Hi, I have two ferns in large containers that I inherited with my house a few years ago. I'm sure that they are Dryopteris, and look possibly like Filix-Max from comparing them with photos.

From coming up over the last 3 months, one of them has
i) developed a black growth on the underside of the fronds
ii) collapsed in structure, so that all the fronds are lying compltely down. I have always kept them moist, so its not lack of water.

Is this somethign that I can treat, shoudl I cut it back and leave it for this year, or should I pull it out completely for fear of contamination with other plants? N.b. it is in an individual container so there's no danger of transmission through the soil