Downy mildew is a fungal disease that affects many plants and trees, including grapevines. It is a serious disease that can lead to the death of vines and the loss of the entire crop. If you want to learn more about the causes of this disease and how to treat it, continue reading this article for a full explanation.
Causes of Downy Mildew in Grapes
Here are the main factors that lead to the appearance of downy mildew on grapevines:
- Rainfall plays a major role in the development of downy mildew, especially when rain follows several humid nights.
- Temperature increases the risk of infection; downy mildew develops within temperatures ranging from 10°C to 29°C.
- The disease becomes more likely when a wet winter is followed by a rainy spring and then a warm summer with rainfall every 8–14 days. Fungal spores spread through rain splash and wind.
- Downy mildew is common in regions where the weather is warm and humid during the vegetative growth phase of grapevines.
Treatment of Downy Mildew in Grapes
The key to managing downy mildew is prevention, as the disease is difficult to control once fully established. However, certain fungicides can be applied immediately after infection and before leaf spots appear. Using fungicides early helps kill the active pathogen within the initial spots and slows disease progression.
Recommended fungicide groups include:
- Phosphonates, such as fosetyl-aluminum
- Phenylamides, such as metalaxyl
- Carboxylic acid amides, such as Mandipropamid
- Strobilurin fungicides with curative effects
- Sequential applications of phosphorous acid products to burn active mildew infections and stop their spread
Note: It is important to rotate fungicide types frequently because downy mildew pathogens can develop resistance over time.
Prevention Tips to Protect Grapevines from Downy Mildew
To reduce the risk of infection, follow these recommendations:
- Plant grapevines in locations with good air circulation, as airflow helps reduce leaf moisture.
- Use well-draining soil, and plant grapevines in rows for better ventilation.
- Avoid overhead irrigation; use methods that keep foliage dry.
- Prune grapevines and use trellising systems to spread and dry leaves, reducing disease risk.
- Grow resistant grape varieties, such as Sultana, Chardonnay, and Pinot Noir.
- Maintain plant health by applying fertilizers according to soil needs.
- Apply fungicides—especially to susceptible varieties—in four key stages:
- Just before flowering
- 7–10 days after flowering
- 10–14 days after the second application
- Three weeks after the third application
- If vines were infected in the previous season, or if they belong to highly susceptible varieties, apply an additional fungicide spray about two weeks before flowering.
