Certified GKH Gardening Expert
Yes, this will be a fungal infection. Each time you transplant a cutting from the plant, the soil will become infected.
This usually happens from overwatering, staying moist for too long, and no drainage in the container.
Soil should be allowed to dry out between waterings AT LEAST down to two or three inches.
Now, on to treatment... This will take a good handful of DOLOMITIC lime and a tablespoon or so of WETTABLE sulfur added to the soil, in tandem.
It will really help to change the soil BEFORE this treatment, but if this is not an option the treatment will still work. It will take much longer, but it will work.