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Oral: Population Dynamics


Emergent rudbeckia downy mildew epidemics caused by Plasmopara halstedii are genetically distinct from the pathogen populations infecting sunflower
Y. RIVERA (1), C. Salgado-Salazar (1), T. Gulya (2), J. Crouch (3) (1) USDA-ARS Systematic Mycology and Microbiology Laboratory and Rutgers University, U.S.A.; (2) USDA ARS Sunflower and Plant Biology Research Unit, U.S.A.; (3) USDA-ARS Systematic Mycolog

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The oomycete Plasmopara halstedii is regarded as the causal agent of downy mildew disease on 35 Asteraceae genera. The pathogen is also a significant problem of sunflower (Helianthus annuus) worldwide and more recently, a destructive pathogen of ornamental Rudbeckia fulgida (rudbeckia) in the U.S.A. As with other recent downy mildew epidemics on specialty crops, little is known about the pathogen causing modern outbreaks on rudbeckia. To determine if modern epidemics on rudbeckia are caused by the same P. halstedii genotypes as those historically found on native rudbeckia or to those causing disease on sunflower, we performed population genetic and phylogenetic analyses. Fifteen microsatellite markers were designed from a draft genome assembly generated from a P. halstedii isolate on sunflower. Phylogenetic and microsatellite analyses of 232 samples collected between 1883-2014 revealed two main groups corresponding to host origin. Isolates infecting sunflower hosts were further separated into eight subgroups with no obvious pattern by year, physiological race or geographical origin. Isolates infecting rudbeckia hosts were further separated into three subgroups, one corresponding to historical pre-epidemic specimens and two modern subgroups. Our results indicate that the emergent populations of P. halstedii infecting rudbeckia are a different species from those infecting sunflower and genetically distinct from pre-epidemic populations infecting native rudbeckia.