The cultivation of alfalfa (Medicago sativa L.) on the Northern Altiplano of La Paz, Bolivia, has increased due to the expansion of dairy farming. However, several leaf diseases affect the yield of this crop. The present work examines the effects in the field caused by Peronospora trifoliorum on 11 varieties of alfalfa in the above region during the 2011-2012 growing season. The results showed disease incidence, determined before flowering, to differ across these varieties. In the following growing season, the protective effects (in terms of disease severity and yield [dry mass ha-1]) of the fungicides Ridomil (metalaxyl) and Curathane (cymoxanil) were examined in the alfalfa variety Bolivia 2000. Both fungicides significantly reduced disease severity (Ridomil by a mean 12.5%, and Curathane by a mean 7%). With Ridomil the yield obtained was 29.94% greater than under the control conditions, and with Curathane it was 28.37% greater. A strong correlation (R2=0.858) was detected between disease severity and yield.