The baking soda won't stick unless you put a little soap in the mixture.

Make sure the soap is Ivory liquid or some other plain, non-antibacterial, non-scented soap.

And lemon might burn your roses even if it wasn't hot outside--I wouldn't use any citrus on plants you want to keep!

I just used a baking soda spray on my new roses for white mold and aphids.
It actualy worked better than the ortho nasty gag spray.t he mold has not been back even though my don Juan clibers only get sun from 1-6pm. I used the ortho four times and it always came back!
This is the spray recipie I used
2 tbls baking soda
2 tbls dish soap
2 tlbs vegetable oil( I actualy only had extra virgin olive oil)
1 gallon water
or use an old 1 quart windex bottle rinsed very well and use 1/2 tbls each.
I would spray when it is sunny because it washes off easy every 2-3 days for fungus and every 5-7 for bugs.
drench plants getting undersides use as much as you want!

I just have Dawn in the house, will it work or is it too harsh for the roses?