Prevent, protect zinnias from powdery mildew

URBANA, Ill. — The summer is always a little brighter in the garden when brightly colored zinnias are among the various plants. Zinnias are heat and drought-tolerant plants that attract a variety of summer pollinators. This flower thrives early in the season, but as summer temperatures rise and humidity increases, growers may start to see a white film on zinnias that looks like they have been dusted with flour. This white film is a powdery mildew fungus that can draw nutrients from the plant.

Why try to limit powdery mildew on zinnias? It causes photosynthesis to be compromised, and plants struggle to develop properly, resulting in smaller leaves and stems. 

The fungus overwinters as a small, black, spore-bearing structure, or as fungal threads called mycelium. Overwintering happens primarily in leaf debris or dormant buds on the plant. For this reason, it is recommended to use good sanitation practices to help contain the spread of powdery mildew by removing any infected plant material at the end of the growing season. Do not add plant material to a compost bin, and clean snips or pruning tools with a mix of one part bleach to four parts water. The overwintering fungi produce spores, which are then moved by either wind or rain that can splash the spores onto the plant. 

 Strategies to Reduce Susceptibility of Powdery Mildew

1. Select resistant varieties for your garden designs
   1. Narrowleaf zinnia (Zinnia angustifolia) is a bushy plant about twelve inches tall with narrow leaves and small, daisy-like flowers. They are used in containers and border plants.
   2. The Zahara series (Zinnia marylandica) or the Profusion series (Zinnia x hybrida ‘Profusion’) has disease resistance to leaf spot and mildew, resulting in longer-lasting plants in the fall. These plants are taller and have a two-to-two-and-a-half-inch wide bloom.
   3. The Oklahoma series (Zinnia elegans) offers medium-sized, sturdy double and semi-double blooms. They are used in cut flower bouquets with longer stems and additional flushes of blooms. This series is less susceptible to powdery mildew than other larger cut flower zinnia series.
2. Regularly inspecting zinnias and their leaves enables early detection
3. Optimize plant spacing to encourage good air circulation
4. Use mulches to increase soil moisture retention and suppress weeds
5. Control soil splash that might contain spores using soaker hoses, drip irrigation, or watering by hand closer to the root zone
6. Use neem oil or a fungicide as treatment according to the label directions

For more on zinnias and additional annuals or perennials blooms, visit the University of Illinois Extension flower site. For questions, contact a local University of Illinois Extension office from the statewide map.

Christina Lueking is an Illinois Extension horticulture educator for Bond, Clinton, Jefferson, Marion, and Washington counties. Gardeners Corner is a quarterly newsletter from gardening experts around the state. Each issue highlights best practices that will make your houseplants, landscape, or garden shine in any season.

Caption: Zinnias with poor air circulation. Photo by: Christina Lueking, Illinois Extension. Photo available for media download, including credits.