If Physan 20 is supposed to be toxic to all fungi, then I would suspect that it would be detrimental to the mycorrhizae as well. I'm pretty confident that fungicide manufacturers do not consider possible effects on mycorrhizae when they are developing and testing their products.
I am not a scientist as LariAnn is, but I would suggest you google info on fungicides that are compatible with beneficial microorganisms such as the Trichodermas and mycorrhizae (I've seen lists on the Plant Health Care sites here in Mexico). Using these beneficials as preventatives as LariAnn suggests, is good practice and if you already have an infection in your your plants, find out what pathogen you're treating (gardening and orchid forums often can help out or your local nursery or biology dept. in a local university) and use a specific fungicide that won't affect the beneficials. I would suspect as LariAnn, that wide spectrum fungicides such as Pysan 20 and Benlate are deadly to these funghi.