As we enter Spring, and after our wet winter, the conditions may be ripe for the onset of one form of pest or disease. In particular, powdery mildew enjoys hot dry days followed by cool moist nights – it’s a rapidly spreading fungus that lies dormant until the right conditions appear.
While it predominantly favours warmer, humid climates such as those in New South Wales or Queensland, South Australia and Victoria’s recent wet winters -coupled with warm weather -may also provide the right conditions for its onset.
Powdery mildew looks like a white, greyish powder spread over the surface of rose leaves. Masses of spores are spread by wind, water splash and insects, making it a particularly virulent disease. The good news is that it is host specific so it can’t be transferred from one plant type to another.
Like many of these diseases, prevention is always better than control. Plant your roses with adequate air circulation and in a spot where they receive at least six hours of light per day. Avoid over-fertilisation – a slow release fertiliser is best.
If you do find powdery mildew on your roses, there are several off the shelf options containing an active ingredient called chlorothalonil which coats the leaf surface with a white milky film.
Organic preparations you may like to try include potassium bicarbonate which has the unique benefit of killing spores on contact. Like baking soda (sodium bicarbonate), it can be made into a solution by mixing a teaspoon of potassium bicarbonate with half a tablespoon of horticultural oil (to spread) and a litre of water which is then sprayed onto the plant.
Another excellent approach to reducing powdery mildew is dusting sulphur on the plants – which is also the best product to control mites. When applying, the temperature must be at least 25 degrees or more for it to be effective. | 
