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Earth laughs in flowers: how emergent downy mildew diseases are affecting the ornamental crop industry and what to do about it

Catalina Salgado-Salazar: Mycology and Nematology Genetic Diversity and Biology Laboratory, USDA-ARS

<div>Downy mildews are some of the most devastating diseases of ornamental plants. While many downy mildews are caused by pathogens previously unknown, others have been around since the 19<sup>th</sup> century, only to recently re-emerge as destructive pathogens exhibiting unwelcome changes in virulence, host range and epidemiology. Newly emergent downy mildew pathogens affecting rudbeckia and impatiens exemplify some of the new challenges faced by the horticultural industry. <em>Plasmopara halstedii</em> emerged as a destructive pathogen of cultivated ornamental rudbeckia in the late 1990’s, although this pathogen was known as an inhabitant of native rudbeckia and presumed to be the same pathogen causing sunflower downy mildew. Using population genetics, we show that isolates infecting rudbeckia are genetically distinct from isolates infecting sunflower, constituting an entirely different species. In the case of the pathogen responsible for impatiens downy mildew epidemics beginning in 2004, we identified seven different genetic populations of the causal pathogen <em>P. obducens</em>, with clear genetic differences between pre-and post-epidemic populations. Here we discuss the knowledge gaps presented by these emergent diseases and the need for further integration of information that would aid disease mitigation efforts. These include the revised methods for species identification and disease diagnostics, identification of common patterns and drivers of emergent and re-emergent diseases, and the development of improved disease management strategies</div>

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