Quantifying arbuscular mycorrhizal fungi in soil is difficult and each approach has its advantages and disadvantages. While use of spore counts and hyphal lengths to quantify AM fungi has received due criticism, bioassays have been used uncritically.
Bioassays represent the interaction between plant and fungus in the field.
We examined the timing of harvest. Data from a series of experiments using a single host plant grown under similar conditions and harvested weekly for 8 weeks were examined. Where mean colonisation at 8 weeks was low, colonisation at 3 weeks was also low but extremely variable. Mean colonisation need not indicate densities of AM fungi. Where propagules are being quantified, the timing of harvests requires carful consideration.