Dissecting genetic resistance to fire blight in three pear populations

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

Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/11/2020
Publication Date: 4/22/2020
Citation: Zurn, J.D., Norelli, J.L., Montanari, S., Bell, R.L., Bassil, N.V. 2020. Dissecting genetic resistance to fire blight in three pear populations. Journal of Phytopathology. 10(7):1305-1311. https://doi.org/10.1094/PHYTO-02-20-0051-R.
DOI: https://doi.org/10.1094/PHYTO-02-20-0051-R

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 the bacterial pathogen Erwinia amylovora, is a persistent problem for pear (Pyrus spp.) growers in the United States. Growing resistant cultivars is one of the best options for managing fire blight. The cultivars Potomac and Old Home and the selection NJA2R59T69 display resistance to fire blight. As such, three mapping populations (‘El Dorado × ‘Potomac’, ‘Old Home’ × ‘Bartlett’, and NJA2R59T69 × ‘Bartlett’) were developed to identify genomic regions associated with resistance to fire blight. Progeny were phenotyped during 2017 and 2018 by inoculating multiple actively growing shoots of field-grown seedling trees with E. amylovora isolate E153n via the cut leaf method. Genotyping was conducted using the recently developed AxiomTM Pear 70 K Genotyping Array and chromosomal linkage groups were created for each population. An integrated two-way pseudo-testcross approach was used to map QTLs. Resistance QTLs were identified on chromosome 2 for each population. The QTLs identified in the ‘El Dorado × ‘Potomac’ and ‘Old Home’ × ‘Bartlett’ populations are in the same region as QTLs that were previously identified in ‘Harrow Sweet’ and ‘Moonglow’. The QTL in NJA2R59T69 mapped proximally to the previously identified QTLs and originated from an unknown Asian or occidental source. Future research will focus on further characterizing the resistance regions and developing tools for DNA-informed breeding.