Distribution and Characterization of Podosphaera macularis Virulent on Hop Cultivars Possessing R6-Based Resistance to Powdery Mildew

Abstract

Host resistance, both quantitative and qualitative, is the preferred long-term approach for disease management in many pathosystems, including powdery mildew of hop (Podosphaera macularis). In 2012, an epidemic of powdery mildew occurred in Washington and Idaho on previously resistant cultivars whose resistance was putatively based on the gene designated R6. In 2013, isolates capable of causing severe disease on cultivars with R6-based resistance were confirmed in Oregon and became widespread during 2014. Surveys of commercial hop yards during 2012 to 2014 documented that powdery mildew is now widespread on cultivars possessing R6 resistance in Washington and Oregon, and the incidence of disease is progressively increasing. Pathogenic fitness, race, and mating type of R6-virulent isolates were compared with isolates of P. macularis lacking R6 virulence. All isolates were positive for the mating type idiomorph MAT1-1 and were able to overcome resistance genes Rb, R3, and R5 but not R1 or R2. In addition, R6-virulent isolates were shown to infect differential cultivars reported to possess the R6 gene and also the R4 gene, although R4 has not yet been broadly deployed in the United States. R6-virulent isolates were not detected from the eastern United States during 2012 to 2015. In growth chamber studies, R6-virulent isolates of P. macularis had a significantly longer latent period and produced fewer lesions on plants with R6 as compared with plants lacking R6, indicating a fitness cost to the fungus. R6-virulent isolates also produced fewer conidia when compared with isolates lacking R6 virulence, independent of whether the isolates were grown on a plant with or without R6. Thus, it is possible that the fitness cost of R6 virulence occurs regardless of host genotype. In field studies, powdery mildew was suppressed by at least 50% on plants possessing R6 as compared with those without R6 when coinoculated with R6-virulent and avirulent isolates. R6 virulence in P. macularis appears to be race specific and, at this time, imposes a measurable fitness penalty on the fungus. Resistance genes R1 and R2 appear to remain effective against R6-virulent isolates of P. macularis in the U.S. Pacific Northwest.

Genomic analysis of powdery mildew resistance in a hop (Humulus lupulus L.) bi-parental population segregating for “R6-locus”

“…Our observations on USDA Newport show two distinct regions that are separated by approximately 1 cM. Previous work by Wolfenbarger et al (2016) could not distinguish between R4 and R6 genes in multiple genotypes possessing the R6 gene. It is possible that cultivar selection has centered on selecting lines that possess greatest fitness and these lines happen to contain both R4 & R6 genes.…”

“…The female parent of this cross, USDA Newport, appears to possess both R4 and R6 genes (Wolfenbarger et al 2014) for resistance to PM. Later work by Wolfenbarger et al (2016) suggests that R4 and R6 may be misclassified, the same gene, or so tightly linked that they segregate as a single group. Our observations on USDA Newport show two distinct regions that are separated by approximately 1 cM.…”

“…Previous research on hop powdery mildew (PM) suggests that resistance is based primarily upon qualitative expression with seven known R-genes sources (Neve 1991;Henning et al 2017;Wolfenbarger et al 2016). QTL analyses on hop powdery mildew resistance performed over the past decade have revealed several possible locations for R-genes (Henning et al 2011McAdam et al 2013;Cerenak et al 2006) although no genes with homology to known ''R-genes'' were identified.…”

“…Unfortunately, different marker systems were used across all these studies and there are no means of comparing results although Henning et al (2017) used genome-based SNP markers and future studies using their genomic data can be compared for genomic locations of R-genes. An important finding in most of these studies showed the presence of highly significant QTL but also showed the presence of QTLs having smaller impact upon expression of disease resistance, leading to the idea that resistance to PM may be conditioned by both qualitative and quantitative genic control Wolfenbarger et al 2016). Henning et al (2017) based their identification of SNPs linked to PM resistance upon the reference 'Teamaker' genome ) and the resulting genetic map was used for QTL identification.…”

Sequence and annotation of 42 cannabis genomes reveals extensive copy number variation in cannabinoid synthesis and pathogen resistance genes

“…G. chicoracearum has been shown to cause powdery mildew (PM) in cannabis while Podosphaera macularis has been reported to cause PM in Humulus lupulus L.(hop) which is a member of Cannabaceae and closely related to cannabis (Wolfenbarger et al 2016; Punja 2018). Cannabis-derived powdery mildew can result in significant crop loss while exposing cannabis trimmers to powdery mildew-induced allergies (Victory et al 2018; Zamir 2018).…”