Elisabeth, while in my friends greenhouse last week, I saw her douse the base of 1 of her plants with a spray bottle of rubbing alcohol... said she'd spotted mold, & the alcohol would kill it off! I've never heard of that before, but I figure she must know what she's doing, as SHE'S the 1 with the greenhouse full of lovely, healthy plants! (I won't say that I'm endorsing her method... just telling you what I witnessed! Considering the fact that mold spores DO go airborne, I will say that there doesn't seem to be a problem with any of the other plants in there... & when I was in her greenhouse again today, the plant she'd zapped with alcohol looked as healthy & happy as the rest!) Go figure!

Myself, I would use $$$$, because it has worked for me on mold/mildew & black spot on 1 of my Rose bushes. I don't know HOW it works... it just does! (& it's QUICK!) Before I discovered $$$$, the way I'd handled sick plants was to totally uproot the plant, strip the old soil from the roots & give them a good rinse, then re-pot the affected plant in brand new soil & a new (or scrubbed/disinfected) container, after powdering the roots with Root tone. (I've never lost a plant by doing this, but have root-shocked a few that took a while to bounce back, cuz it's a pretty extreme method!)

I just now, happened to read your posts from Sept. ("prune hibiscus?") & it sounds like your poor plant has been in distress for quite some time! For this reason, I'm guessing that it may not have the energy to survive a radical replant just now, so I DO NOT recommend uprooting it! I DO, however, strongly suggest that you look into $$$$, as I think it may be your only hope at this point. (Unless you want to try the alcohol thing!) I'll PM you with details about $$$$, & I wish your Hibiscus the best of luck.

(Meanwhile, DO water it! Plants can recover from all sorts of diseases, & the mold spores are already in the soil... & aren't going to go away before your Hibiscus dies of thirst!)

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About the cornmeal: Earlier this year when I was having trouble with a fungus on the leaves I was advised to make a cornmeal tea, spray the entire tree and then add the remaining cornmeal to the soil as it was a fertilizer and fungicide. I did so and the fungus disappeared in no time at all and the tree immediately began to replenish the leaves it had lost and sprout blooms like crazy.
About size: the Hibiscus is about 4 ft. tall and still in the pot it came in - 10".
About current trouble with mold: this began in September while we were away and I left my neighbour in charge. Unsure whether or not she would give it the attention it needed I pulled it inside and placed the pot and part of the trunk in a plastic bag (a make-shift terrarium idea) to help keep the soil from drying out between her visits, only she was admitted to the hospital and the bag never opened.... I returned to mind the plant, soil and pot covered in mold and the plant in a state of deep shock. Only one of the braids has recovered though a few weeks ago when I finally trimmed the other two back there was some green in the branches close to the trunk so I'm hoping... More details on this stage of the drama are in my post ("prune hibiscus?" from September 24)
And this morning when i watered it I noticed there are some tiny fly like things hanging around at the base of the trunk where I have the collar (just a section of a pop bottle to keep the trunk dry and hopefully keep the mold off of it)