Leaf blotch of wheat (Septoria tritici Rob. ex Desm., teleomorph Mycosphaerella graminicola (Fückel) Schr？t. in Cohn) causes significant losses in wheat. During the last decades studies about the genetic variability of the pathogen and location of the resistance have been intensive around the world. The knowledge about the genetic variation of M. graminicola is very important because it could allow us to determine which genotypes predominate within a geographic area. It also can be used to evaluate the germplasm resistance of wheat cultivars with isolates with high genetic differences. In addition, the knowledge of the genes conditioning resistance in different genotypes allows getting precise combination in new germplasm. The incorporation of the known genes in new cultivars could contribute to broadening the resistance to the pathogen. A paper about genetic variability of the pathogen and location of the resistance, with special emphasis in the work carried out in Argentina, is presented. 1. Importance and Biology of the Disease Leaf blotch of wheat (Septoria tritici Rob. ex Desm., teleomorph Mycosphaerella graminicola (Fückel) Schr？t. in Cohn) causes significant losses in wheat. In Argentina, yield losses from 21 to 37% [1], from 20 to 50% [2], and from 16 to 45% [3] have been found. In some other countries, yield reductions range from 31 to 54% [4], from 10 to 45% [5], and even reductions >60% have been reported [6]. Mycosphaerella graminicola is a hemibiotrophic pathogen; early infection is biotrophic, followed by a switch to necrotrophic growth just prior to symptom expression. The sexual stage is known to play a role in the disease cycle. It has been reported to cause most of the initial infection of winter wheat crops during the autumn in the UK [7] and in the USA [8]. An increase in ascospores at harvest time has been reported, suggesting that the sexual stage may be important to initiate the infection in the next growing season [9]. In Argentina, the sexual stage was also found [10]. Unburied crop residue is the major source or primary inoculum for Septoria tritici infecting wheat [8]. Ascospores are produced and released on this substrate [11]. Pseudothecia mature during winter and remain viable until early spring. Only 30？min of moistening stubble are necessary for ascospore release and dispersal [12, 13]. Different studies [9, 14] have confirmed that during spring and the beginning of summer, the severity of the epidemic was conditioned by pycnidiospores produced in the crop; nevertheless, ascospores were present from the time the first
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