Certified GKH Gardening Expert
Unfortunately, it looks to have sustained an infection from the soil remaining too wet for too long without a period to dry out very thoroughly down to about 3 inches.
Clay soil is very deceptive. the top half inch can be bone dry, while just below is saturated. Unless that plot is amended before the next planting, it will be good to get a moisture meter. This way you can be sure that it is dry enough to water.
For now, I would treat with a fungicide. At the end of the season, I would amend that plot with organic material.
Here are some articles that will help: