Plant Pathology Infobytes

September 20, 1999
Powdery Mildew - Zapping Zinnias

"Just when my zinnia bloom crop was at its peak, white patches of growth appeared on the leaves. Then my flowers became infected. Since I've watered, fertilized, and pulled them through some tough growing conditions, I'd like to keep my zinnias flowering as long as possible. Can you help me with the problem?"

The County Extension Office has received calls about this problem over the past week, and in checking with plant pathologists at Mississippi State University, we've confirmed our diagnosis of powdery mildew. The problem springs up on zinnias in the form of patches of fungus growth on the top surface of zinnia leaves. This particular powdery mildew fungus attacks not only the foliage, but generally spreads to flowers. Infected flowers are unsightly, and wither away within a few days. The only good news about this fungus is that it's host range is confined to zinnias.

The powdery mildew fungus begins to show up in zinnia plantings during late summer, but the disease really takes off when cooler nights (50 degree range) are followed by mild days with low humidity. These conditions are ideal for development of zinnia powdery mildew. The disease is quite distinctive in its appearance, but if you're not sure about identification of the problem, bring several specimens by the County Extension Office for examination.

Homeowners who want to extend the bloom period of their zinnia planting still have time to slow down the disease, provided appropriate control measures are followed. Extension plant pathologists suggest the application of a fungicide such as Spectracide Immunox (available at most garden supply stores) at the rate of one fl. oz. per gallon of water. Apply to the plants to the point when spray just begins to run off the leaves. You should plan to spray several times (every two weeks) through September and into October. If you follow this approach, the disease can be "knocked back" and the flowering period of your zinnias should be extended.

Other fungicides (those containing chlorothalonil), and wettable sulfur, are also cleared for use for zinnia powdery mildew. However, these materials generally are not as effective on the disease, unless applied earlier in the season when early symptoms are first observed.

If you have questions concerning pest control in the home landscape, please feel free to give us a call at the County Extension Office.



Written by the late Dr. Frank Killebrew

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