Some workers advocate using single spores rather than a batch of spores to start a monospecific culture. Spores from an active culture are much easier to work with than those from a stored culture. I started from spores to establish a monospecific culture, and the culture remained pure for three culture cycles. Suddenly, I found spores of another species present after I had taken extra precautions to prevent contamination.
Some cultures just stop sporulating after several propagation cycles.
If the contaminant species matches a fungus in our records, the problem is ours and we will immediately correct it. If it doesn't, then you need to check your culturing conditions. Thus, two or more fungi can be colonizing a root system, but only one is a consistent sporulator in one environment. When this environment changes, the other fungi present in low mycorrhizal biomass may be favored, spread more, and then sporulate. Cultures grown in Florida never produced spores of these contaminant fungi.