Downy mildew of grapevines

Page last updated: Tuesday, 4 May 2021 - 11:52am

Please note: This content may be out of date and is currently under review.


Good management of downy mildew is dependent on good monitoring of favourable weather conditions for primary and secondary infection events and of disease progress in the vineyard.

Automatic weather stations can be used for monitoring and predicting weather events. Weather stations collect information on temperature, rainfall, leaf wetness and humidity (suitable for downy mildew) and process the data for the likelihood of a primary or secondary infection event. Alternatively a maximum/minimum thermometer and a rain gauge can be used as a guide.

Monitoring in the vineyard is important to confirm possible infections from weather events. Physical examination can confirm whether a pre-infection or post-infection fungicide is required and will also indicate if spray coverage was effective after an event.

Monitoring in the vineyard should occur every 7-14 days when weather conditions are favourable from 3-4 weeks after budburst (that is, when shoots are approximately 10cm long).

Some general guidelines on vineyard monitoring include:

  • Vines to be monitored should be representative of the block being assessed.
  • Monitor for possible source areas of downy mildew infection such as wetter more sheltered parts of the vineyard (for example, near windbreaks and sheds), vines with dense canopies or areas that have previously been infected.
  • Inspect both sides of 200 vines by scanning the foliage between mid-morning and mid-afternoon.
  • Spend about 30 seconds per vine. Less time will be required early in the season when vine canopies are small.
  • Focus on the canopy near the ground in lower lying areas where the soil may remain wet for extended periods.
    • The disease is difficult to detect in dense shaded canopies. The foliage may need to be parted to scan the inner leaves.
    • Use a hand lens to check suspect spots on leaves for evidence of the fungus.
    • Take samples of suspect leaves and bunches and conduct a bag test (The bag test for downy mildew of grapes).
    • Tag suspect sample sites so they may become the reference points if the sample is confirmed positive.

Monitoring for downy mildew can be combined with monitoring for other pests (for example, diseases and insects).

Note: Other fungi may infect leaves and bunches. Downy mildew spores are white and raised. Powdery mildew spores are ash-grey and are a uniform thin layer. Botrytis spores are cream to grey-brown. Penicillium, Aspergillus or Rhizopus are white and raised but change to green, blue, black or brown. If there is any uncertainty suspect samples may be submitted to a pathology laboratory for confirmation DDLS - Plant pathology services.

Oilspots that have already developed 'white down' on their underside (whether this fungal growth is fresh or old) may not develop any more 'white down' even after a bag test is conducted.