odd trunk floor deterioration over spare tire

[coupe74]

When checking the spare tire (which was near flat), I found a tire-sized-and-patterned ring of mold (appears to be mildew - dry and powdery) and deterioration in the plywood floor above the spare.
I found nothing on this in the archives.

Its a '74 CS, always dry-barn-stored with a car-cover too (I think the local barn-spiders s*** epoxy), no leaks in the barn. Never driven in the rain or on wet roads, stored with trunk slightly open to allow electrical cord to the trunk-mounted battery trickle charger. There is a rubber-backed fitted carpet in the trunk above the elephant-skin-covered plywood floor. No evidence of mold or anything in the rest of the trunk, or anywhere else in the car, or below the tire. Only above the tire.

WTF?

 

[Stevehose]

was the tire moldly? Perhaps a rubber induced mildew

 

[adawil2002]

Moisture in the air will condense on cool metal surfaces during the summer, just like a cold drink on a humid day. I have a a plate joiner for carpentry work in my unheated carriage house that has never seen rain yet it has rusted inside the small barn from ambient humidity.

 

[Gazz]

Organic compounds in the rubber?