Yes, it is powdery mildew. Some people pull off the affected leaves if there are only a few or cut/pull out the plant if it is more widespread. Don't put in compost pile. Throw in trash. Clean up the soil area well so fallen leaves and stalks don't harbor the spores for next year.

To help prevent pm thin plants so there is good air circulation and don't water over the foliage. There are also fungicides one can use throughout the season. Also home remedies with baking soda or with milk. Google "preventing powdery mildew" for specifics.

Thanks, mnwsgal. After posting that note earlier today, I was walking in my neighborhood I started seeing it everywhere, on shrubs and even on tree leaves. I think it took my writing about it to start seeing it.

I know this is the time of year for it. The Coreopsis for some reason has suffered from it since the moment the plants appeared in May, but the Zinnias are just now getting it and I think that's the most painful to see.

A year or two ago I stopped growing zucchini because it seemed that each season I'd get a good month and a half of good fruits but then in August the powdery mildew would always overtake the plant and ultimately kill it.

OK, so having good air circulation is important. I guess that's a good argument for having the type of garden that's not too thickly planted or crowded, and where each plant has plenty of room to grow.

Yup, I just looked on the web and found some interesting preventive techniques and potential remedies

If you grow from seed, there are some zinnia varieties available that are resistant to powdery mildew. I've had good luck with them.

Please ignore ken regarding his advice to ignore it because it's Fall. He's been singing that tune non stop. It rarely makes good horticultural sense to leave diseased or insect infested plants to remain in the garden.

Fall cleanup of PM diseased plants is extremely important in preventing an outbreak next year. I'm sure that your research indicated that spores will overwinter on plant tissues.

I keep PM spores rinsed off foliage with a quick spray of water. A light application of horticultural oil can prevent infection if applied early in the season. Neem oil has both preventative and curative effects. But the oils cannot be applied during the heat of the summer.

Jumping back into this discussion a few weeks later. Out of sheer laziness I didn't pull out the zinnia plants that were infected with powdery mildew. Hopefully I'll get around to that tomorrow. But today I noticed something interesting. Because of my gut aversion to thinning (which I've been trying to overcome), most of my zinnia plants ended up fairly close together so that they're touching, and yes, a bit crowded. All of those plants have powdery mildew.

But the three plants that are well-spaced out and have had room to grow have no powdery mildew. This sure confirms what everyone has been saying about the importance of air circulation to prevent these kinds of fungal diseases. It's quite amazing to see that principle manifested in front of my eyes.

Thanks for reporting back with your observations, Rick. I'm a firm believer that PM can be prevented entirely by plain and simple good gardening practices.

I have one tall Zinnia plant standing by itself and nothing else around it within 9ft and it started getting PM when the weather changed...I have another Zinnia (same exact type plant) at the side of the house which is getting PM... It gets alot of good air circulation also...This type of Zinnia seems to get PM later in the season for us here...

Here's what I have planted:

http://parkseed.com/parks-picks-deep-red-zinnia-seeds/p/02237-PK-P1/