Downy mildew in grapes

In short... 

• Downy mildew can significantly impact grape yield and vine health if not properly managed. 
• Integrated Pest Management (IPM) practices, like canopy management, pruning, and monitoring, can help reduce disease risk and improve control. 
• Rotating fungicides with different modes of action is essential to prevent resistance and maintain long-term effectiveness. 

Downy mildew, caused by Plasmopara viticola, continues to be one of the most significant diseases impacting grape production in Eastern Canada. Special attention to management of this water mould (Oomycete) disease is important to preserve yield, quality and as P. viticola is at high risk of resistance development.  

Left unmanaged, downy mildew can lead to significant crop losses, both directly through fruit rot and indirectly by weakening vines through premature defoliation. Tools to battle this disease are limited, that's why adopting an Integrated Pest Management (IPM) approach and fungicide resistance management strategies is crucial for long term success. 

Disease development 

Optimal grape downy mildew development occurs at 20–25°C with frequent rain or dew. Early foliar infections, especially on leaves that are not yet fully expanded, are especially aggressive and can lead to defoliation that predisposes vines to winter injury and long-term yield reduction.  

Initial infections are caused by overwintering spores, or primary inoculum, found in leaves, soil, or debris in the vineyard that are moved to newly expanding tissue by splashing rain or wind. These primary infections can re-infect, causing more lesions under humid conditions and spread by wind or rain splash. 

Where infections occur 

1. Leaf infections are the most common and lead to premature defoliation, weakening vines and reducing sugar accumulation in fruit, but may not damage the current crop. Leaves are most vulnerable to infection before fully expanding. 
2. Shoot and inflorescence infections can cause early season damage and loss of potential fruit, but the impact of the infection depends on timing and severity. 
3. Fruit (berry) infections are the most detrimental type of downy mildew for a marketable grape crop, especially when it occurs from bloom to four weeks post-bloom (when berries are most vulnerable). 

Prevention and Integrated Pest Management 

Ideal cultural practices to reduce the risk of downy mildew include maximizing sun exposure and managing the canopy to improve air circulation and spray coverage. 

Pruning, and leaf removal help reduce shading and improve airflow, lowering humidity levels that favour disease development. Effective weed control and proper soil drainage can also reduce free moisture, limiting fungal spread. 

Diligent vineyard monitoring is necessary, especially under high humidity or where dense foliage occurs. Monitor for early signs of infection – light green to brown oil spots on leaves – to stay ahead and prevent more impactful infections like, distorted shoot tips and white, downy fungal growth on fruit. 

Applying fungicides prior to the appearance of infection symptoms is crucial, as it both supports resistance management and optimizes fungicide efficacy. 

Downy mildew solutions for grapes 

*Ridomil Gold 480SL must be applied with an effective non-Group 4 tank mix partner.
**Revus should be applied with an effective non-Group 40 tank mix partner and used in alternation with other effective modes of action.

Fungicide resistance 

Special attention to the management of grape downy mildew is now more important than ever, especially after a recent provincial survey in Ontario that found widespread resistance to Group 11 fungicides, as well as a small pocket of Group 40 resistance in one specific Ontario region. The long-term effectiveness of downy mildew management will depend on rotating fungicides with different modes of action. 

Monitoring supports resistance management efforts and the preservation of crop protection tools, and Syngenta Canada is proud to support such collaborative resistance monitoring projects in grapes. To learn more about fungicide resistance in grape downy mildew, read The Lowdown on Downy Mildew in Grapes, by Wendy McFadden-Smith of the Ontario Ministry of Agriculture and Food. 

For more information, reach out to your local extension specialist or Syngenta Representative.