Dogwood Anthracnose

 

The following is an edited version of an article published in April 2008 by
Dr. Sharon M. Douglas of the Department of Plant Pathology and Ecology,
The Connecticut Agricultural Experiment Station.

 

Dogwood anthracnose is considered the most serious disease of flowering
dogwood (Cornus florida) in Connecticut and the Eastern Seaboard. It is
also an important disease of Pacific Dogwood (Cornus nuttallii) in the

West. The causal agent is the fungus Discula destructiva, and as the name

suggests, this pathogen is highly destructive. It is believed that the fungus
was probably simultaneously introduced into the East and West coasts of
the United States in the mid-1970s. Since its introduction, dogwood anth-

racnose has resulted in the death of many dogwoods in forests, woodlots,

and landscapes.

 

SYMPTOMS AND DISEASE DEVELOPMENT

 

Initial foliar symptoms develop in May and June as brown spots up to ¼
inch in diameter that are visible on both the upper and lower leaf surfaces.
These spots can be circular or irregular in shape and frequently develop
distinctive smoky, purple-brown margins.

 

The flower bracts are also susceptible to infection and develop reddish or
brownish spots or blotches. These are most prevalent when wet conditions
occur during flowering.

 

Under certain conditions, pinpoint, brownish-black fruiting structures can
be seen in the centers of the foliar spots or lesions. Spots on the leaves
usually become so numerous that they coalesce, which results in the
development of large, dead areas on the leaves.

 

When entire leaves become necrotic, they usually droop and rather than
falling off, they remain on the tree throughout the fall and into the winter.
The persistence of infected leaves on the tree during winter is a distinctive
characteristic and can help in diagnosis. The presence of infected leaves

on the tree also serves as an important source of overwintering inoculum,

since fungal spores capable of initiating new infections in spring are pro-

duced on these leaves.

 

When the whole leaf becomes infected, the fungus grows into the petiole

and then into the twig where it causes cankers. Cankers are often tan,

slightly sunken, elliptical areas of bark and are readily distinguished from surrounding healthy bark. The fungus can also directly infect shoots dur-

ing spring and fall. These infections develop into very small cankers. If

left unchecked, these cankers increase in size and eventually girdle the

affected tissues (e.g., twigs, stems, branches, or the main trunk). Symp-

toms and branch dieback typically begin on the lower limbs and move progressively up the tree. This pattern of dieback appears to be associated

with poor air circulation in the lower canopy, which results in tissues stay-

ing wet for longer periods of time. This makes them more susceptible to

infection. Some trees attempt to compensate for the loss of limbs by send-

ing out sprouts from the trunk (epicormic sprouts) but these sprouts are

highly susceptible to infection. Sprout infections usually spread quickly

to the trunk and cause severe cankers and splits in the bark. These cankers

readily develop into tree-killing cankers.

 

 

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