I scrub old clay pots with a soap/chlorine bleach (Clorox Clean-up), let them dry out in the sun, which evaporates the chlorine then dunk them in a Physan solution 1/2tsp. per gallon before using them to pot any orchids again. I like to be careful about combining chemicals in case the one cancels out the other, or something . ..
Can't tell if it's helping against the black rot just yet, but it certainly helps keep the pots from growing green algae on the outsides.
Trouble with living in Florida is that every fungus spore known to man is floating around in the lovely warm, humid air, ready to land on the leaves of your plants. I also sprayed my whole pool cage with Physan after it was power-washed last September. I do think that slowed down the incidence of black rot starting. I didn't have any new cases after the cleaning/spraying. Just had to battle with the ones already infected.
