Here's what the Colorado experts say about pH. See the link. Lots of other good soil info precede this particular page as well.
Our soil is fairly acidic out here in the first place, I haven't bothered to test it. I go through hundreds of lbs of coffee grounds in my compost and, although the debate rages on whether or not that actually drops pH, I think it does. Acid loving plants seem happy.

Here is a link that might be useful: Lots of info on soil for dahlias including pH

Thanks Poochella

Now I am adding plenty of compost.

Gary

Who did your testing?. I am surprised that it is that high in your part of the state, what with the sand dunes and jack pines, etc. Not doubting you, just surprised. Grow iris!! They like it at 7 or so. One can never have too many vices/addictions in the plant world. All the better to trade with me. LOLOLOLOL

You are absolutely correct about iris, they grow like weeds. I had the tests done by Michigan State. Even my water is 7.5 pH. So far I have only gotten pink hydrangeas. Last year I used 100 pounds of aluminium sulfate.
There is a quary mine not too far from me that is called the Alkali. That may be a hint of high ph.
Gary

I have been putting lots of manure in....

And get lots of foliage. Any relationship between manure and mildew on the plant?

tc

More like a relationship between the nitrogen in the manure producing foliage vs flowers. What kind of manure is it?
Powdery mildew is a function of watering: that just bends my little blonde head. Water roots adequately, allow for air passage between plants by removing the bottom foot of foliage in mid season and hopefully mildew can be avoided. I had not one leaf affected this year. Usually, I get some mildew very late season by the garage bed which has limited airflow, but not this time.

I hate powdery mildew ! when the rainy season starts down here all that moisture is not good for my dahlias. But I use a simple remedy my granny taught me when mildew attacked her roses, baking soda, 2 teaspoons in a pint of water and a light spray and mildew dies in an eyeblink.

My soil pH is about the same as yours. My dahlia do fine. I do amend the soil as already stated to help it stay loose and hold the water better.