Unfortunately, this isn't an issue you can ignore. If the case is mild, there's a slight chance that it might resolve itself on its own. However, it's best to intervene as quickly as possible. A small infection of powdery mildew can quickly manifest into a bigger one, which can ultimately ruin an entire crop of tomatoes.  Of course, you want to be smart with your time and efforts. This is a condition that usually affects plants later in the season. If you've already harvested a good number of tomatoes and you're satisfied with your crop, it might be more effective to focus your attention on preventing powdery mildew from occurring next year. Collect all of the diseased leaves, and cut the affected stems back. This way, the fungus is effectively removed from your garden and will not remain in the soil over winter. Dispose of all the landscape waste in a paper bag and do not mix it with the rest of your compost.