Mapping fire blight resistance in three pear populations

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

Submitted to: Plant and Animal Genome Conference
Publication Type: Abstract Only
Publication Acceptance Date: 10/31/2018
Publication Date: N/A
Citation: N/A

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 pears (Pyrus spp). Growing resistant cultivars is the most effective disease control measure. Unfortunately, all major cultivars grown in the U.S. are susceptible to this disease. Thus, this important disease was targeted in the USDA-NIFA-Specialty Crop Research Initiative-Funded ‘RosBREED’ Project. A recently developed 70K Pear Axiom® Genotyping Array was used to construct genetic maps and conduct QTL analysis for three populations (‘El Dorado × ‘Potomac’, ‘Old Home’ × ‘Bartlett’, and NJA2R59T69 × ‘Bartlett’) expected to segregate for fire blight resistance. Multiple actively growing shoots of field grown trees were inoculated with E. amylovora Ea153n in 2017 and 2018. The proportion of current season’s shoot that was blighted was calculated. Data for each population were severely positively skewed 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. An integrated two-way pseudo-testcross approach was used to map QTLs. A single significant QTL (a = 0.05) was identified for each population. Interestingly, all three of these QTLs mapped to a similar region on chromosome 2. QTLs have been 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’. Additional work is needed to characterize this unique genomic region.