New gardener, new roses, weird brown petals quickly

Hi guys, recently I've planted roses in a small patch in my garden, I'd say around 5 weeks ago, they flowered nicely and the next set of buds came, two of three bloomed and then I cut them off when they started wilting, the third one took much longer to open, started blooming looked nice, I sprayed the roses with a mixture of potassium bicarbonate this morning and this has happened to my rose. I'm at a loss as to what's happened. This was the rose two days ago

This is it today about 6 hours after spraying it

Any idea what it is? Or how it's happened? I'm assuming it's something to do with the potassium bicarbonate.

Thank you for any help!

 

From the short time period and the marking on the flower I suspect the potassium bicarbonate was to blame. How strong a solution did you use? Potassium bicarbonate solutions are rather alkaline.
I'm interested in why you sprayed with potassium bicarbonate as it is not a chemical I've come across in the garden before.

 

It was one tablespoon to a 5 liter sprayer along with a little bit of original dish soap. I've been having an issue with powdery mildew and the potassium bicarbonate solution can apparently be used as a fungicide, having pets and what not I'm
Not fond of using any harsh chemicals