Casuarina cunninghamiana Miq. (Fam. Casuarinaceae) is native to Australia and was introduced to Argentina and used as a windbreak, shade tree and for ornamental purposes. In autumn 2009, the potted seedlings in the nursery of La Plata University were found to be infected with a powdery mildew. Symptoms first appeared as thin white colonies, which subsequently developed into an abundant growth on the leaves and young stems. The damage caused by the powdery mildew infection has been observed every year after this initial infection and was again confirmed in April, 2011. There have been a few reports on the presence of powdery mildew on Casuarina spp. (Mantz et al., 2008; Farr and Rossman, 2011). In all the reports, the pathogen was cited to be Oidium sp. Thus, the aim of this work was to identify the causal agent of the powdery mildew observed on casuarina in Argentina.