Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 22, 2002
Publication Date: N/A
Interpretive Summary: Economic production of strawberry fruit is often limited by disease. Bacterial angular leafspot is an emerging disease for which no resistant cultivated variety is available, though two resistant wild strawberry plants have been identified. In order to develop resistant varieties, breeders must use these wild strawberry plants in cross-pollinations with cultivated varieties or advanced breeding program selections. Understanding the genetic control of resistance in the wild strawberry will help the breeders determine the number of seedlings that must be produced and the most efficient method to identify resistant seedlings with horticultural traits more like the cultivated varieties and less like the wild strawberry. We made cross-pollinations between a cultivated strawberry variety and each of the two resistant wild strawberry plants. We analyzed the seedlings from the resulting two populations for resistance to bacterial angular leafspot to determine how many were resistant and how many were susceptible. The results allowed us to determine that resistance is controlled by three to four strawberry genes and is recessive to susceptibility. This means that breeders must produce large populations in order to identify resistant seedlings with desired horticultural traits, and that selection for resistance before selection for improved horticultural traits is more efficient than the reverse order of selection. Therefore, these findings should be of value to strawberry breeders interested in speeding up the development of varieties resistant to bacterial angular leafspot disease and to growers interested in reducing economic losses.
Technical Abstract: Bacterial angular leafspot disease (Xanthomonas fragariae Kennedy and King) of strawberry (Fragaria species and F. × ananassa Duch. cultivars) has become increasingly important to strawberry fruit and plant production. Strawberry cultivars and species vary in susceptibility to infection. However, little is known regarding epidemiology of the disease and resistance to infection. Two octoploid genotypes, a native F. virginiana (US 4808, tested as SG-89) and an F. virginiana (SG 26) × F. × ananassa ('Earliglow') hybrid (US 4809, tested as 80-4-38), previously were found to be highly resistant to two differentially pathogenic strains of X. fragariae representing two of four genotypic strain groups. Our objective was to determine the number of genes involved with resistance for these two strawberry genotypes, whether strawberry resistance is conferred by dominant or recessive alleles, and whether or not the heritability is high enough for breeders to reliably make selections of resistant individuals in breeding populations. Approximately 120 F1 seedlings from crosses of susceptible 'Sweet Charlie' with each of the two resistant genotypes were clonally propagated and challenged with each of four X. fragariae strains. These strains were selected to represent four genotypes of X. fragariae defined by repetitive element based PCR: ATCC 33239, Xf-3, Xf6, and Xf-1425. Plants were quantitatively rated on a scale of 0 (resistant) to 5 (susceptible) in replicated evaluations. High estimates for broad sense heritability support the conversion of the quantitative disease scores to qualitative scores and the classification of genotypes as resistant or susceptible. The qualitative ratings were used to estimate the number of genes involved with resistance. Some segregation ratios fit a 7S:1R ratio, and others fit a 15S:1R ratio, indicating that three or four unlinked loci could explain the inheritance of resistance in these populations. The high estimates for broad sense heritability show that resistant progeny can be selected with confidence, though large populations will be needed to identify enough resistant progeny from which to select for other important traits.