Knowledgebase

Downy mildew on zinnias #793441

Asked May 31, 2022, 11:42 AM EDT

I have several zinnias in my small greenhouse that have downy mildew. I want to plant them outside. Should I spray them, and with what? The downy mildew has spread to my dahlias as well, and I'd like to rescue them if I can.

Montgomery County Maryland

Expert Response

Hi Leila!

You're likely seeing powdery mildew, as downy mildew does not impact zinnias or dahlias. The humidity and still air of a greenhouse can provide ideal circumstances for this fungus to develop on plants. 

We suggest moving the plants outside of the greenhouse, which will provide them with better air circulation and reduced humidity. When planting, ensure you space them for adequate airflow and you can pick off diseased foliage and prune for better circulation.  

If you want to spray, you can try horticultural oil products or a copper fungicide. Check the label registration on horticultural oil products for powdery mildew control listings and do not use if temperatures are above 85 degrees F. 

Jamie

Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 01, 2022, 10:41 AM EDT

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