Macrophomina phaseolina(Tassi) Goid. is reported as a pathogen of guar in North America for the first time. AlthoughM. phaseolinais seed borne in other crops, the fungus was not recovered from seeds of naturally infected guar plants. Since the pathogen is found in desert and cultivated soils throughout southern Arizona where concurrent heat and water stress favor disease development, charcoal rot should be considered a potential constraint to guar production in arid regions.