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Rhizoctonia solani, Pythium aphanidermatum, and Macrophomina phaseolina were isolated from Euphorbia lathyris grown in fields near Tucson, Arizona. R. solani occurred as a damping-off organism in the fall. P. aphanidermatum infected seeds, seedlings, and mature plants in laboratory and greenhouse tests. Although P. aphanidermatum was infrequently isolated from field plants in Arizona, it was a major pathogen of greenhouse plants growing at high temperatures in nonsterile soil. M. phaseolina was a major pathogen. Infection of E. lathyris roots occurred within 1 mo of an October 1980 planting, but symptoms were not significant until June. Sclerotia of this fungus ranged in numbers from 1 to 246 sclerotia/g field soil. Population densities of 0.2 sclerotium/g soil were sufficient to cause more than 90% plant death in field plots. Some plants infected with M. phaseolina were growing in an area newly cleared of native desert vegetation. Subsequently, M. phaseolina was found in uncultivated soils from four vegetative communities in southern Arizona at elevations from 600 to 2,000 m; the fungus also was recovered from roots of several symptomless native plants.