Chalkbrood is a fungal disease that has no approved treatment, Apiguard has no effect on the fungus.
Chalkbrood usually does not kill a colony, but does weaken the adult population. Chalkbrood is usually associated with cold and moisture, the brood is chilled and the fungus infects the larvae. If the disease persists when the weather warms re-queening is often used as a treatment and is usually effective.
Do your inspection of the lower box on a warm day with little wind. If there are few adult bees in the box, and the comb has mummies in the cell, remove the comb. Tap the frames over a trash can to catch the mummies and remove what you can. Spray the comb with a water/bleach mixture and store it away for a week until the odor of the bleach leaves and they can be put back in service.
If the hive is not on a stand put it on one that is 6 to 8 inches off the ground, and in full sun if possible.
42 + years - 24 colonies - IPM disciple - Naturally Skeptic