| Cornus species are riddled with leaf diseases. After
the discovery of dogwood anthracnose that
devastated the dogwood population in the
northeastern U.S., it has been increasingly more
significant to be on alert for disease outbreaks. Three
leaf spot diseases that show themselves in varying
degrees are dogwood anthracnose, spot anthracnose,
and powdery mildew. Dogwood anthracnose and
powdery mildew are of particular concern.
Symptoms and Diagnosis
Symptoms of Dogwood Anthracnose: Dogwood
anthracnose is a disease caused by the fungus,
Discula destructiva. Since its discovery in the 1970s,
the spread has been swift and the results disastrous
to the native Cornus florida. Symptoms include leaf
spots that are soft and very wet with purple borders,
twig blight, dead leaves that cling to the tree,
epicormic shoots (water sprouts with many shoots
occurring near the base of the tree), lower branch
dieback, and eventually death. Conditions that favor
disease include low light levels, high humidity, and
temperatures of 60 F.
Symptoms of Spot Anthracnose: Spot anthracnose is
caused by the fungus, Elsinoe corni. Symptoms
include small, dark, pinprick lesions with purple
borders on leaf surfaces. The lesions are numerous
and usually occur on dry leaves. The disease is
unsightly, but typically not very harmful.
Symptoms of Powdery Mildew: Powdery mildew is
caused by the fungus, Microsphaera pulchra. Early
symptoms include round, white or gray powder-like
spots that appear on upper leaf surfaces. These spots
form a dry white layer on leaves. As young leaves
become infected, they appear twisted, dry, and
leathery. Older leaves look bronzed. Moderate
temperatures, low light, and high humidity
accelerate this disease. High nitrogen levels cause
the disease to flourish.
Integrated Pest Management Strategies
1. Choose disease resistant varieties. A soon-to-be
released cultivar for dogwood anthracnose
resistance is called ‘Appalachian Spring’. Cultivars
less susceptible to spot anthracnose include
‘Cherokee Princess’, ‘Cherokee Sunset’, and
‘Springtime’. ‘Cherokee Brave’, C. kousa, and C.
kousa x C. florida crosses are resistant to powdery
mildew.
2. Water the roots of the tree. Avoid using overhead
irrigation in order to reduce the incidence of
leaf spot.
3.Remove infected debris. Remove any infected
leaves, including dead material on the tree, and
compost the material thoroughly or dispose of it.
4. Apply fungicides. If you suspect dogwood
anthracnose, get it diagnosed by a professional
prior to a fungicide application. Powdery mildew
should also be controlled, especially if the
disease has occurred previously. |
