Technical Abstract: Predisposing conditions incite chalkbrood disease (Ascosphaera apis) in honey bee colonies either by cooling or warming brood and correspond to an abundance of mummies where temperatures fluctuate widely. This Chapter describes a novel technique for estimating the temperature where the incidence of chalkbrood is the highest, which may vary according to species and different colonies. In the laboratory, growth rates for pure fungal isolates of A. apis were determined by measuring the spread of the mycelium in a Petri dish at different temperatures. Graphical analysis shows a temperature where a rapid increase in growth rate occurs, defined as critical transition temperature (CTT). Of interest is CTT of A. apis matches the temperatue where chalkbrood symptoms are seen in a bee colony. Thus, determination of CTT of the fungus can be an effective tool for gauging the onset and offset of a fungal disease.