Could be the mold spores got onto your wood in foreign ports and have slowly grown to a visible size. The difference in origin could explain why the wood construction of your garage is not affected.

The usual treatment for mildew is to use a solution of a couple of tablespoons of borax in 1/2 gallon of water. Unlike bleach it should not affect the color of the wood.

If you use chlorine bleach then you should dilute it. Household laundry bleach does it's work on your clothes when used at a rate of about a cup of bleach to 20 gallons of water. If you are only using a gallon of water to clean your plywood, then you only need to use an ounce or two of the bleach.

With either solution, swab it on freely, let it soak a few minutes, and while the wood is damp use a brush or nylon pot scrubber to dislodge any loose scum. let itRinse with some more of the cleaning solution and scrub some more. When it is as clean as you can get it, rinse thoroughly with clean water and let the wood dry in bright sunlight.

Either way, after treating the wood and letting it dry a light sanding will flatten any swollen grain and give you a good surface for epoxy.

hope this helps.

PGJ