Dogwood Diseases

University of Rhode Island GreenShare Factsheets

Dogwood Diseases

Our native flowering dogwood, Cornus florida, is frequently beseiged with dogwood anthracnose and powdery mildew. Both of these fungi can sometimes be controlled with multiple and well-timed fungicides on otherwise unstressed trees. Frequent fungicide, water and fertilizer applications, however, are laborious, expensive and represent potential hazards to the environment.

To reduce resource inputs and promote sustainability in the landscape, we recommend planting one of the dogwood species or cultivars that is resistant to these diseases. The dogwood species currently most resistant to anthracnose and powdery mildew is Cornus kousa, the kousa, or Korean, flowering dogwood. The kousa dogwood flowers a bit later than C. florida, but otherwise fits the same niche in the landscape as C. florida. The kousa dogwood offers showy (mostly white but some pink) flower colors and a beautiful scarlet to purple fall leaf color. The red and edible fruits, borne on long pendulous stalks, and the mottled, exfoliating bark, are additional attractions of the kousa. Although the species is generally anthracnose-resistant regardless of cultivar, there a few susceptible cultivars to avoid: C. k. 'Autumn Rose,' C.k. 'Moonbeam' and C.k. 'Wolf Eyes.'

Attempts to combine the ornamental characteristics of C. florida with the resistance traits of the kousa dogwood have led to several florida/kousa hybrids. Resistance studies of florida/kousa hybrids are still continuing, and results seem quite dependent on microclimate (particularly light and humidity). However, the best indications at this time give us four hybrids that are relatively resistant to anthracnose: C. x. 'Auruora,' C.x. 'Celestial,' C.x. 'Stardust' and C.x. 'Stellar Pink.' With the exception of 'Stardust,' these hybrids are also resistant to powdery mildew. Beautiful in flower, intermediate in timing between their C. kousa and C. florida parents, these hybrids are sterile and do not produce fruit.

The University of Rhode Island is working closely with nurseries to increase the availability of these resistant cultivars,--in the meantime, they can all be obtained from Shadow Nursery in Tennessee, with the exception of 'Stardust.' 'Aurora' is known for its extremely dense flowering that at is peak almost completely hides the foliage. 'Celestial' has sparkling white flowers that form a deep cup at first and a more erect habit with uniform spread. 'Stellar Pink' is known for its pale pink flowers with deeper pink veining and its wide, rounded habit.

The resistance among cultivars of C. florida are dependent on microclimate. Some research indicated that C.f. 'Cherokee Red' and C.f. 'Cherokee Sunset' are resistant to anthracnose ('Cherokee Sunset,' however, is susceptible to a less fatal "spot anthracnose"). Note that resistance to anthracnose does not hold for the entire 'Cherokee' series. 'Cherokee Chief,' in particular, is very susceptible to anthracnose. At least one new anthracnose-resistant C. florida cultivar is being bred and tested; look for it in the next couple of years, possibly by the name 'Presidential.'

For closely related Cornus species that are considered resistant to anthracnose, try C. alternifolia, C. officinalis and C. mas. However, be aware that these species do not have the same ornamental or ecological landscape use as Cornus florida.

If you opt to stick with C. florida, here's how to recognize the common diseases, as well as practical cultural techniques that will help to reduce disease incidence. Careful site selection and plant management are crucial to achieving success with a species or cultivar that is not genetically resistant.

Anthracnose is a fungal pathogen that forms purple or tan blotches on the leaves and causes significant leaf and twig dieback. The entire tree can be killed within two or three years after the first signs of infection. Powdery mildew is another leaf fungus, indicated by white, powdery blotches on the plant. Powdery mildew can cause stunted or distorted growth and will decrease the aesthetic value of the tree--but because it occurs later in the growing season, it will rarely cause tree death. Both of these fungi thrive in cool, humid areas with poor ventilation. See GreenShare Factsheets on Powdery Mildew and Powdery Mildew on Shrubs for more information.

Microclimate is a critical factor in avoiding dogwood anthracnose. Traditional recommendations have been to plant C. florida in partial shade. However, for plants highly susceptible to anthracnose, you might try planting in full sun, where ventilation is better and cool moisture does not linger long. Avoid using mulches such as leaves or partially decomposed compost, as both are likely to harbor diseases. Prune out dead wood and suckers on a regular basis. Water in the morning, only when necessary to avoid drought and avoid wetting foliage. Do not transplant trees from the "wild;" they are more likely to already be infected with anthracnose.

To avoid anthracnose in nursery situations, consider switching to drip irrigation rather than overhead sprinklers, and try spacing container stock significantly apart from each other. Field-grown dogwoods often seem to have less trouble with anthracnose.

By Renee Stoops, Plant Sciences Department, URI

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