Strong evidence for a fire blight resistance gene of Malus robusta located on linkage group 3

Abstract

Fire blight (FB), caused by the Gram-negative bacterium Erwinia amylovora is a dangerous disease on pome fruit, including apple. The FB-susceptible cultivar ÔIdaredÕ was crossed with the resistant wild species clone Malus · robusta 5. A segregating population of 146 progeny has been tested by artificial shoot inoculation for susceptibility to FB. Progeny were infected from 0% to 100% of the shoot length. To identify chromosomal regions or loci responsible for resistance to FB of Malus · robusta 5, a set of microsatellite markers (simple sequence repeat, SSRs) was chosen covering all linkage groups of apple. Up to eight different microsatellites were bulked to one mutliplex PCR using four different labels and a fifth label for a size standard. Fifty-nine microsatellite markers out of 72 SSRs were polymorphic. Fifty-four of 66 loci detected could be mapped and were useful for the detection of related resistant loci. Alleles of microsatellites Hi03d06, CH03g07 and CH03e03 originating from the resistant donor M. robusta were associated with resistance to Erwinia amylovora. Up to eighty percent of the phenotypic variation could be explained by the interval spanned by SSRs CH03g07 and CH03e03, indicating the presence of a major resistance gene. All three microsatellites are located on the distal part of linkage group 3, spanning 15 cM. The SSR marker CH03e03 can be regarded as diagnostic marker for FB resistance. Only seven progeny expressing allele b (184 bp) of CH03e03 showed blighted shoot lengths of more than 30% and only nine progeny lacking allele b showed blighted shoot lengths of <30%. By setting a threshold of 30% shoot necrosis for resistance to FB, the 146 individuals segregate into 71 susceptible and 75 resistant plants, and resistance to FB maps 9 cM away from marker CH03e03.

Assessment of fire blight tolerance in apple based on plant inoculations with Erwinia amylovora and DNA markers

“…We found a total of 10 alleles in this material; 184,185,188,198,200,202,204,206,213 and 217 bp. Malus robusta 5 (184 bp according to Peil et al 2007) was the only one that had an allele really close to 184 bp in our study (actually 184.57 and 184.61 in two different runs). Thirteen cultivars instead had a somewhat larger allele, here denoted as 185 bp (185.18-185.23) which is, in all likelihood, not associated with resistance to fire blight.…”

“…The cultivars in this subset were screened with 12 SSR loci to obtain a data set for estimation of similarity (and putative relatedness) among all the cultivars in general, as well as similarity to 'Cox's Orange Pippin' in particular. Some additional samples, references M. robusta 5, 'Discovery' and 'Prima' (Peil et al 2007(Peil et al , 2008 and 8 cultivars, 'Elise', 'Filippa', 'Goldparmäne', 'Laxton's Superb', 'Mio', 'Norrstack', 'Ribston' and 'Spässerud' that had been inoculated with fire blight for a previous study (Sehic et al 2009) were screened for one of the SSR loci, CH03e03, which has proved to be associated with a fire blight QTL on LG 3 in M. robusta 5 (Peil et al 2007(Peil et al , 2008.…”

“…Since many cultivars obviously have only one of the markers, some of the cultivars with both markers may carry these on separate chromosomes and therefore may still lack the QTL. One SSR locus, CH03e03, which has previously proved to be associated with a fire blight QTL on LG 3 in M. robusta (Peil et al 2007(Peil et al , 2008, was screened in the 23 cultivars chosen for inoculation tests in the present study, as well as in 8 additional cultivars and three references. We found a total of 10 alleles in this material; 184,185,188,198,200,202,204,206,213 and 217 bp.…”

“…Considerable differences among apple cultivars in susceptibility to fire blight have been reported, and heritability estimates for field resistance have been determined (Hampson and Sholberg 2008). Major QTLs for fire blight resistance have been found on linkage group (LG) 3 in M. robusta 5 explaining up to 80% of the phenotypic variation (Peil et al 2007(Peil et al , 2008) and on LG 12 in the ornamental cultivar 'Evereste' (possibly a derivative of a wild Malus species) and in M. floribunda clone 821 (Durel et al 2009). Promising sources of fire blight resistance in M. fusca and M. sieversii are also being investigated at present Peil et al 2010;Baumgartner et al 2011).…”

Excised Leaf Bioassay: Phenotypic Expression of Natural Resistance in Apple against the Obliquebanded Leafroller

“…The major fire blight QTL has been mapped to linkage group 3 in M. robusta, and explains up to 80% of the phenotypic variation [74] [75]. However, the QTL mapped to linkage group 7 for fire blight resistance in 'Fiesta' (M. domestica) explains up to 46% of the phenotypic variation, and has been tracked back to its parent, 'Cox's Orange Pippin' [76] [77].…”