|Pathology of Cornus|
|Dogwoods in the United States are constantly being threatened by a number of fungal pathogens leading to a variety of diseases including powdery mildew, septoria leaf spot, spot anthracnose, and dogwood anthracnose (Hansen, 2000). Powdery mildew is caused by a number of fungal species that grow on leaves and buds, resulting in stunted growth, reddish discoloration and curling of the leaves. Other parts of the plant are also susceptible to the disease including roots, which may lead to an overall stunting of the tree. Septoria leaf spot is caused by the fungus Septoria cornicola and generally affects leaf surfaces. Symptoms include relatively small, brown leaf spots uniform in size that begin to appear in infected areas late in the growing season. Symptoms are usually not severe and leaf blight (necrosis of the entire leaf) does not normally occur. Spot anthracnose is another disease that affects dogwoods and is caused by the fungus Elsinoe corni. Symptoms are generally not severe and are similar to those observed in Septoria leaf spot. Small circular lesions uniformly in size form on leaves and bracts. The lesions are lighter in color than the surrounding leaf surface and are bordered by a purple outline. On the forefront of dogwood diseases is dogwood anthracnose, which is caused by the recently described species of fungus, Discula destructiva Redlin (Redlin, 1991). As the name of the species implies, the effects of D. destructiva can be detrimental to trees that it infects. Various studies have shown the severity of the disease, reporting mortality rates ranging from 48 to 88 percent in certain regions of the Appalachian mountains of Maryland and North Carolina (Sherald et al., 1996; Rossell et al., 2001).
History of dogwood anthracnose
The effects of dogwood anthracnose were first observed in 1976 and 1977 on native populations of Cornus nuttallii, the Pacific dogwood, in Washington state (Daughtrey et al. 1996). About the same time, similar reports of a mysterious disease affecting Cornus florida, the flowering dogwood, came from regions in the northeast United States. Dieback of lower branches along with leaf spotting and defoliation was observed. Reports of the disease increased during the 1980's with its distribution spreading throughout the current ranges of C. nuttallii in western North America and C. florida in eastern North America. By the mid 1990's, the disease had spread among native C. florida populations as far north as Vermont, and in the south to Alabama (Daughtrey et al. 1996).
Once thought to be caused by a currently known fungal pathogen, D. destructiva was later distinguished from other Discula species that cause anthracnose in oak, ash and sycamore and described as a distinct species (Redlin, 1991).
Although the exact origin of the pathogen is unknown, it has been suggested that the fungus was recently introduced near ports of entry in Seattle and New York (Daughtrey et al. 1996, Zhang and Blackwell, 2002). In a population genetic analysis, Zhang and Blackwell (2002) used amplified fragment length polymorphisms (AFLPs) to create a genetic profile of D. destructiva from both eastern and western North American isolates. In this study, they found that eastern and western isolates were genetically distinct and that eastern isolates were more genetically diverse than western populations. Furthermore, isolates from the New York area were more genetically diverse than surrounding isolates, supporting that New York might be a possible place of origin.
A recent phylogenetic study determined that Discula belongs to the fungal order Diaporthales, which contains other plant pathogens such as Cryphonectria parasitica, the cause of chestnut blight (Zhang and Blackwell, 2001).
Symptoms and Resistance
Dogwood anthracnose affects many parts of the tree including leaves, stems, bracts, fruits, and bark (Daughtery et al. 1996). Symptoms on leaves can appear as leaf spots, leaf blotch and blight. Leaf spots are usually light brown in color with purple to red rims and scattered across the blade. Although similar to leaf symptoms in spot anthracnose, the leaf spots caused by D. destructiva are generally more pronounced and irregular in shape. Leaf blotches are more severe lesions, usually affecting the leaf tips or margins. The most extensive damage to leaves is leaf blight, in which the entire leaf surface becomes necrotic. The infection can invade the petiole and eventually move into the cambium of the branch and trunk. This invasion can lead to cankers that cause twig dieback and possible death of the tree.
Although dogwood anthracnose has been predominantly observed in C. florida and C. nuttallii, other dogwood species are susceptible to the disease. A recent study tested various species and cultivars for resistance including C. alternifolia, C. amomum, C. kousa, C. kousa var. chinensis, C. controversa, C. mas, C. sericea and C. alba (Brown et al., 1996). They found that during 1991 and 1992, the disease was detected in all tested species or cultivars, although a relatively high degree of resistance was found among C. kousa, C. mas, C. alternifolia and C. amomum as compared to C. florida, C. controversa and C. sericea. In a three year study, Sherald et al. (1996) tested the resistance of eight species of Cornus by exposing eight to ten plants for each species to native populations of C. florida infected with the fungus. Their experiments detected transfer of the pathogen from infected trees to all C. florida tested, but none of the other species showed signs of the disease. It was suggested that the degree of susceptibility was probably influenced by environmental conditions and that a range of susceptibility in Cornus was possible.
Factors that influence the susceptibility, severity and spread of the disease have been studied over the past few decades. Various environmental conditions have been shown to influence the susceptibility to and severity of dogwood anthracnose. For example, Erbaugh et al. (1995) found a significant increase in disease severity among dogwoods growing at low light intensities in combination with drought conditions. Carr and Banas (2000) showed that smaller trees generally have greater infection than larger trees and that fruit production in infected trees is adversely affected. Trees of the understory are generally subjected to conditions that reduce evaporation such as higher humidity, lower temperatures and less wind (Chellemi and Britton, 1992). These conditions provide a favorable environment for the growth of D. destructiva on smaller, more shaded trees.
There are a variety of control strategies that may help to prevent or treat infections by D. destructiva that include controlling growing conditions, pruning and treatment with chemicals (Hansen 2000). Dogwoods in landscapes for ornamental purposes should be planted in areas that are not prone to wet conditions for long periods of time. Fungi usually thrive in moist, shaded conditions. However, trees should be watered during drought conditions to prevent stress that may make them more susceptible to infections. Pruning of infected branches may help keep the disease from spreading to other parts of the tree or neighboring trees. Fungicides such as propiconazole and chlorothalonil can also be used to treat infections of foliage, but should be used before the twigs become infected. Preliminary work has been attempted to selectively breed or genetically transfer resistance in ornamental dogwood species (Daughtrey et al. 1996), but natural forest populations would still remain at risk for the disease.
David T. Thomas
Brown, D.A., M.T. Windham, and R.N. Trigiano. 1996. Resistance to dogwood anthracnose among Cornus species. Journal of Arboriculture 22(2): 83-85.
Carr, D.E. and L.E. Banas. 2000. Dogwood Anthracnose (Discula destructiva): Effects of and consequences for host (Cornus florida) demography. American Midland
Naturalist 143: 169-177.
Chellemi, D.O. and K.O. Britton. 1992. Influence of canopy microclimate on incidence and severity of dogwood anthracnose. Canadian Journal of Botany 70: 1093-1096.
Daughtrey, M.L., C.R. Hibben, K.O. Britton, M.T. Windham, and S.C.Redlin. 1996. Dogwood Anthracnose: Understanding a disease new to North America. Plant Disease 80(4): 349-358.
Erbaugh, D.K. 1995. Light intensity and drought stress as predisposition factors for dogwood anthracnose. Journal of Environmental Horticulture 13(4): 186-189.
Hansen, M. 2000. Foliar diseases of dogwood. Viriginia Cooperative Extension. Publication 450-611W.
Redlin, S. 1991. Discula destructiva sp. nov., cause of dogwood anthracnose. Mycologia 83: 633-642.
Rossell, I.R., C.R. Rossell, Jr., K.J.Hining, and R.L. Anderson. 2001. Impacts of dogwood anthracnose (Discula destructiva Redlin) on the fruits of flowering dogwood (Cornus florida L.): Implications for Wildlife. American Midland Naturalist 146: 379-387.
Sherald, J.L., T.M. Stidham, J.M. Hadidian and J.E. Hoeldtke. 1996. Progression of the dogwood anthracnose epidemic and the status of flowering dogwood in Catoctin Mountain Park. Plant Disease 80: 310-312.
Zhang, N. and M. Blackwell. 2001. Molecular phylogeny of dogwood anthracnose fungus (Discula destructiva) and the Diaporthales. Mycologia 93(2): 355-365.
Zhang, N. and M. Blackwell. 2002. Population structure of dogwood anthracnose fungus. Phytopathology. 92(12): 1276-1283.