Hi, 

The mildew that basil is particularly susceptible to is downy mildew. We are not aware of reports of basil infection by powdery mildew.

Eleanora is the only sweet (Italian) basil variety we are aware of with some resistance to basil downy mildew. All other varieties are hybrids. They are crosses between sweet basil varieties and wilder, resistant types.

The resistance genes would be passed on to the next generation through collected seeds. However, it is likely that the F2 (second generation) plants from saved seed would not "come true." You should expect to see a variety of expressed traits that may reduce culinary quality (e.g., bitterness, leaf hairiness etc, that are more common on wild basil types).

Thai, holy, lemon, and cinnamon basil varieties have varying degrees of tolerance or resistance to basil downy mildew.

Prospera has been the best basil downy mildew performer in MD trials but since that is a hybrid, you would want to grow plants purchased from a garden center and not collected seeds. 

Let us know if you have further questions. 

Emily