Lifecycle of Grape downy mildew

Plasmopara viticola is an obligate biotrophic pathogen of grapevines with sexual overwintering and asexual cycles during the growing season.

The pathogen overwinters as oospores in fallen, infected leaves. In mild climates, it can also survive as mycelium in grape buds and canes. In spring, oospores germinate when temperatures reach 10°C and soils are wet, forming macrosporangia that release zoospores within 24 hours.

The zoospores are dispersed by rain, encyst near stomata, and form germ tubes that penetrate through stomata. Inside the plant, hyphae spread intercellularly, forming haustoria that parasitize mesophyll cells. Symptoms appear 4-21 days after infection, depending on temperature, with the shortest period on young leaves at 19-24°C.

Management

Disease pressure varies significantly with weather. Rigorous management is essential in wet climates and during unusually wet seasons in typically dry areas. Most grape cultivars are highly susceptible to downy mildew. Breeding programs started in the 1930s and have focused on incorporating resistance from wild Vitis species.

Cultural practices alone are insufficient for complete control, but they can reduce disease pressure. These practices include:

• Promoting air circulation and minimizing leaf surface wetness through pruning, trellising, and proper vineyard layout.
• Using furrow irrigation instead of overhead irrigation to reduce leaf wetness.
• Ploughing to bury oospores in leaf litter and avoiding prolonged irrigation to prevent oospore germination.

Disease forecasting and modeling tools have been used since the early 1900s, with modern computer-based systems developed in various countries (e.g., Italy, France, USA, Australia). Models incorporate temperature, rainfall, humidity, leaf wetness, host growth stage, and varietal susceptibility, aiding in optimizing spray timing.