance of various cinquefoil species cultivated in gardens and nurseries.

Biology and life cycle: The pathogen propagates through asexual sporangia and sexual oospores. Oospores are thick-walled structures that allow the pathogen to survive harsh winters in soil or plant debris. In spring, when moisture and temperature conditions are favorable, these structures germinate to initiate the primary infection cycle, followed by rapid secondary spread through wind-dispersed sporangia.

Conditions for development and spread: Peronospora potentillae thrives in cool, humid environments. Temperatures between 15°C and 22°C, combined with prolonged periods of leaf wetness, are ideal for the infection process. Rain splashes and wind are the main drivers for local and regional dispersal, making this pathogen difficult to contain without strict hygiene measures.

Harmfulness and control measures: The infection leads to premature leaf senescence, reduced plant vigor, and potentially plant death if left untreated. Management strategies include:

• Removal and destruction of infected plant debris.
• Improving site airflow to reduce leaf moisture duration.
• Using certified, disease-free planting material.
• Applying preventative or curative fungicides, such as those containing metalaxyl or fosetyl-aluminum, when symptoms first emerge.