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Interpretive Summary: Because obligately parasitic plant microbes must by nature grow in tight association with their plant host, isolating RNA specifically from the parasite is challenging. For microbes like grape powdery mildew that grow superficially, separation from the host and subsequent isolation of RNA should be possible. To this end, we used nail polish to facilitate removal of all developmental stages of grape powdery mildew (Erysiphe necator) except for the haustorial feeding structure and showed that RNA isolated after removal was specifically from E. necator and lacked contaminating grape RNA. This approach can be applied to expression analyses throughout fungal development and could be extended to other microbes growing on plant surfaces.

Technical Abstract: RNA expression profiling of obligately parasitic plant microbes is hampered by the requisite interaction of host and parasite. For superficial pathogens like grape powdery mildew as well as for epiphytic saprophytes, growth along the outside surface of the plant allows separation from the host and subsequent isolation of RNA. We used nail polish to facilitate removal of conidia, appressoria, hyphae, conidiophores, and developing chasmothecia of grape powdery mildew (Erysiphe necator) and showed that RNA isolated after removal was specifically from E. necator and lacked contaminating grape RNA. This approach can be applied to expression analyses throughout fungal development and could be extended to other epiphytic pathogens and saprophytes.