Certified GKH Gardening Expert
It is likely the plants are being affected by aphids. They are very small insects that suck the sap out of the plant. The ants use aphids like cows, and will "milk" them for the sweet honeydew they excrete. The ants will set up a colony near the aphid population to make the trip to care for them shorter.
This article will help with the aphids:
Once the aphids are gone, this article will help with the ants: