It looks like good old fashioned powdery mildew.

hmm..what an education I am getting....spraying, squishing..wonder what I'll learn next year...anyway..I did a search for powdery mildew and read everyone's ideas...This is what I've learned...Fungicides are for prevention...neem might be curative...good spacing of plants..etc..etc..if late in season rake dropped leaves and be prepared for next season..

So..seeing as this is mid season..should I try and do anything..this is a very large plant that still looks good...will the mildew spread to the surrounding plants..(I've seen it on another plant but in the back yard)..should I just cut it down and be prepared for next year? Sorry to ask so many basic questions..especially since I just admitting to reading previous responses!!!

As far as spreading to other plants....maybe, maybe not. The confusion comes from this: Saying that you have powdery mildew is kind of like saying, "I have a cold." You can get a cold with all the familiar symptoms from many different kinds of germs. Here's the thing, people often see powdery mildew on one type of plant and then another different plant next to it gets hit and assume that the pathogen has spread. It is only occasionally the same fungi. However, the same conditions that allows powdery mildew to take hold in one plant allow powdery mildew of different species to infect other plants. That is to say, it isn't so much a case of a pathogen spreading as it is, the conditions favoring the disease in general are common in an area and so plants in that area are prone to get that disease from whatever species of fungi happens to attack them. i.e., powdery mildew pathogens and hosts:
Erysiphe cichoracearum on begonia, chrysanthemum, cucuribis....
Erysiphe polygoni on legumes, beets, crucifers AND cucurbits
Microsphaera alni on many shade trees and ornamentals
Spaerotheca macularis on strawberry, gooseberry, and curant
Uncinula necator on grape and horsechestnut
etc. etc. etc....
So, the best way to stop the disease is to create conditions that do not favor the pathogen. When it comes to that, it's pretty much the same old story: Prune out dead stuff and thick growth to increase air circulation. Try not to overhead water. Water in the morning so that plants have a chance to dry. Monitor for disease. At first sign, spray with light horticultural oil or potassium bicarbonate. You'll never eliminate it from your life. Control is your best shot.

GREAT information, except for the fact the PM is one of the FEW fungus diseases that does not live in the presence of water. It won't be spread by water droplets and can even be subdued by spraying the foliage with water. Today's information suggests that overhead irrigation can prevent major outbreaks of PM, as the spores are killed my water rather than encouraged to grow. I keep the PM from an old fashioned Crape myrtle by daily spraying of the foliage. Works like a charm.

Lack of sunlight, poor air circulation, excess nitrogen fertilization, excessive pruning....and other factors can all contribute to powdery mildew infestation. And those things are all added to the fact that you may have a susceptible plant species. Some plants are prone to PM, while other species will NEVER 'catch' it.

Be sure you do a GOOGLE search on PM. You will find some excellent ideas on the prevention of this disease. It is almost entirely preventable, in my opinion.

Here is a link that might be useful: Powdery mildew information

Right, the thing with powdery mildew is that regardless of species, it needs free moisture on the leaves for the haustorium to penetrate the tissue of the host. (hence the need to avoid evening watering) The mechanics of the invading fungal peg requires a glut of water to create the posotive pressure neccessary to push through the epicuticle of the host. (which happens with free water on plants at night when temperatures are cool). Once it has gained entrance to the tissue, a dry environment and high temperatures (during the day) favor the fungus. Even though they act a little bit different from other fungi, it's best to avoid free water and increase air circulation on soft stuff like vegetables and annual flowers. I'd never really thought of the idea of blasting the stuff off with water early in the day. It seems so obvious but just never entered my mind in relation to fungus. That's a pretty cool idea. I likes the way you think, rhizo.

Oh yes, and I am aware that most species are inhibited by extended periods of free moisture.
But they, like all living things on earth, require water to go about their busniness. Engorgement of the haustorium is only one of the things that they need water for. Extended periods of moisture would indeed wipe out powdery mildew, but also your plants. Timing and balance are of the utmost importance when it comes to powdery mildew.

Wow..thank you guys so much..I had googled PM and read that same article you linked to rhizo_1 and have read the same as cramapple points out. Again..what an education I'm getting. I do try to read everything I can before I post as the instructions suggest...but I really like the personalized help you guys give...

I've decided that I've have contributed to my own problem. Since I know nothing, I follow the little tags given with the plant about their care. The care tag on my rudbeckia says "partial sun" however..more specific reading suggests they prefer a full sun..it is getting about 4 hours of full sun per day but I think it would like more..so I'm going to have to read on transplanting soon as I'd like to do everything to save it only because it was a gift and still is pretty healthy...once it starts to look bad tho...I'm chopping!

Funny..those little care tags...it also says it is disease resistant..!!! Again thanks..hope you guys can help with the other problems I'll be posting..!!!

This information on PM is very interesting, thanks. Now I have a question. I discovered PM on my lilacs and serviceberry a week ago and immediately gave them a thorough spray with a mixture of copper soap fungicide mixed with a little bit of Eco-Oil (dormant and growing season oil). I went out to do another application yesterday and was pleased to see the PM didn't look nearly as bad as it did last week. Obviously, I couldn't prune off the affected foliage without cutting the shrubs down to the ground, so I had no choice but to spray.

So my question is, do you think it was the copper, the oil or the water that helped the most? I never really thought PM would clear up once it was on the foliage.

It was said: " the thing with powdery mildew is that regardless of species, it needs free moisture on the leaves"

Powdery mildew doesn't require free water. It can get by with only 98 percent humidity -- and that's easy to attain within the canopy of a plant, even during dry weather.

After looking around at the encouragement of rhizo and jean, I stand corrected. Thanks guys, it's always good to learn a new thing or two.

And I have the same sentiment about YOU! It's a learning process and that makes it ALL good!