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Development of laboratory bioassays to study powdery mildew pathogens of Phlox in vitro.

Coralie Farinas: The Ohio State University

<div>The genus<em> Phlox </em>consists of about 65 species that include some of the most prevalent ornamental plants in the temperate zone. These popular ornamentals are extremely susceptible to powdery mildew caused by the biotrophic pathogens <em>Golovinomyces magnicellulatus</em> and <em>Podosphaera </em>sp. Our long-term goal is to identify <em>Phlox</em> species that may be resistant to the disease by screening a germplasm collection stored at the Ornamental Plant Germplasm Center in Columbus, OH. To do so, we developed a set of laboratory tools to study the pathogens <em>in vitro, </em>including a detached leaf and a plantlet bioassay. In each experiment, we used colony diameter and spore production to assess pathogen growth under different experimental conditions, which included the use of different inoculation techniques, media, age of pathogen inoculum, and phenology of the host. As a result, the following protocol was established for germplasm screening. Single pathogen spores are transferred using an eyelash glued to a stick on a detached disinfected <em>Phlox</em> leaf maintained on ½ MS medium supplemented with benzimidazole. The pathogen is allowed to grow for 18 days after which colonies are transferred using a paintbrush on the entire surface of micropropagated plantlets. Disease severity and spore production are then used to assess plant susceptibility to the disease. These tools will facilitate studies on powdery mildew of <em>Phlox</em> and possibly other ornamental species attacked by the same fungi.</div>