Anything that will 'hold' the moisture will rot the wood. I try to have ANY wood that touches the ground or concrete be Pressure treated.
IF the wood is already there. Plastic or NONE the wood will rot at the same speed. Best solution if you have some good penetrating wood sealer (like thompsons, though I hate that brand) apply it liberaly to the wood the concrete will be touching. I wont hurt the concrete either.
I would do that or NOTHING.
IMHO.
