The leaves on my red-twigged dogwoods are spotted and beginning to fall off. Why?
The spotting and premature leaf drop on your dogwoods is probably due to Septoria leaf spot. Septoria leaf spot is a common fungal disease of dogwoods in Iowa. The disease typically causes irregularly shaped grayish spots with dark purple or reddish borders. Heavily infected leaves drop prematurely. Wet spring weather favors Septoria leaf spot development.
Fortunately, Septoria leaf spot does not kill dogwoods. The damage is mainly aesthetic. The heavily spotted foliage and partially defoliated dogwoods are not very attractive.
The severity of Septoria leaf spot in future years can be reduced by raking up and disposing of the infected leaves on the ground. It is possible to control Septoria leaf spot with 3 or 4 fungicide applications in spring. However, most home gardeners choose not to apply fungicides as the damage is mainly aesthetic.