Chalk brood can occur during warm dry conditions too like happened to me in summer.
The warmest and driest period of the year precipitated an outbreak. Certainly I had an underpopulation issue and when I happened to find Michael Bush's advice (as per his link above) it really helped control it.
The Australian Beekeeping Guide by Goodman and Kaczynski (page 92), suggest, (following them noting that it can occur in warm dry periods), that, "poor conditions such as limited supplies of nectar and pollen may be a factor. Moving hives to good conditions or feeding colonies sugar syrup and protein supplements may help recovery. While high protein pollen is preferred, any pollen is better than none at all. Ideally the bees should have access to pollen from several plant species for a balanced nutritional diet."