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Downy mildew

Plasmopara viticola
Downy mildew is a sporadic disease that can have a devastating impact on grapevines when warm, wet conditions trigger the rapid spread of secondary infection.
Downy mildew

 

Description

The easiest way to identify downy mildew is by the characteristic yellow ‘oilspots’ it causes on young leaves, and the white down that forms on the underside of leaves in warm, humid conditions. The symptoms on older leaves are more subtle: small patches of diseased cells between the veins. Young bunches of grapes, which are highly susceptible to the infection, turn brown and quickly die.


Control

There are two chemical application strategies for managing downy mildew: a scheduled spray program using a range of preventative fungicides; and a ‘wait and see’ approach, which involves waiting until the primary infection occurs and then immediately applying post-infection products.

The second approach requires close monitoring of the weather to anticipate rain events at crucial timings, and equally close monitoring of the vines for early signs of infection.

As a general rule, there is a high risk of primary infection if 10 mm of rain falls while the temperature remains over 10º C through a 24-hour period (the ‘10:10:24’ rule). And a single warm, wet night following primary infection is enough for secondary infection to spread rapidly.