Fungal diseases are often associated with intermittent rains and mild weather and the long hot days of late summer don’t seem ideal for fungal growth, but they are perfect for the powdery mildews. This is a large collection of fungi that infect leaves and produce spores in a powdery looking mass (photo shown here is powdery mildew on zinnia, also a problem at this time of year). The fungi may be able to grow on a wide variety of plants, but some occur only on specific plants.
In flowering dogwood, these fungi are often noticed in late summer and early fall, as they can cause distortion and reddening of the leaves. The powdery growth is less obvious than on some plants, but can be seen in the areas of reddish tissue when examined closely.
Powdery mildews that occur at this time of year will not do much damage to landscape plants such as dogwood, since the plants will be losing their leaves in a few months anyway. For commercial nurseries where the disease may develop earlier, intervention is desirable.
Visit the PNW Plant Disease Management Handbook to learn more.