The best advice I can give is to be proactive and give your plants a spray before symptoms even occur. Try a few recipes online, or buy a ready-made product.
I am an organic gardener, and prefer to make my own stuff, because I live growing the green, and don’t work. It might be worth buying a product, such as the above, Dr. Zymes (which I have heard good things about but never tried), or other products, if time is short, or you’re lazy, though 
And, there is always plain old sulfur, which this forum seems to praise a bit.
