Copper Sulphate (CuSO4) Solution as a Fungiside ?

[Ray]

Yes, copper sulfate can be used as a systemic and topical fungicide and bactericide, but there are some dendrobiums and thin-leaved orchids that find it to be toxic. (It damaged a coelogyne of mine once.)

For what it's worth, Phyton 27 is a copper sulfate product, with the equivalent of 200g CuSO4-5H2O per liter of water, and that is used by further diluting it to a 0.5%-2% solution.

Back to your original query - water, by itself, is not an issue, and I have found that plants in potting media that are very water-retentive do OK as long as it stays warm and rains a lot. Over the last two months, we have averaged over 1" a day of rain, and my phalaenopsis in sphagnum are doing great.

That, of course, has no bearing upon your outcome, but I think I'd look at changing the potting medium or protecting the plants from the rain before I'd resort to preventive fungicide use.