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Dogwood Anthracnose: How Collaboration Was Used In The Southern United States To Effectively Deal With A New Tree Disease
Robert L. Anderson - Forest Pathologist, USDA Forest Service, Region 8, Forest Health Protection, Asheville, NC 28802.
From: Exotic Pests of Eastern Forests, Conference Proceedings - April 8-10, 1997, Nashville, TN, Edited by: Kerry O. Britton, USDA Forest Service & TN Exotic Pest Plant Council
Abstract. Dogwood anthracnose, caused
by the fungus Discula destructiva was found in the Southern United States
in 1987. Since that time millions of flowering dogwoods have been killed
and disfigured by this disease. As soon as the disease was discovered a
group of state and federal personnel formed a working group to develop an
action plan for dealing with the disease. Collaboration was the key word
from the beginning of the working group. A key to the success of the working
group was a spirit of cooperation with out concern for who was going to
get credit. Each time the working group met information was shared and cooperative
action plans were developed to address the most pressing questions. The
group established a network and mailing list where information was shared
back and forth on a daily basis. The formation of a steering committee provided
additional direction and organizations such as the Southern Appalachian
Man in the Biosphere added additional support. As the issues on impact and
rate of spread were addressed the focus of the working group shifted to
research. The working group still meets to coordinate activities.
Dogwood anthracnose was first reported as a disease of flowering dogwood
Cornus florida L. in the United States in 1978. Since that time it has caused
serious losses to flowering dogwoods in the forest and in ornamental plantings
over large portions of the Eastern and Southern United States. The fungus
that causes the disease was fully described and identified as Discula destructiva
sp. nov. in 1991 (Redlin 1991). This paper briefly describes the symptoms,
distribution, impacts, and control procedures. Most of the paper will be
devoted to a discussion of how collaboration was used in the Southern United
States to effectively deal with this disease.
Dogwood Anthracnose
Symptoms
Initial symptoms of dogwood anthracnose are small tan leaf spots that
develop into large tan blotches. Often a purple border occurs between dead
and healthy tissues and occasionally the entire leaf is killed. In many
cases, infected mature leaves are aborted prematurely; in other cases, infected
leaves cling to the stems after normal leaf fall. Infections often expand
from leaves into small twigs and symptoms typically start in the lower crown
and progress up the tree.
The dieback of twigs and branches in the lower crown led to the original
name of "lower-branch dieback" (Pirone 1980). Numerous epicormic
shoots form along the entire length of the main stem and on major branches
of infected plants. These shoots frequently become infected and die and
the fungus proceeds from the shoots into the main stem.
The fungus causes cankers that can kill the tree. Cankers may not be
present on all the dead trees. Larger trees often die 3 to 4 years after
the first symptoms are found in the leaves while young trees die the same
year they are infected.
The disease kills dogwoods of all sizes, but it is most severe on young
seedlings and in understory forest dogwoods. Infection of dogwoods is most
likely to occur during cool, wet weather in spring and
fall, but can occur
at any time during the growing season. Ornamentals are often disfigured without being killed, particularly if they are growing in open, sunny sites (Anderson et al. 1994; Mielke and Daughtrey 1989).
Distribution
The following map shows the natural range of flowering dogwood and distribution of dogwood anthracnose in the eastern United States. For a county to be
recorded as affected there only has to be one infected tree in a county.
Therefore, the counties reported as affected can range from severely affected
to a few trees. In general, the disease is more common in cooler wet environments,
especially at higher elevations. The map is from the ATLAS forest health
protection data base maintained by the USDA Forest Service in Asheville,
NC. |
 Data from 1995 Conditions Report -
USDA – Forest Service, Forest Health
Protection, Asheville Field Office. |
Impact
Dogwood anthracnose has spread rapidly and covered a significant part
of the flowering dogwood range. The inpact of dogwood anthracnose has varied
from slight to total mortality. In the South, above 3,000 feet in elevation
most of the trees have died. Below 3,000 feet elevation the most significant
damage has occurred to trees on cool wet areas. Dogwoods on dryer sites,
especially in the sun, have sustained less damage. Those in full sun show
little damage and are doing well. The reason for this cause/effect relationship
is not clear but it may be due to environmental conditions that are condusive
for disease development (Windham 1990).
Mortality estimates vary from 79 percent at the Catoctin Mountain National
Park in Maryland (Schneeberger and Jackson 1989) to 56 percent at the Great
Smoky Mountain National Park to 23 percent in a southwide survey conducted
from 1988 to 1993 (Knighten and Anderson 1993).
The disease impacts seem to be less severe on the hotter and dryer sites.
Trees below 3,000 feet elevation in full sunlight are expected to survive
and do well.
Control Procedures
Control procedures are not available at this time for dogwoods grown
in the forest environment. However, a number of techniques are available
to deal with the disease in generally high-value settings, such as recreation
sites or urban settings.
Managers and homeowners can consider planting new flowering dogwoods
if they are willing to follow the Decision Key and the Ten Essential Steps
outlined in the diagram on the next page:
Dogwood Anthracnose Decision Key
(1) Apply 10 essential steps; omit fungicide and monitor.
(2) Use 10 essential steps, use other tree species or consider resistant trees when they become available.
Ten Essential Steps to Prevent/Control Dogwood Anthracnose
- Know the symptoms of dogwood anthracnose and other problems that commonly
affect dogwoods. Inspect trees frequently to detect the presence of the
disease in its early stages.
- Select healthy planting stock. Never plant diseased stock. Purchase
trees from a reputable nursery. If symptoms are seen on the planting stock,
dispose of the infected trees. Avoid transplanting trees from the forest,
especially from mountainous areas.
- Select reasonably well-drained planting sites with fertile soils. Avoid
sites along streams, lakes, or ponds where moisture will remain on the
foliage for many hours after sunrise. In high-hazard areas, plant flowering
dogwoods only in full sun.
- Planting holes should extend well beyond the root system of your planting
stock, and should be filled with a rich mixture of soil and humus. Be sure
the root collar is placed at ground level.
- Mulch around newly planted and existing trees to a depth of 2-4 inches.
Be sure the mulch does not touch the stem, and avoid using dogwood leaves
or chips.
- Prune and completely remove or destroy dead wood in the tree and leaves
on the ground yearly. Avoid flush cuts, being sure to leave the branch
collar. Prune all epicormic branches in late summer.
- Water weekly during droughts. Water in the morning and avoid wetting
the foliage.
- Fertilize to provide nutrient-rich soil. Have soil tested to be certain
what quantities of nutrients are needed.
- Avoid mechanical and chemical injuries to the trees. Lawnmower and
string-trimmer wounds are particularly troublesome.
- Apply fungicides registered for prevention or control of dogwood anthracnose
when it is necessary to do so. Fungicides should be applied as buds are
breaking in the spring and at least twice thereafter as the leaves are
expanding. Check with your local Extension Service about registration and
use before applying any fungicide (Knighten and Anderson 1993).
Collaboration in Southern United States
Dogwood Anthracnose was first reported in 1978. It was causing a widespread,
rapid deterioration of flowering dogwood in New York and Connecticut. In
1983, Daughtrey and Hibben reported a lower branch dieback disease with
the same symptoms on flowering dogwood in New York , Connecticut, New Jersey,
and Pennsylvania. They made observations on trees in Planting Fields Arboretum,
Oyster Bay, Long Island, and a woodland site at the Brooklyn Botanic Garden
Research Center in Ossining, NY. They reported the cause of the disease
to be a species of Discula sp., and that the reason for a sudden onset of
anthracnose over part of the northeastern range and its coincidental outbreak
on western flowering dogwood was unknown.
In October of 1987, unusual numbers of dogwoods were reported dying on
the Cohutta Ranger District on the Chattachoochee National Forest in northern
Georgia. All of the symptoms matched those of dogwood anthracnose. Foresters
estimated that the affected area covered about 30,00 acres of Cohutta Wilderness.
Discula sp., the causal organism of dogwood anthracnose, was isolated from
samples from the affected area.
The Chattahoochee and Oconee National Forest Supervisor and State Forester
of Georgia were notified of the occurrence. Soon thereafter, a professor
from Clemson University reported an unusual problem with the dogwoods in
Cashiers, NC. This area was checked and Discula sp. was found. In this case,
the affected area was much larger than 30,000 acres. The State Forester
of North Carolina was notified, and a meeting of state and federal personnel
from the affected and adjacent States was held in Dillard, GA in February
1988. A key factor in the success of this group was the open sharing of
information and a spirit of collaboration. All agreed to cooperate and share
information openly without fear of who was going to get credit. A mailing
list was created and updated where the most current information was shared
on a frequent basis. This group became a dogwood anthracnose working group
and agreed that the top priority in 1988 was to assess the disease distribution.
The Southern Region of the USDA Forest Service distributed a southern version
of the dogwood anthracnose pest alert.
By the second meeting of the working group in May of 1988, the disease
had been found in Georgia, North Carolina, South Carolina, Tennessee, and
Virginia. It was reported that the disease affected trees of all sizes and
was more common in the mountains and cool, wet valleys. Six nurseries in
North Carolina and one in South Carolina were reported to have diseased
trees. Fungicide trials were started in Georgia and Tennessee by the University
of Tennessee and University of Georgia and a joint pilot-test proposal was
prepared by the working group for submission to the Washington Office. The
North Carolina pest control forester proposed that permanent plots be established
on a 15-minute grid across the affected area to assess the current and future
impacts. These plots were installed by state and federal personnel in each
of the respective states. In June of 1988, the fungus had been found in
so many locations that the USDA Forest Service and the University of Georgia
began to provide sample identification services. In September, the working
group developed a news release, but it was decided not to send the release
until more information was collected. After this point, the information
became known to the press and public. As a result, the group news release
was never issued. The National Park Service did distribute a news release
from the Great Smoky Mountain National Park. Dogwood anthracnose and its
impact received major media coverage.
At this time, a lot of work was being done, and the working group concept
was producing results. A third meeting of the dogwood anthracnose working
group was held in October 1988. By this meeting a funding proposal had been
submitted to the USDA Forest Service, Washington Office for consideration
(Found in 49 counties in the South). Sixty permanent plots had been established
to assess impact. Birds were discussed as possible vectors, and fungicide
studies in Georgia and Tennessee had not produced positive results. One
important concern was the inability to inoculate trees under controlled
conditions. A high priority was placed on this task by the working group.
In November of 1988 the Great Smoky Mountain National Park, in cooperation
with the University of Tennessee, completed a survey of the park and found
the disease was widespread.
In January of 1989, the Regional Forester for the Southern Region of
the USDA Forest Service called a meeting of the federal and state cooperators
to discuss dogwood anthracnose. At this meeting, a list of priorities was
developed for survey, impact assessment, and research, and a dogwood anthracnose
steering committee headed up by the State Forester of Georgia was developed
to help with the biological, political, and funding aspects of the problem.
The steering committee met two times and helped establish political support,
priorities, and funding.
In March 1989, eight national forests were surveyed to assess the distribution.
At the same time, a greenhouse inoculation test was completed with positive
results. It was found when seedling leaves were pretreated with an acid
mist, fungus spores routinely produced infections on them. Results led to
a controlled acid rain study where a positive correlation was established
between simulated acid rain and infection in the greenhouse. Funding was
approved by the USDA Forest Service, Washington Office for a pilot test
of control techniques, and several studies were started by the State Foresters,
USDA Forest Service, the University of Tennessee, University of Georgia,
and the National Park Service.
In the spring of 1989, the media coverage picked up considerably. The
story ran in dozens of newspapers and on radio and TV. CBS National News
did a Saturday segment on the disease. Realizing the need to provide the
best information possible to the public, the USDA Forest Service, the University
of Georgia, and the State Forester of Georgia developed and published a
booklet on how to manage dogwoods. To keep public officials informed, a
briefing package was developed by the USDA Forest Service. The package included
a briefing paper and a list of people to be contacted. The Southeastern
Forest Experiment Station assigned one person to work part-time on the disease
in 1989 (found in 57 counties in the South).
In September of 1989, the working group met again to discuss progress.
The impact plots showed the disease had increased from 1/2 million acres
in 1988 to 2.2 million acres in 1989. The National Park Service announced
that it would investigate mycological aspects of the problem. Forest Service
research officials reported that they would be working on epidemiology.
In 1989, the Southeastern Forest Experiment Station added a full-time scientist
to work on dogwood anthracnose.
By 1990, considerable information was accumulating. The acid rain study
was repeated and showed the same result. It was noted that the fungus seemed
to remain active and grow down the dogwood shoots in the winter. The disease
was more common at high elevations and in cool, wet coves was able to spread
over large areas very quickly (127 counties now had diseased trees), and
the fungus preferred cool temperatures. Pilot-test data showed that the
fungicides Benlate and Daconil were providing effective control and that
other fungicides showed promise. Fertilization and mulching seemed to improve
tree vigor, while not increasing the disease in the field. Other greenhouse
and field tests were showing that phosphorus tended to increase and lime
tended to decrease disease symptoms.
Early literature reported that there was no resistance in the native
flowering dogwood populations, but people were noting some trees in the
field that seemed to show resistance. Resistance became a high priority
in 1990.
At this time, the University of Tennessee formed a research task force
composed of horticulturists, plant pathologists, entomologists, plant physiologists,
foresters, and genetists. Their mission was to join forces within and outside
the University to solve the dogwood anthracnose problem (Southards 1995).
In the fall of 1990, the Southern Appalachian Man and Biosphere Cooperative,
consisting of the U.S. Environmental Protection Agency, USDA Forest Service,
Park Service, Southeastern Forest Experiment Station, Fish and Wildlife
Service, Department of Energy, Economic Development Administration, and
the Tennessee Valley Authority, organized two dogwood anthracnose conferences.
One was held in Knoxville, TN and the other was held in Asheville, NC. These
conferences consisted of representatives from Federal, State, and private
concerns, and featured speakers from a number of these agencies. The program
reflected a diversity of views held by various groups throughout the South.
These meetings increased the awareness of and understanding of dogwood anthracnose,
and helped define specific goals, such as effective information dissemination.
A follow-up meeting was held in Roanoke, VA in 1991.
USDA Forest Service, Forest Pest Management, took the lead for maintaining
incidence maps. Since there are so many mimicking symptoms, it was decided
that for a county to be designated "affected," disease presence
had to be confirmed in the laboratory. In 1990, the plot data were added
to a Geographic Information System to generate maps displaying both severity
and incidence (163 counties now had diseased trees).
In January 1991, another working group meeting was held. Members reported
progress in all areas. Five hundred thousand copies of an updated version
of "Growing and Maintaining Healthy Dogwoods" including revised
control methods were published. This was a model of cooperation where the
USDA Forest Service, Carson-Newman College, Champion International Corporation,
Georgia Forestry Commission, Izaak Walton League of America, Southern
Appalachian Man and the Biosphere, Southern Nurserymen's Association,
Tennessee Valley Authority and the University of Georgia collaborated to
produce and distribute the copies (Bailey and Brown 1991). Also in 1991,
the fungus causing dogwood anthracnose was described as "Discula Destructiva
sp. Nov." (Redlin 1991) and dogwood resistance screening was developed.
For impact assessment, some 210 permanent 10-tree dogwood plots had been
established in North Carolina, South Carolina, Tennessee, Alabama, Virginia,
Kentucky, and Georgia by state and federal cooperators. These plots were
selected in a random stratified sample on a 15-minute grid. Data showed
that the disease increased dramatically (from about _ million acres in 1988
to 17.3 million acres in 1993), and the severity in the permanent plots
had increased.
The working group continues today where state, federal, and other group
collaborate on understanding the disease and developing strategies for control.
Literature Cited
Anderson, R.L., Knighten, J.L., Windham, M., Langdon, K., Hendrix, F.,
and Roncadori R., 1994. Dogwood Anthracnose and its spread in the South.
Forest Service Protection Report 26, p. 19.
Bailey, K.R., and Brown, E.A., II. 1991. Growing and maintaining healthy
dogwoods. For Rep. R8-FR14.
Daughtrey, M.L., C.R. Hibben, .O. Britton, M.T. Windham, and S.C. Redlin.
Dogwood Anthracnose: Understanding a Disease New to North America. Plant
Disease 80:349-358.
Daughtrey, M.L., and C.R. Hibben. 1994. Dogwood Anthracnose: A new disease
threatens two native Cornus species. Annu. Rev. Photopathol. 32:61-73.
Daughtrey, M.L. and C.R. Hibben. 1983. "Lower Branch Dieback, a
New Disease of Northeastern Dogwods." (Abst.) Phytopathology 73 (1983):365.
Knighten, J.L. and R.L.Anderson. 1993. Results of the 1992 Dogwood Anthracnose
Impact Assessment and Pilot Test in the Southeastern United States. USDA
Forest Service Southern Region Protection report R8-PR24. p. 61.
Mielke, M.E. and M.L. Daughtrey. 1989. How to Identify and Control Dogwood
Anthracnose. USDA Forest Service, Northeastern Area, Broomall, PA. NA GA-18:
8.
Pirone, P.P. 1980. Parasitic fungus affects region's dogwood. The New
York Times. February 24: 34-37.
Redlin, S.C. 1991. "Discula Destructiva sp. Nov., Cause of Dogwood
Anthracnose." Mycologia, 83(5), 1991, 633-642.
Schneeberger, N.F., and W. Jackson. 1989. Impact of dogwood anthracnose
on flowering dogwood at Catoctin Mountain Park. Plant Diagn. W. 10:30-43.
Southards, C.J. 1995. Battling Dogwood Anthracnose In: Tennessee Agri.
Science, University of Tennessee Agricultural Experiment Station Number
175, Summer 1995, p. 7-10.
Windham, M. 1990. Dogwood anthracnose research at the University of Tennessee
in Knoxville. pp. 261-268. In: 23rd annual Tennessee nursery short course.
Tennessee Nursery Industry, Nashville, TN. 29-34 p.
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