I pruned as much of the infected parts of the plants that I could, and popped in to the Fitzroy Nursery on Brunswick St to buy some Eco-Rose and Eco-Oil. The Eco-Rose is based on potassium bicarbonate which according to the Cornell fungicide study is more effective than sodium bicarbonate (ordinary bicarb soda) and still friendly to the rest of the garden inhabitants. The Eco-Rose works better if you mix it with a bit of the Eco-Oil to make it stick to the leaves - as you do with the bicarb, oil, soap recipe. I also purchased their brand of sprayer which has a twistable attachment on the nozzle for spraying the underside of leaves - definitely worth the $12.
Sprayed everything infected, plus the zucchini because it also helps ward off powdery mildew. Will repeat in 7 days.