Genetically susceptible plants mildew when the soil is dry but there is fog, dew or another moisture source getting the tops damp (but not drenching them). That is why it is seen during summer, and why we see it often out here on the west coast.

What you have seen with this exercise is that lilacs do mildew - under the right circumstances. Since the source plant has been clean you might assume that your starts will do likewise when planted in a new spot. Clay soil is moisture retentive so that might help with the mildew, unless it is the same soil they are planted in now.

Thanks for the info. I think I will try it by the house and just see what happens. Sometimes plants surprise you!!! We have had heavy dews this summer.

The soil they are in now is very rich. The soil by the house is clay with constant improving.