Downy mildew of grapevine

Plasmopara viticola is an oomycete pathogen classified within the kingdom Chromista. It is the causal agent of grapevine downy mildew, a major fungal-like disease that poses a significant threat to viticulture worldwide, particularly in regions with humid and temperate climates.

The pathogen infects all succulent parts of the grapevine, including leaves, tendrils, flowers, and young berries. Early symptoms are characterized by yellowish, oily-looking spots on the upper leaf surface, commonly referred to as oil spots. Under high humidity conditions, a white, downy fungal-like growth appears on the underside of these spots.

The disease cycle begins with the overwintering of oospores in fallen, infected leaves on the vineyard floor. During spring, when temperatures exceed 10°C and rainfall occurs, these oospores germinate to release zoospores. These mobile spores are splashed onto the lower leaves by rain, where they enter through the stomata to initiate primary infection.

The development and spread of Plasmopara viticola are highly dependent on environmental conditions. Rainfall, high humidity, and moderate temperatures between 20°C and 25°C are ideal for rapid infection and secondary cycles. During these favourable conditions, the incubation period can be as short as four to five days.

Downy mildew causes severe economic damage by defoliating vines and destroying inflorescences or clusters, which can lead to near-total crop loss if left uncontrolled. Integrated pest management (IPM) is essential, combining cultural practices such as canopy management and proper drainage with the strategic application of contact and systemic fungicides.

• Sanitation: removing and burying infected plant debris.
• Canopy management: improving airflow to reduce leaf wetness.
• Chemical control: using copper-based and systemic fungicides.