RESISTANCE TO POWDERY MILDEW IN PERENNIAL PHLOX

Forty-nine cultivars of perennial phlox, Phlox paniculata L., grown in a field plot at Ottawa were compared for resistance to powdery mildew in 1961, 1962, and 1963. None of the cultivars was immune. A seedling, designated by the number 60–75, showed field resistance to mildew that was significantly better than the other cultivars for 3 years but subsequently became susceptible. A second cultivar, Pyramid White, initially showed field resistance to powdery mildew but it became susceptible to the disease more quickly than 60–75. Results obtained under the conditions of our experiments suggested that first-year plants were more resistant to powdery mildew than plants of the same cultivars that were at least 3 years old. In breeding for resistance, only a few seeds from crosses germinated but one apparently resistant seedling was obtained. However, this seedling subsequently became susceptible.