Schlumbergera bridgesii disease or pest

[Sova1970]

Hello Cacti Lovers,

In the past three months, my Christmas cactus (Schlumbergera bridgesii) has developed a disease or is being attacked by a pest. The damage occurs only on the underside of the leaf segments, which then fall off. I have attached a photograph of the disease/pest, in the hope that somebody can identify the problem and suggest a suitable treatment.

I shall thank everyone in advance for any advice.