Summer 2001 Volume 10, Number 2
Flowering Dogwood - Is It Dying Out??
Phil Marshall, Forest Health Specialist, Vallonia State Nursery
This question will arouse concern and awareness from forest landowners and people who value flowering dogwood. The answer to the question is not easy.
One disease that may be involved is dogwood anthracnose. This disease was found in the state between 1992 and 1994. It was detected on nursery stock shipped into the state. Since that first find of the disease, it has been detected in 17 counties primarily on nursery stock. The disease was also found in Brown County on forest understory dogwood. The disease is also present on dogwood in the landscape plantings in northern counties (St. Joseph and LaPorte). The history of the disease in these counties is also attributed to infected nursery stock as the source of the disease.
In the early 1990's about the time dogwood anthracnose was detected in nursery stock, a powdery mildew was observed on flowering dogwood in the forest of southern Indiana. The role of powdery mildew in the death of dogwood is unknown. Powdery mildew is suspected as a possible cause because it is wide spread and is easier to observe than it is to find the symptoms of dogwood anthracnose.
A recent report by researcher's at Purdue offer another reason for dogwood dying out in the forest. Their study of Ross forest found a change in forest diversity may be involved in the loss of dogwood. The change of the forest from oak and hickory to sugar maple may not allow enough light to the forest understory that is needed by dogwood. Also the dense canopy of sugar maple may provide an environment more conducive for dogwood anthracnose to survive, infect and cause mortality.
Another agent that may be a factor in the loss of dogwood is drought. The stress from drought can directly kill the tree or make it more susceptible and vulnerable to anthracnose and powdery mildew.
Thus, four factors may be involved in the health of flowering dogwood in the forests of Indiana. Although the question of dog-wood health was returned to forefront with the recent report from the study of Ross Forest, I do not have any information or reports from foresters indicating they have noticed dogwood dying out in the forest. I suspect that a decline in the flowering dogwood population, if it is occurring, is a "chronic" situation, i.e. occurring very slow over time and wide areas, and not an "acute" situation, i.e. occurring dramatically over a short period of time is specific areas or in concentrated situations. It is more difficult to recognize a "chronic" problem because not enough symptoms occur in a manner easy to recognize. An "acute" problem is easier to recognize because it suddenly appears in a noticeable way and is easy to see.
From my observations, reports from foresters, prior forest impact surveys for dogwood anthracnose, I am concerned that powdery mildew may have a larger role in the death of flowering dogwood than dogwood anthracnose. Surveys to study the role of powdery mildew have not been done. Dogwood anthracnose has received more attention and as such is given first credit as the reason for any decline in dogwood. I suspect that any decline in the dogwood population of Indiana's forests, if it is occurring, is the result of a complex of possible causes - powdery mildew, dogwood anthracnose, drought, and changing forest composition. More observations and surveys to demonstrate a decline in the dogwood population would address the question and concern for flowering dogwood. They would also provide some information on the four factors listed above. For the present, monitor the flowering dogwood in your forest. If you observe widespread mortality in your forest, please advise your district forester or myself.