Patty,

Powdery Mildew spores can persist on plant debris or mulch, but I don't think the spores actually persist in the soil. So cleaning up the plant debris and adding another inch or two of mulch to the top of the current mulch should help.

I definitely would not grind up the stalks of the zinnias and use them as mulch anywhere. One problem with powdery mildew is that the spores are wind-blown so they can be blown around your yard. In general, I compost garden and lawn debris, with the exception of any plants that had disease issues and any weeds that have set seed. The last thing you want is for your mulch or compost to spread disease or weed seed.

I bag up diseased plant debris and weeds with seedheads and dispose of them along with the household trash.

With zinnias and other plants prone to powdery mildew, it helps to plant them where they have good air flow, to water the ground only using soaker hoses or drip irrigation, and to be prepared to use a fungicide (both chemical and organic ones exist) in periods of heavy rainfall and high humidity. It also helps to space them a little further apart than the seed packet recommends, just to give them a little better air flow.

A few of the newer named varieties, including the entire Profusion Series, are resistant to powdery mildew. In general, of the roughly 20 different species, only a couple of the older ones (Zinnia angustifolia and Zinnia haageana) have strong natural resistance to powdery mildew. Zinnia elegans, which is the main species grown in our gardens here in the USA, is not naturally resistant to powdery mildew. However, some of the newer flowers do have powdery mildew resistance bred into them including the following series (all are available in single colors or mixes): Profusion, Kneehigh Profusion, Dreamland and Magellan. I think the new Zahara series coming out on the market for 2009 is resistant to powdery mildew as well. Other zinnias that are resistant to powdery mildew are "Mexico", "Old Mexico", "Star White", "Blue Point" and "Orange Star".

I am not sure if powdery mildew can persist on saved seed, although I guess it might. You might try treating your seed with a fungicide before you plant it.

I let some of my zinnias reseed every year, but I also rogue out plants that develop a serious PM problem. Each year, I plant fewer and fewer of the Z. elegans and more of the newer PM-resistant varieties.

Dawn

Thanks, Dawn. I'm glad I asked first.

Patty,

You're welcome.

Zinnias are one of my favorite flowers, but even here in hot and dry southern OK, they often get PM.

Dawn