While I have grown zinnias for many years, almost all of them suffer from bad mildew at some point in the season. Many times, they look so bad that I end up pulling them from the garden.
I have found three versions which seem to be resistant to mildew.
Zinnia – Forecast from Burpee.com $5.99 for 50 seeds.
I started these in mid-June and they not only have been blooming ever since, but even though they have been surrounded by plants with mildew, these flowers have not caught the disease. They claim to have different colors, but most of mine are shades of pink. I direct seeded these into the garden.
Zinnia angustifolia – Yellow – Summerhill Seeds. $2.50 for 50 seeds.
These zinnias only grow 12-18 inches tall and don’t develop mildew.
They seem to come true from seed and they are easy to germinate. I planted some seeds in my beds and they came up just fine.
My final favorite is Profusion Zinnia.
Park Seed or your favorite nursery will probably have these. They have larger flowers than Angustifolia, but are about the same height. You can see the difference in the above photo.