It is hard to know what to recommend without knowing the pathogen. Can you describe how the leaf is affected more? Are there spots? a coating? totally black? scorched? Can you post a pic? Are they only affected in one part of the yard? Do you have healthy plants elsewhere? Personally, if they have a fungus that reappears each year, I would dig them up, throw them out and get another plant. If there is one plant that gardeners try to find homes for, it is their divisions of black eyed susans.

Black-eyed Susan (Rudbeckia) often suffers from mildews. The best defense is a good offense: buy mildew-resistent varieties and keep the plants spaced apart to encourge good airflow. Rudbeckia is considered to be a short-lived perennial, usually blooming best during its second, third and fourth year -yearlings sometimes bloom as well :) Since rudbeckia usually multiplies nicely, it is not difficult to have new plants available to replace very diseased or elderly plants. Because of the ease with which it gets mildew, I usually pull up & discard infected plants as soon as they show signs, although many gardeners simply prune the mildewy stems back and the plants apparently will bloom the following year; while other gardeners often simply purchase new plants on a triennial basis.

What growing conditions are they in? They like lots of sun and even moisture IME. I've never grown salmon colored ones and am not growing any yellow ones this year to check how long blossoms last, but the years I have grown them, it's been in a large deep pot (more than 1' in all directions) in a sunny spot with regular watering. They are a bit slow to start, but by this time of year mine were generally multistemmed and most of the way up a 3'-4' trellis and covered with blooms. Since there were a lot of blooms, even if each individual bloom didn't last long, it still had lots of bloom. So I am not sure why, but yours seem like perhaps they aren't getting all they need or perhaps the salmon colored plants aren't as vigorous as the yellow and orange ones that I've grown. If you add info on your zone and location, it helps with questions like this since different plants behave differently in different locations. Here's how: Go to Your Houzz in the upper right of every page, click Edit Profile, and on the left side click advanced settings. Well down the advanced settings page is a blank labeled Climate Zone for Garden Forums along with a link to find your zone. If you add info on your state or the nearest large city you will get even better information since zone only relates to average coldest winter temperatures. Then return to the top of the page and click Done Editing.

I have never found hardy hibiscus difficult to move, so I'd dig up whatever is necessary and, if they are intermingled in what you dig up, soak to rootball until it is easier to separate the different roots, pull them apart and plant the individual types of plants were you want them to be. They are both common plants, so if one died, it would be easy to replace it with a new one, but they are both pretty tough and unlikely to die.