Hello! Thank you for your question and for including the photos.

Yes, it's quite possible that overwatering is contributing to the problem. Veronica is sensitive to overly wet soils, and excess moisture can lead to root rot and other fungal diseases.

I recommend reducing or stopping supplemental watering for now and allowing the soil to dry slightly between waterings. Prune out and discard any brown or dead leaves and stems, and clean up any fallen plant debris around the base of the plant. This will help improve air circulation and reduce the amount of fungal spores that can overwinter and reinfect the plant.

Based on the symptoms, I suspect a fungal disease, which is often made worse by prolonged wet conditions. You can apply a fungicide labeled for ornamental plants, but keep in mind that fungicides do not cure existing infections. Instead, they help protect healthy new growth from becoming infected. Fungicides are most effective when applied preventively in spring, before symptoms develop.

A copper-based fungicide may provide adequate protection. This University of Minnesota Extension resource explains when and how to use fungicides effectively. Although the article focuses on rust diseases, the guidance on proper fungicide use applies to many fungal diseases: https://extension.umn.edu/plant-diseases/rust-flower-garden#using-pesticides-1169012

I hope this helps! Please let us know if you have any other questions.

-Victoria

University of Minnesota Extension Master Gardener Volunteer