Certified GKH Gardening Expert
Yes, unfortunately. Overwatering has left the soil saturated and lacking oxygen, thus, infection has taken hold.
Treating with a fungicide, and solarizing for the rest of the season and through winter until next growing season will help restore the soil. Before solarizing, make sure to test your pH and nutrient content, making note of any deficiencies and swings in pH. Add organic material and compost and then solarize.
Here are some articles that will help: