Downy mildew of viburnum

The causative agent of this disease is the oomycete Plasmopara viburni, a specific pathogen that targets various species within the Viburnum genus. As an obligate parasite, it disrupts the physiological processes of the host plant, leading to significant foliage damage.

The primary hosts are different types of viburnum, including ornamental varieties often grown in nurseries and landscaping projects. The disease is particularly destructive in regions with high humidity, where it can rapidly spread throughout a collection of susceptible shrubs.

Symptoms typically present as chlorotic, yellowish spots on the upper leaf surface, which eventually turn necrotic and brown. Under high moisture conditions, the underside of these spots develops a dense, downy white to greyish layer of mycelium and spores.

The development and spread of Plasmopara viburni are driven by environmental factors, particularly prolonged periods of high humidity and rainfall. The pathogen thrives in mild to moderate temperatures, typically between 15°C and 20°C, when dew or fog allows for successful spore germination and infection.

Managing this disease requires a combination of hygiene and chemical intervention. Recommended strategies include:

• sanitation by removing and destroying all infected fallen leaves;
• ensuring adequate spacing between plants to promote air circulation;
• avoiding overhead irrigation to keep the foliage as dry as possible;
• applying appropriate copper-based fungicides or systemic anti-oomycete treatments at the first sign of symptoms.