Phal Bellina and black spots

[Fredmax]

Hi, I have 3 small bellina's and they all seem to exclusively develop black spots that my other Phal species (basitani, deliciosa, parishii, stuartianii) don't encounter. One leaf was lost 12 months back but the plant recovered though seems an ongoing issue. Violacea has some wet spot issues but no black spots.

I'm assuming they're fungus related so am doing my best with hygiene and circulation, but right now am trying to determine the best treatment. I've tried mancozeb but not convinced it's helped, next is a Physan equivalent or Phosacid.

Alternatively I'm unsure if it's a result of cooler conditions than it appreciates (12-13c) that's exposing it to fungus.

Has anyone encountered this on this specific species and an effective treatment, or is it just my batch.

Cheers
Fred