For Downy Mildew to infect vines it needs to be ‘triggered’ in the soil by rain and soil wetness over a 24 hour period.
Downy needs rain keeping the soil wet for 16 hours, so that its spores can germinate ready to cause infection in the leaves.
|Above: Downy from Spring 2006. This season the weather has not been suitable to start the disease.|
Any breaks in wetness over 2-4 hours stop Downy’s lifecycle. The spores need to be splashed to the leaves after the 16th hour and the leaves then need to remain wet for at least 2-3 hours, for infection to occur. In McLaren Vale windy conditions often prevent this from happening. In the Adelaide Hills, where valley and local conditions limit wind, Downy Mildew is more likely.
Downy can move quicker than 24 hours if the temperature are above 10oC. Also, Downy can complete its infection below 10oC (down to 7-8oC) but takes longer than 24 hours.