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by the Washington State University sweet cherry breeder, Dr. Thomas Toyama (Toyama et al., 1993). This mildew-resistant seedling was named Powdery Mildew Resistant-1, or PMR-1 (Toyama et al., 1993). Inheritance analyses revealed a single allele with a domi nant effect was responsible (Olmstead et al., 2001), named powdery mildew resistance fac tor-1, or Pmr1 (Olmstead and Lang, 2002). The resistance-associated Pmr1 allele appears to be capable of conferring powdery mildew resistance in foliar as well as fruit tissues and pedigree analysis of mildew resistant cherry plants indicates other, Pmr1 -like alleles might be present (Peace et al., 2018). Recent pedi gree analysis for PMR-1 revealed it to be the offspring grandchild of the cultivar ‘Moreau’ (Demir, 2019), which has also independently been identified as phenotypically resistant to mildew infection and carrying the Pmr1 al lele that PMR-1 inherited from it (Peace et al., 2018). ‘Chelan’, an offspring of ‘Moreau’, is also phenotypically resistant to mildew infection and inherited the Pmr1 allele from ‘Moreau’ (Olmstead and Lang, 2002; Demir, 2019). Examination of diverse breeding germplasm of Washington State University determined other cultivars, advanced selec tions, and wild crop relatives are resistant to mildew infection, shown in Table 1 (Olmstead et al., 2001; Peace et al., 2018; Zhao, 2013). Whole-genome genotypic profiling of single nucleotide polymorphisms (SNPs) using a 6K SNP array developed for cherry (Peace et al. 2012) genetically mapped the Pmr1 locus to the proximal end of chromosome 5 (Zhao, 2013; Demir, 2019). While the underlying mechanism govern ing genetic resistance to fungal infection has not been fully explored in sweet cherry, preliminary assessment in sweet cherry’s more economically valuable relatives, peach [ Prunus persica (L.) Batsch] and apple have provided some insight. Resistance to infec tion establishment for both Prunus and Malus was reported to be the product of a pathogen associated recognition cascade, culminating in programmed death of the affected cell (Al-

inadvertently purchase and then plant infected plants (Swamy et al., 2019). Powdery mildew management has histori cally relied on the physiological strategies of canopy management and application of fun gicidal sprays, but with drawbacks. Because lack of light and air movement promotes fun gal infection establishment, pruning branches to increase both of these factors within the canopy reduces powdery mildew incidence (Calabro et al., 2009). However, pruning must be repeated throughout the year to maintain canopy structure and thus can be costly (Ca labro et al., 2009; Hubbard and Probst, 2017). With fungicidal sprays, a major problem is the efficacy of an applied fungicide to control disease often diminishes over the multi-year duration of its use (Colucci et al., 2008; Hub bard and Probst, 2017), requiring constant at tention by scientists to develop new fungicide chemistries (Vielba-Fernández et al., 2020). Additionally, application of fungicides can significantly alter orchard microbial commu nity dynamics and exert artificial selective pressure on fungal pathogens that can favor development of fungicidal-resistant types (Loland and Singh, 2004). Further alterations or damage to biological community structures can occur when sprayed fungicides or their residues contaminate soil and groundwater (Bedos et al., 2010; Nettles et al., 2016). Such off-target effects resulting from accidental misapplication or off-site movement have led to marked changes in orchard ecosystems (Lo land and Singh, 2004). Therefore, rather than exclusive reliance on pruning or sprayed prod ucts, powdery mildew management might be best achieved via an integrated approach that also encompasses host genetic resistance (Ol mstead and Lang, 2002; Dreistadt, 2016). While most sweet cherry cultivars grown commercially are susceptible to infection from powdery mildew, certain cultivars and selections exhibited reliable disease resistance (Olmstead et al., 2000 and 2001; Olmstead and Lang, 2002). The first cherry tree to be recorded as mildew resistant was a chance seedling found growing near Prosser, WA,