Fungal growth visible around the ramjets of a B-1B Hussar.
Because air flows past the jet faster than the speed of sound, any mold spores present on the craft will be super-enriched by the oncoming airflow, which at high altitudes is over 90% Oxygen because the heavier Nitrogen atoms fall to lower sections of the atmosphere.
The oxygen enriched fungus can grow at spectacular and dangerous speeds, up to one meter per hour. Needless to say this is a risk as the fungus can clog the ramjets and lead to stalling, or even an explosive blast if a mushroom should form and hit one of the ramjet’s many fragile spinning blades. Anti-fungal paint and regular cleanings help prevent such disasters, but the real line of defense is the Fungal Obliteration Damage search before flights, or “FOD Walk” in which crew will manually inspect each jet for signs of mold.
The practice was introduced after the destruction of the Hindenburg Zeppelin, which is suspected to have caught slime mold in its ventral ramjets. So outraged at this event was the scientific community that slime molds were even robbed of their fungal status and reclassified as Myxogastria, latin for “The mold which shall not be named”.