This was floated in Australia as the be all and end all of stopping
chalkbrood.  It was based on a medical paper where bananas were used to help
stop some forms of ulcers as I recall.  However, the beekeepers did not
realise that ulcers are caused by bacteria and chalkbrood by a fungi.

Some beekeepers started using bananas cut in half and placed on the top bars
above the brood area.  The skin and all was used.  The theory was that it
was the potassium in the banana that was doing the trick.  There had been
stories of using lite salt to stop chalkbrood i.e. salt that has the sodium
replaced by the potassium, and this is where the potassium idea came from.

After several seasons, the reports are that the hives are still getting
chalkbrood just as bad.  So it seems that there is no need to rush out and
buy a banana plantation.

Trevor Weatherhead
AUSTRALIA