Josh Hancock pitched eight innings to earn the victory for the Reds, who have won five of their last seven. Hancock (5-2) permitted three runs on seven hits.
The Cubs, who have lost three of their last four, began play one game ahead of San Francisco and 1 1/2 games ahead of Houston in the NL wild card standings.
Dunn's homer was his 44th of the season.
Greg Maddux (15-11), the Chicago starter, absorbed the loss after being charged with six runs on six hits in five innings.
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