Where can I find information about dogwood hybrids, especially crosses between Cornus kousa and C. florida? Won't these trees be more resistant to the mildew affecting many dogwoods?

In addition to powdery mildew, many dogwoods can suffer with anthracnose. In his book Dirr's Hardy Trees and Shrubs (Timber Press, 1997), Michael Dirr mentions Rutgers Hybrids (which are a cross of the kousa and florida species of Cornus). These trees were developed at Rutgers University by Elwin Orton, and are resistant to dogwood anthracnose. here is an article about these cultivars, written by Orton. This article from North Carolina State University Extension discusses powdery mildew resistance. Scroll to the second table at the end which charts cultivars and their resistance or susceptibility to powdery mildew.

Oregon State University provides information about each of the six hybrids of C. florida x C. kousa. Two of the trees on this list are resistant to powdery mildew.

Clemson University Extension offers further information about the insects and diseases affecting dogwoods.