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Plant Pathology Circular No. 359
May/June 1993
Fla. Dept. Agric. & Consumer Services
Division of Plant Industry
S. A. Alfieri, Jr.(2) and N. E. El-Gholl(3)
Cornus florida L. is a small tree up to 35-ft. tall with clustered, petal-like, white or cream-colored bracts. Flowering in the spring, it is grown as a popular ornamental (3). Its wide distribution includes southeastern Maine to southern Michigan, generally southward to central Florida, west to southern Missouri and eastern Texas.
Flowering types with pink bracts are rarely cultivated in the southern part of its range, presumably because it is not adapted to this area (3).
CAUSAL AGENT AND DISTRIBUTION: Phytophthora nicotianae Breda de Haan var. parasitica (Dastur) G. M. Waterhouse, alternatively designated as Phytophthora parasitica Dastur (1,6,7) was recently established as the cause of leaf blight of flowering dogwood. This fungus has an extensive host range (2) and is able to overwinter in soil and plant debris (4).
SYMPTOMS: The disease is characterized by scattered, rapidly enlarging, irregular, brown, water-soaked lesions with characteristic gray-green borders. Symptom development occurs particularly during and immediately following periods of heavy rains and high relative humidity (Fig. 1).
Scattered distribution of lesions on blighted leaves of flowering
dogwood, Cornus florida, following 3 days incubation in a
moist chamber after inoculation with 1.5 x 105 zoospores/ml of Phytophthora
nicotianae var. parasitica.
Fig. 1a A) Leaf blight on 1-yr-old
seedling;
Fig. 1b B) Close-up of blighted leaf.
(DPI File #702192, Jane Windsor).
CONTROL: Reference to University of Florida/IFAS Pest Control Guides
(1) Contribution No. 685, Bureau of Entomology, Nematology, Plant Pathology - Plant Pathology Section.
(2) Emeritus Plant Pathologist (deceased), FDACS, Division of Plant Industry.
(3) Plant Pathologist, FDACS, Division of Plant Industry, P. O. Box 147100, Gainesville, FL 32614-7100.
LITERATURE CITED
1. Alfieri, S. A., Jr., W. H. Ridings, D. J. Mitchell, and N. E. El-Gholl. 1987. Leaf blight of dogwood (Cornus florida) caused by Phytophthora parasitica. Plant Disease 71:555-556.
2. Farr, D. F., G. F. Bills, G. P. Chamuris, and A. Y. Rossman. 1989. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, APS Press, St. Paul, MN. 1252 pp.
3. Godfrey, R. K. 1988. Shrubs and woody vines of northern Florida and adjacent Georgia and Alabama. University of Georgia Press, Athens, GA. 734 pp.
4. Kuske, C. R., and D. M. Benson. 1983. Survival and splash dispersal of Phytophthora parasitica causing dieback of Rhododendron. Phytopathology 73:1188-1191.
5. Simone, G., T. Kucharek, M. Elliott, and R. S. Mullin. 1993. Florida Plant Disease Control Guide. Institute of Food and Agricultural Sciences, Florida Cooperative Extension Service, Department of Plant Pathology, University of Florida, Gainesville, FL. 546 pp.
6. Tucker, C. M. 1931. Taxonomy of the genus Phytophthora de Bary. University of Missouri Agricultural Experiment Station, Bulletin No. 153. 208 pp.
7. Waterhouse, G. M. 1963. Key to the species of Phytophthora de Bary. Mycological Papers, No. 92. Commonwealth Mycological Institute, Kew, Surrey, England. 22 pp.
