I will let the experts pop in and give you an opinion but if it were my plant, I wouldn't be all that concerned by that small amount of whatever it is. Truthfully having plants just go through normal everyday living stress they get little blemishes...I'd say it could have been as simple as a sun scald maybe there was water ...or I've created that sort of thing from sprinkling fertilizer on my plants.

Thank you for your help. I went out yesterday and got some fungicide... it has been pretty humid lately. The spots haven't gotten worse, so I feel better.

Your white spots are just sun scald. No problem. You'll find several discussions about it over on the Growing Tomatoes forum linked at the top of this forum's front page.

Dave