Genetics of Powdery Mildew Resistance, Bacterial Canker Resistance, and Albinism in U.S. Sweet Cherry Germplasm

Alexandra M Johnson

doi:10.7273/000005217

Sweet cherry is an economically important crop grown in the U.S. Pacific Northwest; however, production is currently limited due to the endemic pathogens responsible for powdery mildew and bacterial canker. A few powdery mildew resistant cultivars are available for use as breeding parents, but it is not clear if these arose from the same genetic source or are different and thus combinable for more durable resistance. Information on cultivars that are resistant to bacterial canker disease are even more vague, with different reports producing conflicting results for some cultivars. Even with mildew resistant parents and greater insight into bacterial canker response, breeding efforts are stymied by deleterious alleles responsible for albinism that leads to an expected one-quarter of seedlings not fit for adulthood when two carrier parents are crossed. Limited studies have only examined a few cultivars for possible carrier status for all three traits. The Pacific Northwest Sweet Cherry Breeding Program in Prosser, WA, grows diverse cultivars and also has access to the USDA-ARS National Clonal Germplasm Repository’s germplasm collection of highly diverse accessions of sweet cherry. Therefore, the objectives of this work were to identify genetic sources and carriers of valuable alleles for powdery mildew and bacterial canker resistance, as well as deleterious alleles for albinism in U.S. cherry germplasm. Over two years, a total of 97 individuals (genotypically unique cultivars, selections, and accessions) were phenotyped for response to powdery mildew. Twenty individuals were identified as resistant to mildew infection and 77 as susceptible. Genetic dissection revealed three allelic sources of resistance arising from the same genetic locus: cultivars Bigarreau Reverchon and Hedelfinger and a group of five “Mildew Immune Mazzards”. Over three years, a total of 90 genotypically unique cultivars were phenotypically screened using an existing laborious assay (whole leaf) and a newly devised streamlined assay (detached leaf disk). Correlation of assay types conducted via a Pearson’s test revealed that the detached leaf disk assay is as robust as the previously reported whole leaf assay for determining response to bacterial canker infection. Phenotypic responses identified two cultivars, ‘Black Eagle’ and ‘Persian’, as fully resistant to infection from bacterial canker for all assays (infection score of 0, using a 0-4 scale, for all leaves, for all assays). Nineteen cultivars were strongly resistant to infection (mean score of <0.5 for both leaf assays) while 32 were somewhat resistant (mean score of 1 to 2 for both leaf assays). Twenty-five cultivars were identified as somewhat susceptible to infection (mean score of 2 for both leaf assays), and 13 cultivars were identified as moderately susceptible (mean score of 3 to 4 for both leaf assays). To investigate albinism, one cultivar known to be a carrier of the allele for albinism (‘Bing’), one suspected to be a carrier (‘Lapins’), and one known to be a non-carrier (‘Rainier’) were used in controlled crosses in 2021 and 2022 with a set of 36 other cultivars chosen for their pedigree positions. Seeds were collected and germinated, and seedlings observed for albinism. Of the 572 seeds collected in 2021, 439 germinated. Seven families had 7 - 53% (16% average) albinos observed, consistent with a single controlling locus with 3:1 segregation if both parents are heterozygous for the recessive albinic allele. Albinism in such families confirmed that ‘Bing’ and ‘Lapins’ are carriers, as is ‘Black Tartarian’, ‘Emperor Francis’, ‘Hedelfinger’, ‘Napoleon’, ‘Stella, and ‘Van’. No albino seedlings were observed in nine families of 2021, indicating the following cultivars are not carriers of the deleterious albinism allele: ‘Black Republican’, ‘Kordia’, ‘Moreau’, ‘Rainier’, ‘Schneiders’, ‘Sparkle’, ‘Walpurgis’, ‘Windsor’, and ‘Yellow Glass’. Genotypic dissection revealed one haplotype, H2, located in Chromosome 1 that associated with albinism for all cultivars identified as carriers by the phenotypes of their seedlings. However, the H2 haplotype was not identified in the cultivar ‘Van’ and therefore indicates a possible second source of albinism in the germplasm. This new knowledge about the genetic control, allelic sources, and carriers of favorable and unfavorable phenotypes will be useful to inform grower choice among current cultivars for mildew and bacterial canker resistance as well as albinism. Outcomes will also inform future breeding efforts targeting mildew and bacterial canker resistance while avoiding seedling loss from albinism.

Genetics of Powdery Mildew Resistance, Bacterial Canker Resistance, and Albinism in U.S. Sweet Cherry Germplasm

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