That fuzzy white fungus on your summer phlox is one of many non-life threatening molds that latch on to a variety of plants ranging from common zinnias to cabernet sauvignon grapes. Powdery mildew and its less common relative, downy mildew, are caused by fungi with very specific appetites. Powdery mildew, the more common unwelcome garden guest, is seldom fatal -- just unattractive.
Powdery mildew caused by the fungi Golovinomyces cichoracearum, also called Erysiphe cichoracearum, afflicts garden phlox. It appears on leaf surfaces, and less frequently on stems, of Phlox paniculata. Inoculation begins with mycelial growth -- fine white hair-like tracks. Powdery mildew then blooms into spore colonies on leaf surfaces that resemble white talc, hence the name powdery mildew. Powdery mildew spores travel on the wind and individual spores do not move around on plants; new spores must land to enlarge colonies. Affected leaves may drop before healthy leaves. Unchecked, the fungus can cause leaves to yellow and die, or stunt new growth.
Another fungus, Peronospora phlogina, or downy mildew, attacks Western native members of the Polemoniaceae family, including Phlox species, Collomia sp., Gilia sp., and Navarretia sp., many of which bloom in summer. Downy mildew grows on the underside of leaves and on stems; it first appears as yellow spots on leaves and grows in branching white to gray spores. Downy mildew can eat through the tops of leaves, creating dry, necrotic areas in its wake. It can also stunt growth, and its leaching of tissues can cause above-ground growth to wither.
Powdery and downy mildews require different conditions for success. Powdery mildew travels and is most successful in shady areas where temperatures range from 60 degrees Fahrenheit to 80 degrees Fahrenheit. Sunlight and heat retard the growth of powdery mildew; water and temperatures above 95 degrees Fahrenheit may kill the spores. Downy mildew gains its hold in cool, moist conditions with temperatures in the 50s and 60s, high humidity and free moisture -- a foggy spring morning fits the bill nicely.
Spacing plants widely improves air circulation and makes dropping from one plant to the next more difficult. Prune affected foliage to limit spreading of existing spore populations, destroy infected plants and re-plant the area with mildew-resistant varieties of garden phlox. Water phlox early in the day when the sun will dry the foliage; water will inhibit powdery mildew, and timing watering when airborne moisture is rapidly dissipated discourages downy mildew. Sulfur-containing fungicides that have labels listing powdery mildew or downy mildew on phlox may also be effective on infections. Horticultural oils such as neem or jojoba oil labeled for use on powdery mildew on phlox may help control mild to moderate infections. Whenever using fungicides or other sprays, follow label directions and control over-spray.
- University of Vermont Extension: Powdery Mildew on Phlox and Monarda
- Lady Bird Johnson Wildflower Center: Plant Database: Polemoniaceae, California
- Pacific Northwest Plant Disease Management Handbook: Phlox-Downy Mildew
- UC IPM Online: Powdery Mildew on Ornamentals
- UC IPM Online: Floriculture and Ornamental Nurseries Powdery Mildew
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