The I. walleriana types are still susceptible to downy mildew. The only ones that I know of that are resistant are the Bounce Series, an interspecific hybrid. As for preventing or reducing downy mildew infection, don't plant where you have had impatiens before, especially if they had downy mildew; minimize water on the plants - therefore water early in the day and preferably at the base of the plants to reduce water on the foliage.