The decline of the flowering dogwood, Cornus florida,
is a common concern. People want to know the reason for the early decline
of their trees. One or more of the following factors may be involved:
fungal infection, insect attack and/or environmental stresses.
The dogwood is an understory plant, living under taller trees or on the
edges of forest openings. It is normally found along the banks of rivers,
streams or ponds where the soil is deep, moist and well drained. Therefore,
dogwoods may not adapt readily to an open lawn situation. In addition,
Connecticut is in the northern fringe of cold hardiness for dogwoods.
Any severe change in the environment, such as drought or long periods
of high temperature, can stress the plant. Such stresses can reduce resistance
of dogwoods to disease and insect attack, further weakening the plant
to the point where it may die.
The key to keeping dogwood trees healthy and living longer is a good cultural
management program. This includes adequate air drainage, fertilizing,
watering and pest control when needed.
The following guidelines may be followed for growing the flowering dogwood.
- Buy healthy trees.
- Plant in early spring.
- Use proper planting technique.
- Be sure site is well drained.
- Maintain low mulch around the tree.
- Water adequately during dry weather.
- Protect the trunk from mechanical injury.
- Prevent borer attack by avoiding wounds and painting pruning cuts
with tree wound paint or orange shellac.
- Do all pruning in early winter when moths are not active.
- Reduce fungal diseases by proper pruning and good air circulation.
Some of the factors responsible for dogwood decline in Connecticut include
the following:
Fungal Infection
Three fungi: Botryosphaeria sp., Discula sp. and Elsinoe sp. are frequently
associated with the decline of the dogwood. These and other fungi infest
the foliage of dogwoods during cool, wet periods in the spring when the
new growth is emerging. The amount of rain does not matter as long as
there is a film of water present on the foliage. Susceptibility of dogwood
to these fungi is increased when the foliage stays wet for extended periods
of time. Controls against the different species of fungi are the same.
Symptoms of fungal attack include reddish to maroon spots on the leaves,
yellow to brownish discoloration of leaves and light gray lesions on the
twigs often followed by progressive dieback of twigs starting in the lower
part of the trees.
-Botryosphaeria sp. can be recognized by its small black spore-bearing
structures found on the twigs and branches.
-Discula sp. produces small, 1/8-inch, reddish to reddish-brown spots
on the leaves. As the spots enlarge, they often join together, discoloring
a quarter or more of the leaf. The fungus may grow through the stem of
the infected leaf and enter the twig turning it light gray.
-Elsinoe corni causes the disease known as spot anthracnose.
Infected flower buds do not open or they produce stunted, malformed bracts
with numerous small, circular to elongated tan spots with purple to brown
borders. There may be up to 50 spots on a bract. Leaf spots have slightly
raised purplish edges paling to yellow-gray at centers. The centers will
often fall out producing a shot-hole effect. There may be 100 spots on
a leaf. Spots on petiole, fruit clusters and stems are similar to leaf
spots.
Control
Have the soil tested to apply the proper amounts of lime and fertilizer,
irrigate as needed. Vigorously growing trees tend to have better resistance
to fungal attack. Remove broken or diseased branches and dispose of them.
Prune in early winter to increase air circulation. Avoid overcrowding,
over-fertilizing with a high nitrogen fertilizer and planting in heavily
shaded or low, wet sites. If fungal diseases have been a problem, rake
up and remove fallen leaves. For additional information, please read the
Horticulture Fact Sheet: Leaf Spot Diseases.
Insect Attack
The dogwood borer, Synanthedon scitula, is the most serious insect pest
of dogwoods. Several other insects attack the dogwood but are not considered
major pests. The dogwood borer makes irregular tunnels under the bark
on the main stem and sometimes on the base of limbs. Small trees or branches
may be girdled. Young trees are frequently killed and older ones are reduced
in vitality, often leading to death. The adult insect is a clearwing moth
which lays eggs from early May to mid-July. Upon hatching, larvae enter
the plant through cracks, wounds and fresh scars. Once inside, they disrupt
the nutrient supply channels of the tree. Young dogwoods are often attacked
at the ground line. Symptoms of the borer attack include drying and dropping
of leaves, dieback of branches, the emergence of numerous watersprouts
near the borer damage and borer holes on the trunk.
Control
Cover all wounds and pruning cuts on the trunk with a tree wound paint
or orange shellac to discourage easy entry of the larvae. A fine wire
can be inserted into the entry hole of the dogwood borer and pushed up
the feeding channel in an attempt to kill the larva. Also see Horticulture
Fact Sheet: Dogwood Borer.
Ecological Factors
Each tree species does best within a given environmental range. The flowering
dogwoods prefer cool, moist soils on the fringe of woodlands. The open
lawn site can be very harsh because soils can be hot and dry in the summer
and the site may be cold and windy in the winter.
Other Cultural Factors
- Improper use of mulches around trunk (material piled several inches
deep).
- The use of broadleaf herbicides near and under the dogwood.
- Too little or too much water.
- Poor fertilizing practices.
- Improper pruning or lack of pruning.
- Poor transplanting and care practices.
Control
Check the soil's pH and nutrient levels by having the soil tested. Apply
fertilizer and lime as recommended on the soil test report. Do not apply
fertilizer around dogwood trees after mid-June. It can stimulate soft
late growth, making the trees susceptible to winter injury. Avoid planting
dogwood in open, exposed areas. Prune dead limbs. Do not use herbicides
under and around dogwoods. Do not apply insecticides or fungicides to
dogwoods with sprayers that were used for applying weed killers. Use not
more than a two-inch layer of mulch around the tree. Dogwoods have shallow
roots, so water thoroughly if a summer drought should last two weeks or
more. Avoid lawn mower injury. Kousa Dogwood might be a suitable replacement
tree. It is later blooming and more resistant to the anthracnose fungi
than the native dogwood.
For pesticide recommendations call the UConn Home
and Garden Education Center at 877-486-6271.
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