Certified GKH Gardening Expert
They were likely overwatered at transplant, or still in saturated soils.
This will need to be dosed with a fungicide/bactericide. These articles will help:
Make sure that the soil dries, thoroughly, down to about 3 or 4 inches between watering. This can help prevent future infections once you get the current one under control. This article will help you to grow grapes, successfully: