By Andi Anderson
Zinnias bring bright colors and joy to summer gardens. These heat- and drought-tolerant flowers also attract many pollinators. However, as summer temperatures and humidity increase, gardeners might notice a white, flour-like layer on zinnia leaves. This is powdery mildew—a fungal disease that steals nutrients and weakens the plant.
Powdery mildew reduces photosynthesis, leading to smaller leaves and weaker stems. It spreads through tiny spores and survives winter in leaf debris or buds as fungal threads or spore-bearing structures. These spores move through wind or rain splash and infect new plants.
To protect zinnias, begin by choosing resistant varieties. The Narrowleaf zinnia (Zinnia angustifolia) is great for containers and borders. The Zahara and Profusion series (Zinnia marylandica and Zinnia x hybrida) resist leaf spots and mildew.
The Oklahoma series (Zinnia elegans) produces strong double blooms and is less prone to mildew than other tall cut-flower varieties.
Proper garden care helps prevent the spread of mildew. Space your zinnias well for better air circulation. Apply mulch to retain soil moisture and reduce weed growth.
Use watering methods like drip irrigation, soaker hoses, or hand watering near the roots to avoid soil splash that spreads spores.
Inspect plants regularly for signs of mildew. If needed, use neem oil or fungicide treatments as directed. At the end of the season, remove and discard infected plant parts—do not compost them. Clean garden tools using a solution of one part bleach to four parts water to stop fungus from returning next year.
To learn more about zinnia care and other flowers, visit the University of Illinois Extension flower site or contact a local Extension office.
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Categories: Illinois, Rural Lifestyle