George schreef
As I mentioned, acorns in the gutter (some of which must have been
under the sitting rain water) did not produce the gray mold. All I
remember seeing is black or dark brown - decaying stuff.

Is it possible that bacteria in the decaying stuff prevented or ate
the mold? My indoors experiment (with just tap water) may not have had
that bacteria.

+ + +
Relationships between fungi and bacteria in decaying material are quite
complex. Very many fungi and bacteria exist, and several strategies exist.
In general it is a matter of first arrived, first served.

A major factor is the presence of oxygen. This is determined by such
questions as submersion (partly submerged, permanently submerged, etc) and
whether the water is in motion.

In theory a book could be written about each such acorn, but research
focuses on economically more important issues
PvR