Looks like it's rotting due to the super heavy soil you have them in.. Sorry to say but there's almost nothing right about the soil. They need very free-draining soil, an easy fix is 50% perlite and 50% soil but if you have the time and the resources then invest in a grittier soil mix with Turface, chicken grit, and other ingredients.

Some of them are pretty much done for, but the newer and shorter ones maybe can be saved if you repot them in time.
Are they being grown outdoors? If they are then watch out rainfall so they're not drenched in wet soil all the time, and also you need to protect them from direct sun all day, seems like there might be sun-burn on these plants too.

Is the plant in full sun? It may just be sunburn. Had the same thing happen to mine - and I, too, thought it was diseased. But don't discount the advice from David on the soil.

Check out this thread.

Here is a link that might be useful: C. perforata sunburn