Chalkbrood back again after a fortnight of Perthshire drizzle — beyond ventilation, what actually moves the needle?

Seven years in Perthshire, six hives. One colony throws chalkbrood mummies every time we get a cool damp spell — and we've just had two solid weeks of it. White and grey mummies on the floor and a few on the landing board again, maybe a dozen a day. Same hive as the flare-up in May; the other five on the same stand are clean. I've done the standard things: mesh floor is open, the stand has the hive tilted slightly forward so rain doesn't pool inside, entrance is full-width for airflow, and the site gets morning sun. It helps — flare-ups fade when the weather dries — but this colony clearly carries the problem while its neighbours don't. Which points at genetics, right? Ascosphaera is everywhere; susceptibility is the variable. Is requeening toward hygienic stock the actual fix here, and has anyone seen a chronic chalkbrood colony genuinely turn the corner from a queen change alone?