As I’m doing this week’s reading for my master gardener class, I think I figured out what’s wrong with our dogwood tree. I’ll have to wait until the leaves are in bloom and see if it comes back, but we’ve had dead areas on the leaves ever since we bought it and transplanted it two summers ago.
I think it’s dogwood anthracnose, which was first noticed in the U.S. in mid-70s. We don’t know where the disease pathogen came from, but we don know that it’s not found in any other country so far.
From the Cornell Plant Disease Diagnosis Clinic:
Shortly after the leaves have expanded (mid-late May and June), spots and blotches of varying shape and size appear on infected trees. These spots have a tan center and a purple or reddish margin. On the opposite side of the leaf, tiny brown or black spots may appear beneath these lesions. The flower “petals” or bracts are also susceptible and show reddish or brownish blotches. Blotches can also occur at the tip or along the margin of leaves; these too have a tan center and purplish margin. In some cases, entire leaves may become infected and die. Many drooping, brown, dried leaves remain on the stem throughout the fall and winter.
The best strategy is cultural–buy a disease resistant variety. Again, from Cornell:
To avoid or lower the risk of dealing with this disease, consider using resistant varieties. The white flowering Kousa Dogwoods, Cornus kousa have shown good resistance and require less input to maintain a healthy tree. A number of crosses between C. kousa and C. florida have been made in attempts to produce the flowering characteristics of the Flowering Dogwood with the resistance of the Kousa Dogwood. These cultivars are available on the market and are known as the ‘Stellar’ Hybrid series, ‘Aurora’, ‘Celestial’, ‘Constellation’, Ruth Ellen’, ‘Stardust’, and ‘Stellar Pink’. A resistant Flowering Dogwood cultivar named ‘Appalachian Spring’ has also been developed from a living tree in an otherwise devastated Maryland forest and may soon be available.
There’s not a whole lot you can do once your dogwoods have anthracnose other than deal with it or use fungicides. But since anthracnose can eventually kill the dogwood tree, chemicals might be the way to go. The USDA Forest Service has some recommendations for chemical controls if you have to. This summer I’ll be on the lookout for it…I hope we don’t have to use chemicals, but….I also don’t want it to die.
Photo 1 courtesy of USDA Forest Service; Photo 2 from the Ohio State University Extension service