Most likely powdery mildew - the disease is common in crowded plantings where humidity is high and air circulation is poor and in damp, shaded areas. Overhead watering doesn't help either. It's a fungus and can attack spireas and ninebarks as well as a host of other things.

Powdery mildew lives over in the soil, on dropped leaves, etc., can be spread by wind, insects, and with rain or watering it can splash back up to reinfect a plant the following season if debris isn't totally cleaned up and disposed of properly - not in compost piles.

There are chemical controls, but it seems late in the season for that. Late winter, early spring is the best time for pruning both of what you mention. Cut them back hard and clean up every scrap of fallen debris. They should bounce back clean.

IME, this stuff attacks thirsty plants growing in leaner soils in high heat. I've never had it kill anything although it can make stuff ugly and shorten the lifespan of annuals, like Zinnias. I areas where more organic matter has been added periodically for a few years, and the soil remains more moist and well-mulched, it's much less prevalent, if it shows up at all.

I wouldn't and don't worry about PM in the compost, or from leaving the plants' fallen leaves at its' base. PM is everywhere, all the time, blowing in the wind. I'm not going to sort leaves or part with perfectly good organic materials. The plants won't have it when they come back in the spring because it's a last-season issue.

there are no controls ... there are preventatives ...

at this time.. you plan on full fall cleanup.. get rid of all fallen leaves.. etc..

and perhaps spray everything down with a proper spray ...

and then in spring.. you prevent.. before it appears. ...

do you spray your plants down after work every night ... if so.. you are the problem.. get a breaker bar ... and water the soil.. and keep the plants dry going into dark ...

and if you can thin out the garden a bit .. so there is air movement.. that will help..

i used to do it all the time.. and when i quit watering the leaves.. 90% of the problem went away ... and the other 10%.. i jsut got rid of the plants ...

also look for PM resistant varieties ...

i have friends in the cleveland area.. you should not be having this problem any greater than anywhere else ... unless something is going on in your particular garden.. which is aggravating it ...

and no.. its ugly... but usually not harmful ....

and like any other garden plague.. we have bad years.. and good years ...

the simplest preventative is baking soda and water ...

ken

Jake, do us a favor and google powdery mildew images and let us know if that's what appears to be on your plants....or not.

The one thing that bothers me about your description is the mention of two or more different kinds of plants. PMs are very species specific; the strain that affects spirea (for example) can't also infect weigela...or anything else but other spirea.

If it is PM it can be one of the easiest of the fungal disorders to prevent and cure (if the infection is not too severe) . Minor problems can simply be sprayed away with plain water. This particular disease does not need water to germinate as do other fungi. Humidity, but not free water.

Neem oil is an excellent product to both protect and cure PM problems and is something I readily recommend for this purpose.
There are also a number of commercial fungicides labeled for this disease, but I've never found them necessary.

As others have mentioned, plain old housekeeping chores are really important...so after you determine for sure that this is the disease that's pestering your plants and then let us help.