Abstract
Dogwood anthracnose caused by Discula destructiva poses a threat to flowering (Cornus florida) and Chinese (C. kousa) dogwood in the eastern United States and to Pacific or western dogwood (C. nutallii) in the Pacific Northwest. Differences exist in the impact the pathogen has on these hosts (subjectively in the order C. florida > C. kousa and C. nutallii). Ten Cornus species, including C. alternifolia, C. amomum, C. controversa, C. alba, C. augustata, C. florida, C. kousa, C. mas, C. sericea, and C. stolonifera, were evaluated for disease reaction in 1991 and 1992 at locations in eastern Tennessee infested with D. destructiva. Greatest disease severity in 1991 was observed on C. controversa, C. florida, C. kousa cv ‘Chinensis’, and C. sericea. In 1992, C. florida, C. sericea, C. kousa cv ‘Chinensis’, C. alba, and C. stolonifera cultivars appeared susceptible to anthracnose. Symptoms developing on each appeared similar to those observed on flowering dogwood, and D. destructiva was successfully isolated from lesions within the canopy of the trees. Other Cornus species tested, including an unnamed Chinese dogwood cultivar, appeared resistant. The results of this study indicate that concerns regarding the anthracnose susceptibility of native and ornamental Cornus species may be raised. Finally, the possibility should be considered that infected Cornus species (other than C. florida) might serve as a mechanism for the introduction of D. destructiva into anthracnose-free areas of the United States.
Dogwood anthracnose has become a serious threat to flowering dogwoods (Cornus florida L.) of the eastern United States. This disease, caused by the imperfect fungus Discula destructiva (8), should not be confused with spot anthracnose caused by Elsinoe corni, a common nonthreatening foliar disease of flowering dogwood. Initially referred to as lower branch dieback, dogwood anthracnose was first observed on flowering dogwood in the northeastern United States (4) and on western flowering dogwoods (C. nuttallii) in the northwestern United States and southwestern Canada (2). Mortality due to anthracnose has been observed within native flowering dogwood populations. Mielke and Langdon (7) reported that 97% of C. florida in Catoctin Mountain Park, Maryland, showed characteristic dogwood anthracnose symptoms and nearly 33% of trees were dead. Schneeberger and Jackson (9) reported 79% mortality of dogwood in that same area. D. destructiva is commonly isolated from blighted leaves, twigs, shoots, and berries from anthracnose-infected dogwoods. Infection of epicormic shoots by the fungus may lead to the development of annual cankers, an abundance of which may coalesce, girdle the stem, and lead to tree death. Anthracnose symptoms have been reported on Chinese dogwood (C. kousa), another Cornus species used in ornamental plantings (1,5); however, controversy exists regarding the susceptibility of Chinese dogwoods to D. destructiva infection.
Given the aesthetic and commercial value of flowering dogwood in the southeastern United States and interest in the development of additional ornamental Cornus species for the urban market, a study was initiated in 1991 at the Tennessee Agricultural Experiment Station to evaluate 10 Cornus species for their relative resistance or susceptibility to anthracnose caused by D. destructiva. An important objective of this study was to describe symptoms of anthracnose infection on susceptible Cornus species and document the presence of the pathogen within symptomatic plants. By doing so, the potential for the introduction of D. destructiva into anthracnose-free areas of the United States via infected Cornus species might be reduced and roles for susceptible species as sources of D. destructiva within the landscape might be identified.
Materials and Methods
Trees were grown in 5-gallon containers in a composted bark mix commonly used in the production of woody ornamental species. In addition, a timed-release fertilizer (Osmocote™ 14-14-14) was incorporated into the growing medium to maintain adequate nutrient status within the containers.
Seven Cornus species (with 5 replications), including C. alternifolia [common name: Pagoda dogwood (3)], C. amomum [common name: silky dogwood (3)], C. controversa [common name: giant dogwood (3)], C. florida, C. kousa, C. kousa cv ‘Chinensis’, C. mas [common name: Cornelian cherry dogwood (3)], and C. sericea [common name: Redosier dogwood (3)], were placed in a disease resistance trial at Ozone, Tennessee, in May 1991. This site was chosen due to its recent history of severe anthracnose infection (Windham, unpublished). Trees were placed under native flowering dogwoods with severe dogwood anthracnose symptoms, watered by hand as necessary, and rated weekly for disease development using the Horsfall-Barratt disease assessment scale (6). Comparisons regarding disease susceptibility or resistance were made in relation to symptoms developing on the flowering dogwood.
Ten Cornus species (with 6 replications), including those species listed above for the year-1 resistance screening test (except C. sericea), plus 4 C. stolonifera [also referred to as C. sericea (3)] cultivars (‘Kelseyi’, ‘Flaviramea’, ‘Ruby’, and ‘Isantii’), 2 C. alba [common name: Tatarian dogwood; closely allied to C. sericea (3)] cultivars (‘Elegantissima Variegata’ and ‘Bloodgood’), C. mas cv ‘Golden Glory’, and C. augustata were placed in field sites at Ozone, Tennessee, and at 2 locations within the Great Smoky Mountains National Park in May 1992. These species were evaluated 3 times over the course of the growing season, as described above.
Results and Discussion
In both 1991 and 1992, significant differences in response to anthracnose infection were observed among species and cultivars. Results from 1991 (Table 1) indicate that—while not statistically different from C. florida—C. controversa, C. sericea, and most surprisingly, C. kousa cv ‘Chinensis’ were at least as susceptible to anthracnose as was C. florida. In 1992 (Table 2), 3 of the 4 C. stolonifera cultivars, as well as C. kousa cv ‘Chinensis’ and C. alba, were as susceptible as C. florida was to anthracnose.
Results of 1991 anthracnose resistance screening study.1
Results of 1992 anthracnose resistance screening study.1
The status of anthracnose resistance among Chinese dogwoods has been debated. Previously, we have reported on infections occurring within C. kousa populations present on Lookout Mountain, Tennessee (1). D. destructiva was successfully isolated from Chinese dogwoods exhibiting characteristic anthracnose symptoms and no unusual environmental condition accompanying the disease was observed. The results of this study (Tables 1 and 2) indicate that C. kousa cultivars indeed vary with respect to their resistance and/or susceptibility to dogwood anthracnose. The cultivar C. kousavar. ‘Chinensis’ proved quite susceptible to dogwood anthracnose. In contrast, the C. kousa seedling material included in this test may possess a genetic basis for disease resistance to anthracnose that could be identified and exploited in future tree improvement programs. The differential response of C. alba and C. stolonifera cultivars (Table 2) also offers additional evidence for a genetic basis for anthracnose resistance.
A key concern raised by our results is the infection of Cornus species other than C. florida, C. kousa, and C. nuttallii by D. destructiva. Three of the four C. stolonifera cultivars included in this study were anthracnose-susceptible (Table 2). Both C. alba selections tested, albeit to varying extents, were infected by D. destructiva. Perhaps of most concern, however, is the development of D. destructiva anthracnose symptoms on C. sericea. Yellow-and red-twig dogwoods are highly valued by homeowners for their characteristic twig color in fall and winter and are commonly found in a wide variety of landscapes. The susceptibility of these ornamental Cornus species to anthracnose raises the possibility that they 1) might play a role in anthracnose epidemics as they become infected and harbor D. destructiva propagules that might in turn infect flowering dogwoods or 2) might serve as a means for the introduction of D. destructiva into anthracnose-free areas through the transport of infected C. sericea, C. alba, and C. stolonifera. Those involved in the scientific, regulatory, and production communities should bear in mind these results as experiments, regulations, and growing practices are developed.
Acknowledgments
The authors wish to acknowledge the financial support of the Tennessee Agricultural Experiment Station and the Nicholson Anthracnose Fund, a private research fund established by Mr. Hubert Nicholson of Commercial Nursery, Winchester, Tennessee. Critical review of this manuscript by Drs. William C. Carey (Auburn University School of Forestry), Pauline C. Spaine (U.S.D.A./Forest Service), and Charles R. Tischler (U.S.D.A./Agricultural Research Service) is gratefully acknowledged.
Footnotes
- © 1996, International Society of Arboriculture. All rights reserved.
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