Technical Abstract: The benzophenones are a new class of agricultural fungicides that demonstrate protectant, curative, and eradicative/anti-sporulant activity against powdery mildews. The chemistry is represented in the marketplace by the fungicide Metrafenone, recently introduced by BASF as FLEXITY®.1 The benzophenones show no evidence of acting by previously identified biochemical mechanisms, nor do they show cross resistance with existing fungicides. The value of microscopy in elucidating fungicide mode of action is demonstrated through identification of the effects of an early benzophenone, eBZO, on mildew development. eBZO caused profound alterations in the morphology of both monocot and dicot powdery mildews, affecting multiple stages of fungal development including spore germination, appressorial formation, penetration, surface hyphal morphology, and sporogenesis. Identification of analogous effects of eBZO on sporulation in the model organism Aspergillus nidulans provides a unique opportunity to elucidate important morphogenetic regulatory sites in the economically important obligate pathogens, the powdery mildews. Benzophenones provide a further example of the benefits of whole-organism testing in the search for novel fungicide modes of action.