An indication of the presence of fungus (e.g., in a sample of grain), when light of an appropriate wavelength is shone on sample. For example, when the fungus Aspergillus flavus infects cottonseed during boll development on the cotton plant, the resultant seed (when harvested) shows BGYF on its lint and linters. That fungus gains entry into the bolls typically via holes made by the pink bollworm (Pectinophora gossypiella).