The most important fungal disease of dogwoods in North America is anthracnose caused by Discula destructiva. This disease affects Cornus florida (flowering dogwood), C. nuttallii (Pacific dogwood), and C. kousa (kousa dogwood). It has not been well studied in Ontario nor anywhere in Canada. In this study, over 2,500 fungal isolates were obtained from symptomatic samples of C. alba, C. alternifolia, C. amomum, C. kousa, C. florida, C. nuttallii, C. racemosa and C. sericea. To help distinguish between foliar symptoms of different etiologies, a “Dogwood Disease Symptom Guide” was produced. Isolates were divided into 13 fungal morphotypes, of which D. destructiva accounted for 39% of all isolates. Pathogenicity testing of Discula destructiva on C. florida satisfied Koch’s postulates, and this fungus was confirmed as the causal agent of dogwood anthracnose in southwestern Ontario (C. florida) and southwestern British Columbia (C. nuttallii). Wounds and leaf trichomes may provide a point of entry and help the pathogen survive endophytically without producing symptoms on “non-host” plants such as oak, maple and pear. The pathogen was found to survive for over 12 weeks at -20 °C, and the optimal growth temperature was found to be between 20-25 °C, but temperatures as high as 30 °C inhibited the growth, and the fungus died after one week incubation at 40 °C. The finding of only one mating type within D. destructiva populations (122 isolates) explains the lack of sexual reproduction of this fungus in North America, and along with the SSR results, reconfirms the low genetic variability within its populations. / Ontario Ministry of Natural Resources

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 72-74). Also available via the Internet.

Thesis (M.S.)--West Virginia University, 1999. / Title from document title page. Document formatted into pages; contains ix, 80 p. : ill. (some col.) Includes abstract. Includes bibliographical references (p. 60-69).

Discula destructiva, the causal agent of dogwood anthracnose, has caused severe mortality in dogwood over the last 30 years. Although considerable research has been done with dogwood anthracnose, the infection process by D. destructiva is still obscure. A resistant cultivar of Cornus florida, ‘Appalachian Spring’, was discovered and released by the Tennessee Agricultural Experiment Station. However, the resistance mechanisms are unknown. The objectives of this research were 1) to determine the sequence of events in the infection process of D. destructiva in C. florida and 2) to determine how host resistance affects infection events of D. destructiva on flowering dogwood. At 3 days after inoculation (DAI), majority of conidia germinated and hyphae were observed on the leaf surface. Direct penetration by D. destructiva hyphae was observed without appressorium formation. At 8 DAI, hyphae were aggregated between the cuticle and epidermis and grew intracellularly in epidermal cells, palisade parachyma, and spongy mesophyll cells. At 16 DAI, chloroplasts were intact but decompartmentalized and infection sites were clearly defined. Acervuli were detected at 20 DAI and were fully developed at 24 DAI on adaxial and abaxial leaf surfaces. Sporulation (ruptured acervuli) was observed at 20 DAI. This clear understanding of the infection process can be used to look for resistance mechanisms in dogwood germplasm. A resistant line would expect to slow or inhibit one or more infection events. There was no statistical difference between the percentages of germinated conidia on susceptible and resistant cultivars of flowering dogwood one day after inoculation (DAI). However, the resistant cultivar, ‘Appalachian Spring’, significantly suppressed the growth of D. destructiva conidial germ tubes at 2 DAI, 3 DAI and 4 DAI when compared to conidial germ tubes on leaves of the susceptible cultivar ‘Cloud 9’. Observed resistance may be due to smoother wax crystals on adaxial leaf surface and significantly thicker cuticle observed on leaves of ‘Appalachian Spring’. An unknown compound, observed highly concentrated in resistant but lower in susceptible cultivars, may be important as a resistance mechanism. These strategies reduced the inoculum potential of D. destructiva and play important roles in why ‘Appalachian Spring’ is resistant to dogwood anthracnose. These results provide new ways to use conidia germination test and germ tube growth measurement for detecting resistant cultivars.

Flowering dogwood (Cornus florida L.) populations have experienced severe declines caused by dogwood anthracnose in the past three decades. Mortality has ranged from 48 to 98%, raising the concern that genetic diversity of this native tree has been reduced significantly. Microsatellite data were used to evaluate the level and distribution of genetic variation throughout much of the native range of the tree. In the first conducted study, we found that genetic variation in areas affected by anthracnose was as high as or higher than areas without die-offs. We found evidence of four widespread, spatially contiguous genetic clusters. However, there was little relationship between geographic distance and genetic difference. These observations suggest that high dispersal rates and large effective population sizes have so far prevented rapid loss of genetic diversity. The effects of anthracnose on demography and community structure are likely to be far more consequential than short-term genetic effects. The second study examined levels and distribution of genetic variation of C. florida throughout Great Smoky Mountains National Park (GSMNP). Significant genetic structure at both landscape and local levels were found. We infer that two genetic clusters exist within the park, mostly separated by the main dividing ridge of the Great Smoky Mountains. The differentiation is statistically significant, but subtle, with gene flow evident through low-elevation corridors. It seems unlikely that recent demographic dynamics have resulted in a depletion of genetic variation in flowering dogwoods.

Thesis (Ph. D.)--University of Tennessee, Knoxville, 2010. / Title from title page screen (viewed on July 13, 2010). Thesis advisor: Robert N. Trigiano. Vita. Includes bibliographical references.