Genetic characterization of fire blight resistance in three pear populations

Authors: Zurn, Jason; Norelli, John; MONTANARI, SARA - University Of California, Davis; Bell, Richard; Bassil, Nahla

Submitted to: American Society of Horticulture Science Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 3/1/2019
Publication Date: 7/21/2019
Citation: Zurn, J.D., Norelli, J.L., Montanari, S., Bell, R.L., Bassil, N.V. 2019. Genetic characterization of fire blight resistance in three pear populations. American Society of Horticulture Science Meeting. American Society of Horticulture Science Meeting.

Interpretive Summary: Fire blight is a devistating bacterial disease of pears. growing resistant varieties is the most effective way to control the disese. Unfortunately, all major pear varieties grown in the U.S. are not resistant. To identify resistant genes to breed into new pear varieties a genetic mapping experiemtn was conducted. Three resistant pear varieties were crossed with non-resistant varieties to identify where the gene or genes are located within the pear genome. The offspring were grown in a field for 10 years prior to infecting them with the pathogen. Plants were infected by dipping scisors in a bacterial mixture and then cutting the leaves on 10 different stems on the plant. At the end of the growing season the stems were measured for how long they were and how much infection was observed. The experiment was conducted in both 2017 and 2018. Genetic information was gained for each of the plants using the Axiom pear genotyping chip. Genetic maps were constructed for each population and a statistical analysis was conducted to find associations between disease resistance and the genetic information. In each population a single gene was found to provide resistance. Interestingly, these genes were found in a similar location in the pear genome. Additional work is needed to determine if the genes that provide resistance are the same or different.

Technical Abstract: Fire blight, caused by Erwinia amylovora, is the most devastating bacterial disease of pear (Pyrus spp). This disease is a constant problem for the primary pear production areas of the U.S. Planting resistant cultivars is the best option for managing fire blight. Unfortunately, all major cultivars grown in the U.S. are susceptible to this disease. The cultivars Potomac and Old Home and the selection NJA2R59T69 are resistant to fire blight and can be used to create new cultivars with traits desired by growers and consumers. Therefore, three mapping populations (‘El Dorado × ‘Potomac’, ‘Old Home’ × ‘Bartlett’, and NJA2R59T69 × ‘Bartlett’) were developed to identify genomic regions associated with fire blight resistance. The populations were initially genotyped with a DNA fingerprinting set to identify progeny that were the result of an unexpected pollination event. Progeny which had the expected pedigree were selected for continued analysis and were genotyped with the recently developed Axiom Pear 70K Genotyping Array. In 2017 and 2018, multiple actively growing shoots of field grown seedling trees were inoculated with E. amylovora Ea153n via the cut leaf method. The percentage of current season’s shoot that was blighted was calculated. The phenotypic distributions for each population were severely skewed toward resistance and the number of genes mediating resistance could not be easily discerned. Chromosomal linkage groups were established for each population using a cross-pollinating mapping approach with JoinMap 5. Each map had approximately 30,000 markers distributed across the whole genome. The average total length of the high quality maps was 1,087 cM. An integrated two-way pseudo-testcross approach was used to map QTLs using MapQTL 6. A single significant QTL (a = 0.05) mediating resistance was identified for each population in a similar region on chromosome 2. Fire blight resistance QTLs have been previously reported in this region for ‘Old Home’ and ‘Moonglow’ (a parent of ‘Potomac’). The presence of the chromosome 2 QTL in NJA2R59T69 is interesting as the resistance originated from the P. ussuriensis selection ‘Illinois 76’ and not a P. communis source like ‘Potomac’ and ‘Old Home’. This could suggest that the resistance in this region may be due to a conserved basal defense response. Additional work is needed to further characterize this unique genomic region.