Certified GKH Gardening Expert
This is one of many common fungal infection. Identifying them will take a bit of work, since they all exhibit similar symptoms. Usually, though, a broad spectrum fungicide will take care of this. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
Some experts say there are two secret ingredients to add to your soil, at least once per year.
DOLOMITIC LIME and WETTABLE SULFUR will keep most issues at bay, while conditioning your soil to handle extended wet periods. These can be obtained very inexpensively at many gardening centers or farming equipment stores.
Just apply them at least once per year or more if the need should arise.