I ask this question because one of the top killers of my orchids is fungus. I've already lost countless plants to fungus over the years, and I'm sick of it. I haven't tried anything because I'm not sure what I can use on my plants.
Trust me - It isn't easy keeping orchids fungus-free when it rains frequently and the infections come from out of nowhere.
Secondly, I suspect your problems are due to something you're doing in cultivation that you may not be aware of.
Third, all the fungicides in the world won't help you get rid of fungal problems. Fungus is everywhere.
Here's a bio/history lesson. Fungus appeared on land prior to plants leaving the oceans. Fungus were the ones that helped make land hospitable for plants. As a result plants and fungus have evolved side-by-side.
So let's try to fix the problem without the big guns first.