Identification of QTL underlying powdery mildew and bacterial canker infection in sweet cherry (Prunus avium L.)

Powdery mildew (PM), caused by Podosphaera clandestina, and bacterial canker (BC), caused by Pseudomonas syringae pv. syringae, are the major diseases of sweet cherry in the USA. Incorporation of natural resistance into elite cultivars would be an effective way to reduce reliance on fungicide and pesticide use and facilitate the transition to sustainable production systems. This study was designed to identify quantitative trait loci (QTL) underlying PM and BC infection to facilitate development of new resistant cultivars. Six hundred pedigree-linked germplasm were used in this study. PM was scored in the field on a 0-5 scale (0 = no visible symptoms and 5 = very severe infection on leaves) from 2011 to 2013. BC phenotyping was performed by inoculating five healthy and newly emerging leaves with 10 µl of 100,000,000 CFU/ml bacteria suspension mixed with 0.5% surfactant by mid-rib wound method in a detached leaf assay. Disease was scored in the lab on a 0-4 scale (0 = no necrosis and 4 = total necrosis). Approximately 1100 single nucleotide polymorphism (SNP) and four simple sequence repeat (SSR) markers were used for determining genome-wide marker-locus-trait associations. One PM QTL mapped on top of linkage group (LG) 5 in the three years while others mapped on LGs 1, 3 and 6 in a single year. For BC, one major QTL was identified on top of LG 5. The co-location of QTL for both diseases on LG 5 will be explored further to develop a breeding strategy for the two diseases combined.