Field apple scab susceptibility of a diverse Malus germplasm collection identifies potential sources of resistance for a
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CABI Agriculture and Bioscience Open Access
RESEARCH
Field apple scab susceptibility of a diverse Malus germplasm collection identifies potential sources of resistance for apple breeding David Papp1†, Liqiang Gao1,2†, Ranjita Thapa1, Dan Olmstead3 and Awais Khan1*
Abstract Background: Breeding for resistance to apple scab (caused by Venturia inaequalis), the most devastating fungal disease of apples, relies on genetic resources maintained in germplasm collections. Methods: To identify new sources of scab resistance, we evaluated 177 Malus accessions, including 27 primary and 13 hybrid Malus species from diverse geographical origins, in an orchard at Geneva, New York. We also screened a differential host set for 2 years to monitor for changes in the effectiveness of ten known scab resistance genes, which allowed us to confirm the presence of virulent pathogen races in the orchard. Results: We found that ~ 37% of the wild Malus accessions and domesticated cultivars were resistant to apple scab in the field. Several of these accessions were unrelated to sources of previously known resistance genes and are promis‑ ing for apple scab genetic research and resistance breeding. Cultivars carrying the Rvi6 (Vf) gene from Malus floribunda clone 821, e.g. ‘Liberty’ or ‘Florina’, remained resistant despite the breakdown of Rvi6. ‘Demir’, a Malus hybrid from Tur‑ key, and ‘Chisel Jersey’, a traditional English hard cider cultivar, showed fewer symptoms than the Rvi6 resistant cultivar ‘Prima’. Races 1 to 7 and 9 of V. inaequalis were present in the orchard, but no scab was observed on the indicator host accessions for races 11 and 12. Conclusions: Detailed and systematic screening of Malus germplasm identified resistant and moderately resistant donor accessions based on resistance reaction types. These accessions are promising for use in future genetic studies to identify novel sources of scab resistance alleles for apple breeding to develop cultivars with durable apple scab resistance. Keywords: Venturia inaequalis, Disease resistance, Core collection, Differential hosts Introduction Disease susceptibility of commercial apple cultivars (Malus domestica Borkh.), and the continual emergence of new pathogenic races that overcome resistance genes, are major threats to the apple industry worldwide. Apple scab (causal agent: Venturia inaequalis Cke./Wint.) is the *Correspondence: [email protected] † David Papp and Liqiang Gao contributed equally to this work 1 Plant Pathology and Plant‑Microbe Biology Section, Cornell University, Geneva, NY 14456, USA Full list of author information is available at the end of the article
most devastating fungal disease of apples in humid areas throughout the world where apple is grown (GonzálezDomínguez et al. 2017). The majority of apple cultivars grown commercially in the USA are susceptible to apple scab. Apple scab lesions on fruit mainly impact their cosmetic appearance, severely limiting their marketability. Growers must apply approximately 12–18 fungicide sprays per growing seas
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