Thanks for the reply Blane.
I am sure that in hives with hygienic bees the number of chalkbrood spores
will be much less than in the average hive.  However this may mean that there
are only millions of spores instead of billions.  From my reading I
understand that it does not take many spores to invade a larva successfully.
It is a very attractive hypothesis that hygienic bees are the solution to all
problems but I remain to be convinced that they are the complete answer to
chalkbrood.  My mind is still open.  Perhaps now that the economic importance
of chalkbrood is starting to be recognised somebody will do some proper
research.
Chris Slade