I have never used fungicides on my
Disa tripetaloides.
I do have issues with leaf blemishes. But I found out why they do that.
It's not fungus. If there's fungus, it's a secondary cause of stress for the plant.
The primary cause would be severe root damage.
I originally planted these in a SuperSphag and perlite mix.
Then I found out that this potting mix went bad very quickly because of how wet they grow. So when I repotted them, I found out that many of the roots were rotting.
Then...
I also checked out how they grow in the wild. It took me a while to realize, that certain parts of South Africa are sandy. This is most consistent with what I see in pictures of the habitats of
Disa tripetaloides and
Disa aurata (they're apparently sometimes found growing side-by-side).
I switched out to "horticultural sand", and I no longer have problems with leaf blemishes with the newer offshoots.
The problem with Disas is that their root systems are fairly sparse. Then there's the issue of each root not taking damage well, unlike the epiphytical orchids. So if you snap one (even if it's a small portion of the root), that's all she wrote for that particular root. The roots of Disas are also pretty rigid and brittle, it can be easy to mishandle and snap off.
However...
They make up for this shortcoming by being extremely fast growing.
I'd like to also mention, Disas generally don't live long lives like the epiphytical orchids do.
Once the plant blooms, it dies.
Again, however...
The evergreen Disas make up for this shortcoming by being one of the easiest orchids to germinate from seed!
As you can guess, the primary mode of reproduction for Disas is not asexual reproduction, it's sexual reproduction. Although, asexually, they're pretty prolific, sexual reproduction is the most efficient method of proliferation.
If you're keeping fish in there, that's even better. Then there'd really be very little need to fertilize.
Disas don't like heavy fertilization. They can get leaf blemishes or die due to root damage as a result of too much fertilization.
Hard water is a big no-no. The lower the concentration of dissolved minerals in the water the better.