A washing machine appliance includes a pretreatment assembly positioned proximate a top panel of the washing machine appliance for spot treating articles of clothing prior to a wash cycle. The pretreatment assembly includes a detergent reservoir defining an aperture and being configured for receiving a fluid additive. A ball socket is positioned at least partially within the aperture and a roller ball applicator is rotatably mounted within the ball socket. The roller ball applicator is at least partially in contact with the fluid additive within the detergent reservoir such that a user may rub an article of clothing on the roller ball applicator to transfer fluid additive from the detergent reservoir to the article of clothing.