- European Hazelnut (Corylus avellana L.) is an important crop in the Pacific Northwest where around 5% of the world's hazelnuts are produced. Hazelnut production in the Pacific Northwest is threatened by eastern filbert blight (EFB) caused by the pyrenomycete Anisogramma anomala. Host resistance is a useful strategy to control disease, and the first EFB resistance was identified in ‘Gasaway’ which transmits a dominant allele at a single locus. In the OSU hazelnut breeding program, ‘Gasaway’ has been widely used as a source of resistance. However, there is a concern about the durability of this single resistance gene since the pathogen may overcome it. Therefore, identification and investigation of novel sources of EFB
resistance are crucial for the hazelnut industry in Oregon.
Microsatellite markers, also known as simple sequence repeat (SSR) markers, have been widely employed in hazelnut genetics studies and breeding. Microsatellites have been identified using different techniques. This study used an in silico approach to mine polymorphic SSRs from the ‘Jefferson’ Pacific Biosciences (PacBio) (version 2) genome sequence in the EFB resistance regions on linkage groups 2 and 7 (LG2 and LG7). A total of 5,455 SSR motifs with at least eight repeats were identified in the 17 contigs predicted to be in the target regions on LG2 and LG7. Di-nucleotide repeats were most common (55%) and only they were pursued. After removal of duplicates, SSR-containing fragments were aligned with the Illumina genome sequences of seven C. avellana cultivars and inspected for polymorphism. Visual inspection identified 373 polymorphic SSR loci, and 96 primer pairs were designed. Primers were screened in 24 diverse hazelnut accessions and 45 showed polymorphism on agarose gels. A set of 48 hazelnut accessions plus the two parents of the reference mapping population (OSU 252.146 x OSU 414.062) were used for characterization of the new SSR markers. Of the 42 SSR loci that segregated in the mapping population, 21 were assigned to LG2 and 9 were assigned to LG 7.
For the investigation of eastern filbert resistance, a total of nine accessions were investigated, including OSU 533.129 from Michigan (US), three Molnar selections (3-04-05, 3-7-11, 3-12-62) from Simferopol (Crimea), four Molnar selections (3-04-23, 3-04-28, 3-04-30 and 3-13-40) from Holmskij (Russia), and OSU 1166.123 from Sochi (Russia). Structure or field inoculation approaches were used for disease inoculation. A set of 46 seedlings from each resistance source were screened with previously developed SSR markers on LG6, LG2, and LG 7 and correlation coefficients of disease response and SSR markers were calculated. Segregation from five Molnar selections (3-04-23, 3-04-28, 3-04-30, 3-13-40, and 3-12-62) fit the expected 1:1 ratio, indicating control by a single locus and a dominant allele for resistance. Molnar 3-04-05 and Farris OSU 533.129 showed an excess of resistant seedlings and fit a 3:1 ratio of resistant: susceptible. Progenies of Molnar 3-07-11 and OSU 1166.123 had 63% resistant seedlings and did not fit either a 1:1 or 3:1 segregation ratio. Resistance from Farris OSU 533.129 and Molnar selections 3-04-23, 3-04-28, 3-04-30, and 3-04-40 was assigned to LG6. Resistance from Molnar 3-12-62 and OSU 1166.123 was placed on LG7. Resistance from Molnar selections 3-04-05 and Molnar 3-07-11 was not correlated with marker allele scores for markers on LG6, LG2 or LG7.