Heat and humidity.
If you store your car in a garage or something it'll likely be fine. That extra layer of weather protection will probably do the job mostly.
But if your car is kept outside, no chance.
If you want to be able to keep games in your car, make sure the rubber strip lining your trunk is in good shape, and then get a moisture absorber of some kind to keep in there.
You can also get dry bag duffle that you can seal up to be airtight.
But neither of these things will protect it from the heat, so ymmv.
Either way, humidity is a much bigger problem.
My sister used to have these damp rid moisture absorbers and they seemed to help with humidity https://www.amazon.com/DampRid-FG100-Unscented-Disposable-10-5-Ounce/dp/B000LM0WIM/ref=asc_df_B000LM0WIM/?tag=hyprod-20&linkCode=df0&hvadid=198129187858&hvpos=&hvnetw=g&hvrand=7928342958651652881&hvpone=&hvptwo=&hvq...
I heard that that de-humidifiers only help with the moisture but not the actual odor problem. Is that true? Like I said, our basement feels dry, but I don’t know if that’s still the normal protocol (this is our first time ever living in a home with a basement). I actually just bought this though like an hour ago:
It had good reviews and some people said it solved their musty basement problems…so I’m gonna give it a shot. Though honestly, I probably should’ve gotten a bigger one considering our basement is like 300 sq. ft lol. But hey I guess we’ll see…
Looks like mold. It used to be a problem with my closet and all my leather shoes were covered with it. You can wipe it down with any alcohol based cleaners then let it sit in the sun. UV will kill any on surface. Then clean and thoroughly air dry the area the shoes were kept. If you have dampness, keep a canister of desiccant around.