Location: Southern Horticultural Research
2008 Annual Report
1a. Objectives (from AD-416)
The objective of this cooperative research project is to develop strategies for breeding new and improved woody ornamental germplasm adapted to the southeastern United State that exhibit host-plant resistance to diseases, insects, and other pests.
1b. Approach (from AD-416)
1)Elucidate the genetic/genomic bases of host-plant resistance to microbes, insects and/or nematodes in woody ornamentals, especially those characteristic of the southeastern United States; 2) Apply the knowledge gained through the preceding research to developing effective genetic enhancement and/or improvement strategies for host-plant resistant woody ornamentals adapted to the southeastern United States utilizing molecular and conventional breding techniques.
3. Progress Report
One hundred and thirty-six cultivars of rose with industry resistance claims were evaluated for black spot and cercospora leaf spot at two locations in Tennessee and in south Mississippi. Nineteen cultivars were rated as highly resistant (> 2% foliage symptomatic over the growing season). For most roses tested, industry resistance claims could not be substantiated. Ninety cultivars of hydrangea were evaluated for resistance to powdery mildew and 20 cultivars have been rated as resistant (> 2% of foliage affected) or moderately resistant (>10% of foliage affected). Increased shading (0, 30%, 60%, 90%) reduced severity of cercospora leaf spot in hydrangea. At 0 and 30% shading, cultivars could be separated by levels of resistance to the disease but disease levels were too low at higher shade levels to distinguish resistance levels. Hydrangea cultivars ‘Nikko Blue’ (susceptible) and ‘Veitchii’ (resistant) were examined microscopy for differences in response to infection by Erysiphe polygoni. Resistance to powdery mildew in ‘Veitchii’ was evident by manifestation of early accumulation of callose, higher percentage of necrotic infected cells and restricted colony development compared to the susceptible ‘Nikko Blue’. Microsatellite loci (simple sequence repeats: SSR) were identified from Cornus kousa ‘National’. Primer pairs for 86 loci were developed and of these eight were optimized and screened using genomic DNA from 22 C. kousa dogwood cultivars. All optimized loci were polymorphic and the number of alleles per locus ranged from 3 to 17. Observed heterozygosity ranged from 0 to 0.31 and expected heterozygosity ranged from 0.38 to 0.91. Intraspecific crosses between C. kousa ‘Galilean’ and ‘Blue Shadow’ were made using honey bee pollination and 173 seeds were produced. Hybridization between the two cultivars was confirmed using the 8 optimized and characterized SSRs. Seedlings were planted at a nursery for evaluation. Attempts at interspecific crosses between the two Cornus species failed. Five SSR markers from C. kousa and four SSR markers from flowering dogwood (C. florida) were used to construct a dichotomous key for 22 and 24 cultivars respectively. A genetic linkage map of flowering dogwood was constructed using 94 individuals derived from a cross of two F1 trees designated 97-6 and 97-7, which were originally from a cross between ‘Appalachian Spring’ and ‘Cherokee Brave’. Out of approximately 800 SSR loci examined, 271 were polymorphism between ‘Appalachian Spring’ and ‘Cherokee Brave’, but were monomorphic between 97-6 and 97-7. These 271 segregating markers were screened to build a linkage map for flowering dogwood (n=11). Eleven linkage groups were obtained and consisted of 255 SSR loci, which spanned a total of 1175 centimorgans (cM) with an average internal distance of 4.6 cM. This is the first linkage map of flowering dogwood and will be a fundamental tool for new gene identification and marker-assisted selection in our flowering dogwood breeding program. The ADODR or his representative meets regularly with the cooperating scientist, and these meetings include site visits.
4. Accomplishments
