Certified GKH Gardening Expert
This appears to be a fungal infection. I can see signs of it on all of the plants in that picture.
You will need to treat with a fungicide. This article will help:
Make sure to allow the soil to dry out down to about 1 inch between watering. Any drier will be bad, as well as leaving the soil wet. You may find that the moisture needs of this plant will not be the same as the surrounding plants. They will likely stay infected, which will keep the Brunnera in a diseased state.
Here is an article that will help you to measure soil moisture:
Here is an article that will help you with the care of the plant: