Powdery mildew will thrive in the hot dry weather. To prevent powdery mildew it may be helpful to mulch your plants in spring and water them regularly to reduce stress and make them less prone to infection. Removing and destroying the infected parts will control the spread of the mildew but if the infection is over a wider area you may have to use a chemical control. Fungicidal sprays are widely available and will normally specify if theyre effective against powdery mildew. To prevent infection next year make sure you clear away any fallen leaves as the fungus may over-winter on these.