Posted: 25/05/2015 at 09:27
If the foliage is dry and it is warm then it will be powdery mildew. You will get mould on both the upper and underside of the leaf. Powdery mildew spores can’t germinate or grow when foliage is wet.
Downy mildew develops spores on the undersides of leaves and appears in cool, wet conditions and is generally stopped by warm, dry, windy weather.
I would use a spray with Sodium bicarbonate add half a tea spoon to a pint of water and spray.
You can also use fresh cow’s milk diluted with water to a 10% solution.