I had an apple pie in an aluminum pan wrapped with aluminum foil. 
After some neglect the pie turned a deep, powdery black with a mold or
fungus.  There was a whitish fuzz towards the edges of the black.  The
curiosity is that the aluminum foil had been "eaten" through in
places.  What caused the foil to disaapear, eaten by mold, or some
other interaction?

You have discovered something scientists have been expecting to
happen:  the mold that develops on good American Apple Pie (and I am
sure yours is good) has finally mutated to eat aluminum foil.  This
has taken quite a long time (Reynolds starting selling foil in 1947),
when one considers how quickly viruses can mutate, but this could be
explained by the fact that most pies never survive long enough to even
see alumninum foil, let alone to let the mold develop a taste for it. 
Just this once, you should be complimented for leaving an old moldy
piece of pie around for so long.  ;-)

No, seriously (and the above isn't, and this is only a free comment,
not an answer), I expect that it has something to do with acid
developing, either through fermentation and/or in combination with the
mold.

http://www.scripophily.net/reymetcom.html

Myoarin

My first thoughts are similar to myoarin-ga's (not that the mold has
mutated to digest foil) that there has been accumulation of acids,
presumably through fermentation of the apple pie. The acids produced
have oxidzed the foil, leaving you with aluminum oxide. You'd probably
find the same thing is you covered spagetti sauce or cola with foil.

There are many molds, fungus and bacteria that are capable of
producing acidic by products under the right conditions. Many of these
produce the worst affects in anearobic conditions i.e. without access
to air. These are more than capable of eating through aluminium and
can also eat through stainless steels.
In your particular case there may be a number of other possibilities.
Aluminium is corrosion resistant due to its oxide coating. If you
scratch the oxide coating and then prevent it from reforming i.e. put
a hot pie on top then the aluminium can corrode fairly rapidly and
quickly. This can typically happen as a result of contact with the
fruit acids in the apple pie. Once the reaction starts it can then
accelerate. Small pitting will lead to concentrated areas of corrision
and in these pits the acidity will increase without access to oxygen
or any circulation. This will further increase the rate of corrosion.

Crevice Corrosion is another well known phenomena and starts at some
defect or scratch on the service.

These are just a few of the possibilities but all of them can result
on the corrosion of the aluminium. If you really want to pursue this
further we can look at the type of corrosion that you have and
probably determine the exact cause. If you want more details
www.bayworth.com