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| Figure 1. Spot anthracnose on dogwood petals. |
The University of Georgia
College of Agricultural and Environmental Sciences
Cooperative Extension Service
PDF
Edward A. Brown II, Extension Plant Pathologist
Kim D. Coder, Extension Forester
Leaf and Flower Diseases
Stem Diseases
Root Diseases
Other Growth Difficulties
Conclusions
Two prevalent leaf and flower diseases of dogwoods are found throughout Georgia. They are Spot anthracnose, caused by Elsinoe Corni, and Septoria leafspot. The most prevalent foliage and flower disease of dogwood is Spot anthracnose. Dogwood anthracnose, caused by Discula sp., occurs in north Georgia and causes similar foliar symptoms as Spot anthracnose and Septoria leafspot but also causes lower limb dieback.
Spot anthracnose and Septoria leafspot, in most situations, do not cause permanent damage to the tree. They are usually unsightly and interfere with the beauty of the flowering dogwood. Severe infections, especially if they occur in consecutive years, eventually weaken the tree. This situation reduces flower production. Dogwood anthracnose can cause significant mortality in dogwoods, especially under ideal environmental situations.
Symptoms: Tissues of the flower bracts (petals) are usually infected first. Eventually infection spreads to other flower bracts, leaves, young shoots, and fruit. The initial symptoms are conspicuous, small, circular to elongated, reddish-purple spots which are first noticed in early spring (Figure 1). These spots are quite small-- usually about 1/25 of an inch in diameter. The centers of these spots are yellowish in color with margins a much darker color--brown to black. As additional infection occurs, the spots become numerous and eventually merge together forming larger leafspots. Therefore 'spot' size cannot be used as the criterion for disease diagnosis. Severely infected flower bracts usually fall prematurely from the plant. Those diseased bracts that remain on the plant are small and distorted.
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| Figure 1. Spot anthracnose on dogwood petals. |
Infections on the bracts lead to infection of the leaves. The spots on the leaves are usually circular, although they may be somewhat angular to elongated (Figure 2). The centers of these leafspots are yellow to grayish in color with a dark purple border. These infected spots or lesions will eventually 'drop out.' This loss of tissue gives a 'shot hole' appearance to the leaves. Infected leaves are usually reduced in size, and some may die. Young shoots and berries may develop elongated, scabby-looking lesions that have a very distinct, purplish margin, Severely infected fruit are usually deformed.
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| Figure 2. Spot anthracnose on dogwood leaves. |
The causative organism of Spot anthracnose is the fungus, Elsinoe corni. The disease incidences are increased by the production of spores (similar to seed in higher plants) produced in fruiting structures located within infected areas. The fungus persists from year to year in infected twigs, fruit, and other infected tissues. Infection occurs on the flower bracts the following spring. If the weather is dry prior to and during the flowering period, very little disease occurs. Therefore infections will vary from year to year.
Symptoms: Infection sites appear as small, uniform, purplish-colored spots or lesions on leaves. These spots may be angular in nature when they occur between veins of the leaves). The centers of these spots become grayish in color, but these lesions do retain the deep purple border. The 'shot hole' effect as seen with the Spot anthracnose rarely occurs. Near the end of the summer, lesions may be quite numerous and may completely cover the surface of the leaves. It is not unusual for Spot anthracnose and Septoria leafspot to occur on leaves at the same time.
Damage to fruit has been reported. Infected berries are discolored (blackened), which detracts from the beauty of the tree. The causative organism of this disease is the fungus Septoria cornicola. The disease commonly occurs in July. Variation in disease incidence is directly dependent upon damp, humid spring weather.
Dogwood anthracnose, caused by the fungus Discula sp., causes leafspots, stem cankers and kills shoots (Figure 3). Eventually infected trees decline and die. This should not be confused with spot anthracnose. The disease was confirmed in northern Georgia, western North Carolina, eastern Tennessee, western South Carolina, and Virginia in 1987. Anthracnose has been present in the northeast and northwestern United States since the early 1980s. Immediate symptoms are medium-large purple-bordered leaf spots and scorched, tan blotches that may enlarge to kill the entire leaf. Blighted leaves often cling to stems after leaf drop in the fall. Trunk sprouts occur during the later stages of disease development. The fungus infects the twigs and goes down the limb and infects the main stem. Cankers that form on the main stems can be detected when the bark is peeled back. Cankers are identified by distinct margins from the healthy cambium tissue (Figure 4).
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| Figure 3. Leafspot phase of Dogwood Anthracnose. | Figure 4. Discoloration and dieback caused by Dogwood Anthracnose. |
Control of this disease is difficult. Dogwood trees that are adjacent to natural stands of dogwoods may be more susceptible. Do not transplant dogwoods into urban areas from trees dug in the woods. You may be transporting the disease into an area where it did not previously occur.
The use of fungicides for control of these diseases must start in the early spring prior to blooming or at bud swell. These foliage and flower diseases require a wet, humid environment during particular times of the year, especially in the early spring. Since weather conditions cannot be anticipated, then it would certainly be wise to follow a regular spray schedule if disease control is desired. Haphazard applications of fungicides is useless and expensive and is discouraged. Fungicide recommendations can be obtained from the Georgia Pest Control Handbook.
Stem and branch disease caused by the Spot anthracnose and anthracnose fungus and the control measures have been discussed above. There are other stem diseases which have been reported in other parts of the country but are rare in Georgia.
A basal trunk canker is a major trunk problem of ornamental dogwood. It is caused by the fungus Phytophthora cactorum, which invades the basal area of the trunk. An injury to this area (lawn mowers and/or insects) is a prerequisite for disease development.
Symptoms: The foliage of infected trees appear stunted and/or color red prematurely. The basal portion of the trunk is usually sunken (canker). The bark eventually peels away which exposes the wood below. Eventually the trunk will be girdled. Some bleeding from the canker may also occur. Basal canker is best controlled by avoiding injury to the base of the trunk and to control insects. If injury does occur, follow procedures outlined in Section 4 of this publication.
Mistletoe is an occasional problem of older trees. Proper pruning of the infected limb controls this situation quite well.
Root rots caused by fungi are usually not a major problem with established trees. Organic root rots are usually secondary in nature. Plants which are weakened by an adverse environment or cultural problem are predisposing factors for root rot. These factors are treated separately below.
One fungus, Clitocybe tabescens, has the capacity to invade and destroy dogwood roots. However, roots must be injured before infection can occur.
The only specific, root rot seedling disease reported is caused by the fungus, Pythium species. Disease development requires wet, poorly drained soils. Death of infected seedlings usually occurs before the woody stage. Survivors will always be of poor quality due to the lack of roots.
Symptoms: Since a root rot disease kills roots, the plant is restricted in the amount of water and nutrients that can be absorbed and translocated up-ward to meet the requirements of the above-ground parts. This results in twig-dieback, foliage discoloration and marginal burning (scorch) of the leaves.
Control: Identification of the predisposing factors to this disease must be determined. In Section 4. Specific problems associated with established trees and their solutions are discussed. If a tree has died because of root rot, remove the tree and all roots from the soil. Remaining roots will serve as a n infection source for other trees and shrubs set in the same site. It is best not to set another plant in the same site. If replanting must be done, be sure all roots are removed, work the soil 12 to 18 inches deep and treat with methyl bromide.
Seedling root rot disease can be controlled by proper sanitation practices, use of well drained soil medium, reduction in watering frequencies, and fungicidal soil drenches.
Flowering dogwoods have a number of environmental and growth problems that can cause structural, aesthetic, or biological problems, In order of importance, diseases, insects, drought, soil constraints, human-caused damage and heavy shade are the most damaging. A few of the cultural and environmental problems are reviewed below.
Dogwoods grow in the forest as a mid-understory tree in upland forest. Dogwoods are tolerant of competition from many other plants. Wild dogwoods tend to form a single layer crown that effectively captures the filtered sunlight. Dogwoods grow best where there is 60 percent full sun and moist, uncompacted soil that is moderately well drained with plenty of leaf litter and organic material on the soil surface. Dogwoods are stressed badly by poorly drained soil.
Dogwoods established in full sun must be properly taken care of for at least three growing seasons. Young trees around home and businesses have severe water problems stemming from the heat generated in the landscape and from the tree's limited root system. Also, many new sites for planting may have poor or compacted soils that prevent good root development and water absorption.
Dogwoods do not use water very efficiently. The top of the tree can lose water much faster than the roots can absorb it, leading to shut-down of food production and a stressed tree. Planting in full sun requires much more water for use just to dissipate the heat built up in the tree tissues. Watering is critical for maintaining healthy dogwoods.
Marginal leaf burning or browning, morning leaf wilting, and upper twig death are all signs of drought problems.
Select a planting site with partial shade. Do not select sites with less than 60 percent full sun. Avoid poorly drained and wet areas. Use 2-4 inches of an organic mulch (i.e. wood chips, pine straw, bark nuggets, etc.) over the rooting area of the tree. Do not pile mulch immediately around the stem base. Allow the stem to be open to the air. Place mulch in young trees over an area that stretches at least 3 feet around the tree. In established trees, an ideal mulch bed would be twice the width of the crown.
For new trees, weekly watering is critical to survival. For older trees during dry periods (even if the dry period in the winter), water deeply once a week. Do not water lightly every day because this prevents the tree from adjusting to drought conditions and wastes water.
Most people plant trees improperly and the trees suffer for the rest of their shortened lives. Select a planting site that has plenty of above ground and below ground room for a tree to grow. Avoid planting over or beneath utility lines. If the site has no other plants, till a bed 10 times the width of the root ball as deep as possible. Dig a hole in the middle of the tilled area and plant your tree at the same level as it grew in the nursery. Do not add any organic material to the back-fill because this hurts root growth.
On sites with establish turf or other plants, dig a saucer-shaped hole with very slanted sides three times the diameter of the root ball. Do not dig a straight-sided hole for planting because your tree will become 'pot-bound' and have many root growth problems. Plant the tree without organic soil amendments and at the same soil level at which it grew in the nursery.
Once the tree is planted, water heavily to settle the soil and provide water to the new tree. Do not prune or fertilize for the first growing season, just continue to add water.
A common problem with dogwoods is planting too deep or filling around the tree with extra soil. Dogwood roots will suffocate when set too deep and this can invite many other growth and pest problems. Always plant at the same depth as the tree grew in the nursery. Also, do not add more than 1 inch of fill dirt around dogwoods in any one growing season.
Root systems of established dogwoods grown under good conditions can be three to five times more extensive than the above-ground parts of the tree. Heavy concentrations of fertilizers over the rooting area can leave dogwood stressed or dead from over-application. A soil test is essential to determine element shortages in the soil before fertilization. Conservative applications of low nitrogen fertilizers can greatly help long term growth of dogwoods on impoverished sites. If the turfgrass around a dogwood is being fertilized, extra fertilizer for the tree is not required. Too much nitrogen fertilizer can minimize flowering and cause the tree to be prone to other growth and pest problems.
Fertilization should be calculated on the amount to usable nitrogen being applied. One to two pounds of nitrogen per year spread evenly over every 1,000 square feet of soil surface area is a good application rate. Avoid fertilizing in late spring or early summer when the leaves are expanding. Do not fertilize within 2 feet of the stem base on mature trees.
The most common mechanical injury of dogwoods are basal wounds and cracks initiated by lawn mowers and string trimmers. Pests and decay organisms can enter into these wounds and cause many long-term problems. Use mulch, short protective stakes or hand weeding to keep damaging equipment away from the trunk. Basal stem root disease (discussed above) and insect borer infestation can result from mechanical damage.
If injury has already occurred, remove excess bark or wood from the wound with your fingers. Do not strip the bark, just pull of dead tissue. Do not use metal tools to scribe wounds and clean the wood. Also, do not use wound paints. Wound paints can prevent the tree from sealing-off and growing over the wound.
Dogwoods and other woody ornamentals are very susceptible to many common herbicides. Do not use a herbicide or herbicide-fertilizer combination over the top of exposed dogwood roots. Do not spray young sprouts or the stem base with herbicides.
Distorted, twisted, yellowing, browning, spotting, or marginal scorch are all leaf symptoms of herbicide damage. Also, twig and branch death can result. These symptoms are most noticeable during periods when the tree is already under stress. Tree death will result if herbicides are improperly used around dogwoods. Do not use spray equipment heat has contained herbicides, even if rinsed, to apply fertilizers or other pesticides.
Before any attempt can be made to rectify any problem on dogwoods, the specific cause or causes must be identified. Do not waste time and money treating a minor but showy problem when a major problem stressing your tree is overlooked. Many times environmental and site problems are difficult to correct. Some pest problems that could be easily treated are not severely damaging to the tree, and so, are not cost-effective nor biologically prudent. The best way to minimize dogwood problems is to plant dogwoods in the right place and take care of them properly.
For additional information on proper care and culture of dogwoods, see Extension leaflet No. 104, Growing Dogwoods.
Leaflet 36/Reprinted August, 1999
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