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R.N. Trigiano, M.H. Ament, M.T. Windham, and J.K. Moulton

Cornus kousa Hance (Korean or kousa dogwood) cultivars are increasingly used as landscape plants because they lack the disease and insect problems typically associated with the native C. florida L. (flowering dogwood). A number of red-bracted kousa dogwood cultivars are now available and several are phenotypically indistinguishable from one another. Plants of six cultivars obtained from three nurseries were characterized genetically using deoxyribonucleic acid (DNA) amplification fingerprinting (DAF) and arbitrary signatures from amplification profiles (ASAP). DAF profiles of three red-bracted cultivars—`Rosabella', `Satomi' and `Heart Throb'—were nearly identical. ASAP also failed to clearly differentiate these cultivars and indicated consistent genetic similarities. In contrast, another red-bracted cultivar `Christian Prince' and two white-bracted cultivars—`Little Beauty' and `Samaritan'—were identified and separated from all other cultivars by both DAF and ASAP techniques.

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R. N. Trigiano, G. Caetano-Anollés, B. J. Bassam, and M. T. Windham

DNA Amplification Fingerprinting (DAF) was used to characterize ten isolates of Discula destructiva Redlin and three isolates of an undescribed species of Discula, the causal organisms of dogwood (Cornus species) anthracnose. Isolates were obtained throughout the disease range in the eastern United States and DAF profiles generated with ten arbitrary oligonucleotide primers. Very few polymorphic loci (27/298) were detected between isolates of D. destructiva; whereas, a greater number were observed between and among the isolates of Discula species. Relationships among and between the two fungal groups were analyzed using PAUP and UPGMA and indicate that the genome of D. destructiva is highly conserved throughout the distribution. In contrast, isolates of Discula species exhibited greater variability. This suggests that D. destructive was recently introduced to the eastern United States.

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James Hill Craddock, R.J. Sauve, S.E. Schlarbaum, J. Skinner, R.N. Trigiano, M.T. Windham, and W.T. Witte

Hand pollinations and honey bees were used to cross Cornus florida cultivars in a series of experiments investigating dogwood pollination biology from a breeding viewpoint and testing the use of insects (domestic honey bees and ladybug beetles as pollinators in dogwood breeding. Experiments were conducted to study possible incompatibility between dogwood cultivars and to determine if self-compatibility and self-fertility occur in Cornus florida. Since 1993, ≈200 seedlings have been produced by hand and insect-mediated pollinations. Honey bees can be used in dogwood breeding. Trees cross pollinated by ladybeetles had lower fruit set than trees cross pollinated by honey bees. Greenhouse forcing to accelerate anthesis and cold storage to delay the onset of bloom of container-grown trees can extend the dogwood breeding season effectively.

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Yonghao Li, Mark T. Windham, Robert N. Trigiano, Donna C. Fare, James M. Spiers, and Warren E. Copes

Infection process of Puccinia hemerocallidis, the causal agent of daylily rust, and resistance responses in eight daylily cultivars, were studied macroscopically and microscopically. After germination of urediniospores, appressoria were formed at the tip of germ tubes and penetrated through stomatal openings. Intercellular hyphae aggregated and formed uredia under the infection sites, and released urediniospores after rupturing the epidermis. In highly resistant cultivars `Prairie Blue Eyes' and `Bertie Ferris', intercellular hyphal growth was restricted and uredia were not formed. No macroscopic symptoms of the disease were present on the leaf surface, although a few collapsed cells were observed microscopically. Both resistant and moderately resistant reactions were characterized by necrotic lesions with many collapsed cells under infection sites. The difference between these two reactions was that uredia and urediniospores were observed on the moderately resistant cultivar `Chicago Apache', but not on resistant cultivars, `Buttered Popcorn' and `Stella De Oro'. Sporulation was observed on both moderately susceptible and susceptible cultivars, but latent periods were delayed and the amount of urediniospore production was reduced on moderately susceptible cultivars, `Mary Todd' and `Chorus Line', compared to the susceptible cultivar `Pardon Me'. The results indicate that the hypersensitive cell death is one of the resistance responses to daylily rust, but necrotic lesions on leaf surfaces are associated with the amount of collapsed host cells. The delayed latent periods and reduced sporulation that resulted from restricted intercellular hyphal growth could represent another resistance mechanism in the daylily rust pathosystem.

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Xinwang Wang, Robert N. Trigiano, Mark T. Windham, Renae DeVries, Timothy A. Rinehart, James M. Spiers, and Brain Scheffler

The genus Cornus consists of many species, of which C. florida, C. kousa, C. mas, and C. stolonifera are four main ornamental species in North America, Asia, and Europe. For example, over 200 cultivars of C. florida alone have been developed for the nursery industry. Microsatellite loci, or SSR, are useful markers for studying genetic diversity and for creating linkage maps of the various species. The objective of this study was to investigate the genetic diversity between these four Cornus species and eight hybrids. Evaulation of the diversity will be useful in assessing the selection pressure of breeders and/or genetic drift of these dogwood cultivars/lines. Fifteen SSR primer pairs were selected to examine 56 Cornus cultivars and/or lines of the four species and hybrids. The study included 28 C. florida cultivars and lines, 15 C. kousa cultivars and lines, four C. stolonifera cultivars, one cultivar of C. mass and eight hybrids between various Cornus species. An exceptionally high level of diversity was detected among the 56 entries in both the number and size range of SSR alleles. A total of 95 alleles with an average of 7.8 alleles per loci were detected among these 56 genotypes. These selected Cornus cultivars and/or lines could be clustered into four to six subgroups. Some Cornus species were integrated into other species groups, suggesting gene flow between species via the breeding or evolution. SSR markers can contribute to the exploitation of genetic diversity for existing Cornus germplasm. For further study, examination of more SSR loci could explain more completely the diversity among these Cornus cultivars and lines.

Free access

Phillip A. Wadl, Xinwang Wang, Andrew N. Trigiano, John A. Skinner, Mark T. Windham, Robert N. Trigiano, Timothy A. Rinehart, Sandra M. Reed, and Vincent R. Pantalone

Flowering dogwood (Cornus florida) and kousa dogwood (C. kousa) are popular ornamental species commonly used in the horticultural industry. Both trees are valued for their beautiful floral display and four-season appeal. Species-specific simple sequence repeat (SSR) loci were used to genotype and assess genetic diversity of 24 flowering dogwood cultivars and breeding lines and 22 kousa dogwood cultivars. Genetic diversity was determined by allele sharing distances and principal coordinate analysis and was high in both species. Molecular identification keys were developed for cultivars and breeding lines of each species using a few polymorphic SSRs loci (four in C. florida and five in C. kousa). Most (18 of 24) of the flowering dogwood and all (22 of 22) kousa dogwood accessions could be distinguished from each other using these SSRs; those that could not were resolved using DNA amplification fingerprinting. The reliability of both keys was assessed using five anonymous cultivars for each dogwood species, which were correctly identified using the molecular keys. The genetic information presented here will be useful for identification and verification of cultivars for nurseries and as molecular markers for breeders and researchers.

Open access

Katherine M. Solo, Sara B. Collins, Madalyn K. Shires, Ron Ochoa, Gary R. Bauchan, Liesel G. Schneider, Alan Henn, James C. Jacobi, Jean L. Williams-Woodward, M.R. Hajimorad, Frank A. Hale, John B. Wilkerson, Alan S. Windham, Kevin L. Ong, Mathews L. Paret, Xavier Martini, David H. Byrne, and Mark T. Windham

The eriophyid mite, Phyllocoptes fructiphilus, vectors the causal agent, Rose rosette virus (RRV), that results in rose rosette disease. Parts of the southeastern United States have remained free of the disease, except for infected plant material introductions that were eradicated. A survey of sampling points through Alabama, Georgia, and Mississippi (n = 204) revealed the southeastern border of RRV. The presence of RRV in symptomatic plant tissue samples (n = 39) was confirmed by TaqMan-quantitative reverse transcription polymerase chain reaction (RT-qPCR). Samples were also collected at every plot for detection of eriophyid mites, specifically for P. fructiphilus. Three different species of eriophyid mites were found to be generally distributed throughout Alabama, Georgia, and Mississippi. Most of these sites (n = 60) contained P. fructiphilus, found further south than previously thought, but in low populations (<10 mites/gram of tissue) south of the RRV line of incidence. Latitude was found to be significantly correlated with the probability of detecting RRV-positive plants, but plant hardiness zones were not. Plot factors such as plant size, wind barriers, and sun exposure were found to have no effect on P. fructiphilus or the presence of RRV. The reason for the absence of RRV and low populations of P. fructiphilus in this southeast region of the United States are unclear.