Thank you all for the information. Keeping the silicone sealant dry seems to
be important.
I am not sure why the silicone sealant used in window frames never seems to
get mouldy: it is often very wet for long periods in rainy weather.

Steve, your suggestion to use materials other than silicone sealant is
interesting. If the choice was between a yellowing seal or a mouldy seal, I
should be happy with the yellowing one.
What do the phrases "does not support the growth of mould" and "resistant
to moulds" mean? Do they mean you will never see mould on such surfaces?

Regards
Sal.