In 1964, Septoria leaf spot was detected for the first time in the United States in experimental pistachio (Pistacia vera) plantings at Brownwood, Texas. A moderate level of the same disease, caused by the fungus Septoria pistaciarum, was first observed in 1986 on leaves of pistachio trees in Arizona. In 1988 a survey of the 800 ha of pistachio orchards in southeastern Arizona revealed a widespread incidence of the disease. Since the initial discovery and identification of the disease, Septoria leaf spot has appeared every year in Arizona pistachio orchards. The onset and severity of the disease is affected by summer rainfall that occurs in this region. Yearly disease management studies conducted since 1992 have shown that as few as two applications of chlorothalonil in July and August can virtually prevent disease development. Applications of copper hydroxide or benomyl alone or in combination also effectively arrested disease development. Leaves around nut clusters not receiving fungicide treatments were senescent at crop maturity, while leaves on treated trees showed no sign of senescence. Pistachio trees infected with Septoria leaf spot and not treated with a fungicide can defoliate in the autumn up to 2 months prematurely.