4 pp. / Mesquite (Prosopis sp.) is a popular tree in landscapes in Arizona because of its drought tolerance and attractive growth habit. Powdery mildew has been observed from late summer until early spring on mesquite leaves. It has been identified as Pleochaeta polychaeta based on morphological descriptions and comparison to herbarium specimens. Surveys were conducted in fall 2008 through winter 2009 at two locations in southern Arizona to determine the susceptibility of different mesquite species to powdery mildew. Twelve mesquite trees representing two species were sampled at Texas Canyon near Willcox, AZ, and 177 trees representing eight species were sampled at the University of Arizona campus in Tucson, AZ. The North American mesquite species P. glandulosa var. glandulosa and P. velutina were infected with powdery mildew at the University of Arizona campus and P. velutina at the Texas Canyon site. No powdery mildew was observed on P. alba, P. cinerea, P. nigra, P. chilensis, P. pubescens and P. chilensis x flexuosa. The powdery mildew affects the aesthetic value of severely infected trees but seems to have little effect on long term tree health.