I had chaulkbrood in most of my colonies last year.
This year I have very little.  Weather clearly plays a large role, as does
old comb, genetics and varroa.  I read recently (can't remember where) that
chaulkbrood is more likely when mite loads are starting to strain a colony.
 Marla Spivak wrote an article suggesting using chaulkbrood as a breeding
criterion since bees bred to be resistant to chaulkbrood will also be
resistant to AFB. (http://www.orsba.org/htdocs/download/spivak.pdf) So you
can view this situation as an opportunity instead of a problem!

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