Progress 01/01/04 to 09/30/06
Outputs
The overall objective of the project is to advance efforts in powdery mildew disease management through host resistance and generate information needed to further powdery mildew resistance breeding and the development of a sustainable disease management system in dogwood. Specific objectives of the project were: 1) morphologic and genotype characterization of resistance selections; 2) identification of molecular markers for powdery mildew host resistance; 3) evaluation of P. guttata and M. pulchra prevalence and their role in disease severity. This project was partly funded by USDA/CREES Capacity Building Grant CRIS NO 0193424 which ended in 2005. Ten plants that displayed high level of resistance to powdery mildew over a five-year period were evaluated in different locations in Tennessee, Kentucky, North Carolina, and Mississippi. Four selections exhibited high resistance at all locations, but six were susceptible at the Kentucky location. Plants that exhibited
resistance at multiple locations represent new sources of powdery mildew resistance for resistance breeding. Only Erysiphe (sect. Microsphaera) pulchra was observed at all test plots. Amplified fragment length polymorphisms (AFLP) showed that the new selections were genetically diverse and none of the selections were identical. This work has been accepted for publication in the Hortscience Journal. Further analysis of the resistant selections using Inter-Simple Sequence Repeats (ISSR) primers revealed molecular markers that were highly specific to different Cornus species and to some cultivars; these may be useful in DNA fingerprinting of Cornus species and identification of hybrids. F1 progeny from resistant x susceptible parents suggested that resistance may be quantitative; AFLP, ISSR or microsatellite primers did not show molecular markers associated with powdery mildew resistance. DNA analysis using E. pulchra and P. guttata ITS- species-specific primers on powdery mildew mycelia
and conidiospores in samples from Tennessee, Kentucky, North Carolina, Virginia, South Carolina, Georgia, New York, Michigan and Louisiana detected E. pulchra in all samples; P. guttata was detected at one location in Kentucky. E. pulchra ascocarps were abundant at all locations; P guttata ascocarps were few and not associated with detectable mycelia. When ascocarps of the two pathogens occurred on the same leaf, the conidiospores tested positive for E. pulchra only. Thus, E. pulchra was the major pathogen causing powdery mildew symptoms; P. guttata had insignificant role. DNA analysis of E. pulchra and P. guttata using microsatellite-primed PCR identified five primers (GAC) 5, (GACA) 5, (TGTC) 4, (GTG) 5 and (GTGTGG) 3 that can distinguished E. pulchra, from P. guttata. In addition, AFLP analysis of the genomic DNA using 16 EcoR I/Mse I primer pairs showed prominent polymorphisms that were distinct to E. pulchra or P. guttata. Both AFLP and microsatellites may be useful in disease
diagnosis of the anamorph stage of powdery mildew and to study E. pulchra genetic diversity, but time and resources did not allow the completion of this project.
Impacts
Identification of powdery mildew resistance will provide a genetic resource that may be used in breeding for host resistance and in the release of new powdery mildew resistant cultivars. Information on genetic analysis of the resistant selections is needed in new cultivar releases to ensure that the new plants are genetically diverse, genetically different from each other and other commercial cultivars. New cultivar releases that are genetically diverse will add to genetic diversity in the production system and this is will help to enhance resistance durability. The use of powdery mildew resistant plants will reduce the need for fungicide applications in dogwood production, reduce production expenses and increase grower income from dogwood sales. The use of host resistance will also reduce pesticide exposures and environmental contamination hazards associated with fungicide applications and result in economic and environmental benefits.
Publications
- Mmbaga, M.T., Sauve, R. and Sheng, H. 2006. Molecular analysis of genetic diversity in powdery mildew resistant dogwood selections. HortScience Journal. 41:(In Press).
- Mmbaga, M. T., Avila, F. J., Howard, E. F., and Myles, L. A. 2006. Accumulation of a protein associated with powdery mildew resistant flowering dogwood (Cornus florida). Envir. Hortscience Journal. 24:115-118.
- Shi, A., and Mmbaga, M.T. 2006. The identification of Erysiphe australiana as the powdery mildew pathogen of crape myrtle (Lagerstroemia indica) in mid Tennessee. Plant Disease Journal 90:1098-1101.
- Mmbaga, M.T., Klopfenstein, N., Kim, M., and Mmbaga, N.C. 2004. PCR-based DNA analysis of powdery mildew pathogens of dogwood (Cornus spp.). Forest Pathology 34: 321-328.
- Mmbaga, M. T. and Suave, R.J. 2004. Evaluation for multiple disease resistance in dogwood for three foliar pathogens. J Arboriculture: 30(2):101-10.
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Progress 01/01/04 to 12/31/04
Outputs
Powdery mildew resistant selections and several commercial cultivars including ten C. florida commercial cultivars and four hybrids between C. kousa and C. florida were analyzed using amplified fragment length polymorphisms (AFLP) technique, and Inter-Simple Sequence Repeats (ISSR). AFLP analysis showed that all resistant plants were distinct. Data analysis using the Phylogeny Inference Package (PHYLIP) and a matrix of pair-wise Euclidean distances showed the associations among genotypes. The new selections of C. florida exhibited high genetic diversity and cluster analysis revealed genotypes that could replace susceptible cultivars and enhance genetic diversity in the production system. Plants that displayed superior resistance and horticultural characteristics have been propagated and evaluation in four locations in Tennessee, one location in Kentucky, North Carolina, and Mississippi. Some plants that displayed high resistance in Tennessee and North Carolina were
susceptible in Kentucky. However, 4 genotypes continued to exhibit high resistance at all locations and three are being prepared for cultivar release. Only one pathogen, Erysiphe (sect. Microsphaera) pulchra was observed on the test plots even in Kentucky where Phylactinia guttata also occurs. Ascocarps of P. guttata are very scarce and inoculation with P. guttata has not been successful. Crosses between the powdery mildew resistant selections and susceptible Cherokee Princess did not produce resistant plants progeny. However, crosses between Cherokee Brave and the resistant selections produced segregations of resistant and susceptible progeny. The gradation of resistant levels showed a continuous trend and suggested that powdery mildew resistance may be governed by several genes and not by major genes. It was thus imperative that a different technique have to be used instead of the AFLP analysis proposed in the work plan. Crosses between Cherokee Brave and resistant selections
produced segregating F1plants that can be analyzed for resistance gene analogs (RGAs).Ascocarps of P. guttata were observed in very few samples at only one location in Kentucky while ascocarps of E. pulchra were observed in all locations and on almost all infected leaves. The few leaf samples that had ascocarps of P. guttata only, showed very mild symptoms with almost undetectable mycelia while leaves that carried E. pulchra ascocarps were consistently covered with white powdery mycelia typical of powdery mildew infections. DNA analysis of the mycelia of powdery mildew fungi using ITS-specific primers for P guttata and E. pulchra showed the occurrence of E. pulchra only, even when ascocarps of the two pathogens were present. Although powdery mildew infection from ascospores has not yet been reproduced by artificial inoculation of healthy plant, observations indicated that E. pulchra is the major pathogen associated with powdery mildew symptoms and P. guttata has an insignificant
role.
Impacts
New cultivars that have powderey mildew resistance will reduce the need for fungicide applications in dogwood production; it will reduce expenses associcated with fungicide applications and thus increase grower income from dogwood sales.
Publications
- No publications reported this period
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