Cleaning is worth a try
I asked the identical question at the site this winter and got several good suggestions.First I removed the covers from the cushions. Then I left the cushions out on our deck for several days to let the sun work on the black spots and current mildew, if any. Then I sprayed the cushions with Lysol and let them set again. After that I let them set out again for several more days and then rinsed two of the smaller cushions with a bleach solution. These two took a heck of a long time to dry out, but I wanted to find out if rinsing them would get rid of the black spots. It did somewhat. I decided not to do the larger cushions because of the dryout time and then put the covers on all the cushions and treated them with over the counter upholstery foam cleaner. I used Bissel and Woolite. Both worked well, cleaned the cushions, and I didn't see any difference in the quality of the products. One of the cans had a built-in brush on top of it. That was handy. The cushions look good and smell good now. My wife warned me against even soaking the covers because of the material, so washing them was out of the question. I was also told not to do this on this site. Before spending $500-700 on new cushions, I would recommend trying this unless the covering is worn out or you just want to change the upholstery. I had maybe $10 in upholstery cleaner and Lysol. That's my 2 cents worth.Tom