Phomopsis has been described in Yugoslavia (Muntañola-Cvetković et al., 1980) as a fungus responsible of the reduction of sunflower yields. However, during the last two decades in Argentina, Phomopsis was not considered as a limiting pathogen of sunflowers fields. At the XI International Sunflower Conference held in Mar del Plata, Argentina, in 1985, a paper of Mihaljčević was presented on the pathogenesis of Phomopsis isolates from wild sunflower. In 1992, National University of Entre Ríos (UNER) carried on a special project on the disease. This report discusses the occurrence of two natural infections recorded during three seasons (2003-2005) in two neighboring locations, one in the central-eastern Argentina (Gualeguay) and another in Uruguay (Mercedes), which are separated by the Uruguay River. The aims of this study were:

a. To determine and register the spreading of Phomopsis in field conditions.

b. To observe the distribution of the disease.

c. To determine whether hybrids posses genetic tolerance to this pathogen.

d. To make a contribution to sunflower disease map in South America.

As a result of observations under field conditions, we can say that in Uruguay severe attacks of Phomopsis are decisive for sunflower yield reduction. The behavior of commercial hybrids was: tolerant - 27%, intermediate - 30%, susceptible - 43%. The distribution of Phomopsis is increased. In Argentina, the presence of Phomopsis is low or null.