1a. Objectives (from AD-416):
The objectives of this project are 1) conduct a QTL analysis of fire blight resistance in the Malus sieversii GMAL4593 mapping population; 2) identify M. sieversii-specific SNPs to add additional markers to the genetic map of M. sieversii; and 3) establish plantings of M. sieversii in Washington and West Virginia for the identification of additional sources of fire blight resistance in the future.
1b. Approach (from AD-416):
Molecular map for Malus sieversii GMAL4593 mapping population is being developed. Fire blight resistance of population will be determined and used with the molecular map to identify a new genetic locus (QTL) conditioning resistance to fire blight in apple. New plantings of 192 M. sieversii accessions will be established in two distinct environments (Washington and West Virginia) in collaboration with Dr. Kate Evans, Washington State University, for future identification of new sources of fire blight resistance.
3. Progress Report:
Fire blight of apple is a devastating disease that can kill young trees outright. The limited number of effective management practices available to growers makes it difficult to slow the progress of fire blight epidemics. Although host plant resistance is one of the most effective and sustainable options for managing this disease, sources of fire blight resistance from wild apple species with small, bitter, or astringent fruit have negative effects on fruit quality that are difficult to overcome by conventional breeding methods. Wild Malus sieversii, previously collected by USDA-ARS in Kazakhstan, is an excellent source of disease resistance for apple breeding because of its large and palatable fruit that facilitate rapid development of new varieties with superior eating quality. In order to identify genomic regions in M. sieversii associated with resistance to fire blight, over 725 molecular markers were screened in a population (171 individuals) resulting from a cross between 'Royal Gala' apple and M. sieversii PI631981. The current map contains a total of 355 markers, with 285 markers mapping in the ‘Royal Gala’ parent, 132 mapping in the M. sieversii parent, and 62 markers mapping in both parents. All of the 17 chromosomes (linkage groups) of apple are covered by the current map with an average marker density of one marker every four cMorgans. This map provides an excellent foundation for additional research on this population and M. sieversii. The map will be deposited and publicly available at the Genome Database for the Rosaceae on the World Wide Web. The population was evaluated for fire blight resistance in the greenhouse and in the field over multiple years using multiple strains of the pathogen to insure that the effects of both environment and genetics were taken into consideration in the genetic analysis. Fire blight resistance in this mapping population was associated with molecular markers in two different regions of the genome. This genetic map will now be integrated with the apple genome sequence and expressed sequence tag databases to identify candidate fire blight resistance genes within these genomic regions. After validation, markers for the resistance genes will be made available to U.S. tree fruit breeding programs to facilitate the development of fire blight resistant apple and pear cultivars by marker assisted breeding. To identify better sources of fire blight resistance for future breeding, all available field and greenhouse data on fire blight resistance and fruit quality of over 1,000 M. sieversii accessions collected by the USDA in Kazakhstan was analyzed, and 193 accessions were selected for evaluation. Replicate trees of the 193 selected accessions and nine control cultivars were propagated, and field plantings were established at Wenatchee, WA, and Kearneysville, WV. These plants will be used to identify superior sources of disease resistance for use in apple breeding programs.