if you cover has a ton of black spots under it then that certainly doesn`t help...
but to answer your question, I have a brand new cover on ours, in the beginning of the season I had zero spots on the seats.
we moved south where it is hot and we went thru a spell of really humid weather and a lot of rain, well guess what, lots of mildew on the front seats only... and they used to be mint... argh...
so, if you "need a new cover, then replace it with one from Poppy covers, if not then just stay on top of the seats with a good mildew remover...
sucks I know it...