I have only ever grown the I. Walleriana from seed, but they did especially well last year. I used them to border the front of a few beds. As I understand it, you don't have to worry about the downy mildew if growing from seed at home. The seed doesn't carry it, the spores are in soils etc. I have certainly never had an issue with the home grown plants.
Showstopper Cherry Flare