Genetic resistance to fire blight in three pear populations maps to chromosome 2

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

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 3/15/2019
Publication Date: 8/3/2019
Citation: Zurn, J.D., Norelli, J.L., Montanari, S., Bell, R.L., Bassil, N.V. 2019. Genetic resistance to fire blight in three pear populations maps to chromosome 2. Journal of Plant Health. 2019 Plant Health Conference.

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 a constant problem for pear (Pyrus spp.) growers in the U.S. Resistant cultivars are the best option for managing fire blight. Fire blight resistance has been observed in the cultivars Potomac and Old Home and the selection NJA2R59T69. Mapping populations (‘El Dorado × ‘Potomac’, ‘Old Home’ × ‘Bartlett’, and NJA2R59T69 × ‘Bartlett’) were developed to identify the genomic regions associated with fire blight resistance in these sources. Progeny were genotyped with the recently developed Axiom Pear 70K Genotyping Array. Phenotyping was conducted in 2017 and 2018 by inoculating multiple actively growing shoots of field grown trees with E. amylovora isolate E153n via the cut leaf method. Severity was calculated by determining the percentage of blight on the current season’s shoot. Chromosomal linkage groups were created for each population and an integrated two-way pseudo-testcross approach was used to map QTLs. In each population, a single resistance QTL was identified at a similar region on chromosome 2. Fire blight resistance QTLs have been identified in this region for ‘Harrow Sweet’ and ‘Moonglow’ (parent of ‘Potomac’). The resistance in NJA2R59T69 originated from the P. ussuriensis selection ‘Illinois 76’ and not a P. communis source like ‘Potomac’ and ‘Old Home’. Additional work is needed to further characterize this resistance region.