Brown Spot Black Spot is fungus due to humidity. I too have to deal with that every spring
Here in Tx.
The leaves get wet and stay wet throughout the day creating a fungus,
Here is what I do,
Cut off all the infected leaves place in a bag and throw away, Do Not let any linger on the ground as the spores will still be active and will infect more leaves so make sure all are removed even on the ground,
You can get a spray called rose Defense By greenlight[
and spray only early in the morning
before the sun shines on the rose
's in the heat of the day. other wise the heat of the sun will "Burn" the remaining leaves with the rose
defense on it,
You can also Buy Neem
oil concentrate and use it in a spray bottle,, Both are found at your local nurserys,, Rose Defense
is found at Home Depot and Maybe Lowe's.
Either One will have to be used repeatedly 2 times a week until No signs of reoccurring fungus.
As a precautionary measure I always spray them Once a week Until the rainy season is over. I also "thin" the roses
( Cutting out crossing Branches in the middle of the rose
bushes ) to allow for more air circulation
I know it's disappointing to have to remove all the spotted, yellowing leaves,, and possibly removing a branch or two , But it must be done to restore it's health.
Bad thing about rains and humidity with roses
,, especially if you have a tree
in close proximity, the leaves on the tree
has rain drops on them that in turn drip on the roses
during the day even when the rain has stopped ,, creating possible fungus ,