Dogwoods are susceptible to a number of fungal leaf spot diseases. Red and yellow twigged dogwoods are particularly susceptible. Most of the dogwood samples arriving in the Plant Disease Clinic have been showing Septoria leaf spot. This disease typically causes angular, grayish spots with a dark purple or reddish border. Symptoms usually occur first on lower leaves. As with many of the fungal leaf spot diseases, wet spring weather favors their development.
Diseased leaves should be raked and removed to prevent overwintering of the fungus. Prune out any dead branches. Improve air circulation around the foliage if possible. Mulch and water as needed to improve plant vigor. Fungicides (such as Daconil 2787 or a product containing maneb, mancozeb, or captan) can be used to protect the leaves in the spring. Sprays are applied just before flower bracts are fully expanded. In wet years, repeat 2 or 3 more times, 10 to 14 days apart.
This article originally appeared in the July 21, 1995 issue, p. 110.