Nature of Work: Garden phlox, from cultivars of the native Phlox species caroliniana, maculata and paniculata, is an important herbaceous perennial for the landscape and nursery trade. Powdery mildew is recognized as a major limiting factor when growing garden phlox in warm, humid areas. Resistant cultivars of garden phlox may reduce production costs and increase landscape success.

Evaluations of garden phlox mildew resistance have been conducted in climates unlike that experienced in the humid southeastern United States (2). Cultivar response is often different when plants considered highly resistant in other areas are exposed to the extended warm, humid growing conditions in the southeast (1).

Erysiphe cichoracearum, a causal organism of powdery mildew in phlox, spores germinate optimally under damp conditions between the temperatures of 59 and 86 F. (3). These temperatures are close to the daily average maximum and minimum temperatures from early June through mid September at MHCREC, Fletcher, NC. In addition, there is morning fog the majority of days from early June through mid September at MHCREC.

In April 1999 a completely random design experiment with 3 one quart plants of each cultivar was planted on 3 ft centers in a soil that had been prepared according the NC Cooperative Extension recommendations following soil tests. Plants were irrigated until established then never irrigated again. Weeds were removed by hand and fertilizer applied according to NCCES recommendations.

Included in the test were 'Blue Boy,' 'Blue Paradise,' 'Darwin's Joyce,' 'David,' 'Delta Snow,' 'Fairest One,' 'Flamingo,' 'Franz Schubert,' 'Laura,' 'Little Boy,' 'Magnificence,' 'Miss Jill,' 'Miss Jo-Ellen,' 'Miss Karen,' 'Miss Katherine,' 'Miss Lingard,' 'Miss Margie,' 'Miss Mary,' 'Mt. Fujiyama,' 'Natascha,' 'Nikki,' 'Nora Leigh,' 'Orange Perfection,' Phlox caroliniana, 'Prime Minister,' 'Robert Poore,' 'Rosalinde,' 'Russian Violet,' 'Sir John Falstaff,' 'Starfire,' 'Tenor,' 'The King,' and 'White Admiral.' The incidence and intensity of powdery mildew infestation on each plant was recorded monthly from June through early September of 1999, 2000 and 2001.

Results and Discussion: By the end of the first growing season, it became evident that disease pressure from powdery mildewwas intense. Many plants were thoroughly infested and did not survive the first winter. Cultivars that did not survive were replanted in spring 2000. When many of the same cultivars as well as others were not alive in spring 2001, we re-evaluated the trial due to severe disease pressure and decided not to collect data if 2/3 of any cultivar was dead. The cultivars 'Blue Boy,' 'Blue Paradise,' 'Darwin's Joyce,' 'Flamingo,' 'Laura,' 'Miss Lingard,' 'Orange Perfection' and 'Prime Minister' had at least two-thirds of plants die during the trial. No cultivar was immune to powdery mildew under our conditions.

To determine overall resistance, powdery mildew infestation ratings from the beginning of June and mid September were averaged. Those with 11 % or less of foliage showing powdery mildew were considered highly resistant. Those averaging 25 to 50 % were considered moderately resistant, and those showing over 50% of foliage with powdery mildew were considered poorly resistant.

Significance to the Industry: Even under severe powdery mildew pressure there are garden phlox cultivars that exhibit a high degree of disease resistance. These include the cultivars 'David,' 'Delta Snow,' ' Natascha,' 'Robert Poore,' 'Speed Limit 45' and the species Phlox caroliniana.

Acknowledgments: Thanks to the following nurseries for providing the plants in this test: Andre Viette Nursery, Holbrook Farm, Wayside Gardens, Niche Gardens, North Creek Nurseries, Pine Knot Farms, Sunny Border Nursery and White Flower Farm.

Literature Cited:

1. Bir, Richard E. and Richard Hawke. 1999. Monarda Mildew Resistance. Proc. IPPS 49: 394-396.
2. Hawke, R. G. 1999. Plant Evaluation Notes: An Evaluation Report of Selected Phlox Species and Hybrids. N. American Pl. Prop. 11(1):16-20.
3. Powell, C. C. and R. K. Lindquist. 1992. Ball Pest & Disease Manual. Ball Publishing, Geneva, IL p. 63-67.

Return to Richard E. Bir homepage
North Carolina State University