Cultural, pathogenic and genetic variability of the causal pathogen of early blight (Alternaria solani Sor.) on tomato (Lycopersicon esculentum Mill.) in Cuba. | Variabilidad genética, patogénica y cultural del patógeno causal del tizón temprano (Alternaria solani Sor.) en tomate (Lycopersicon esculentum Mill.) en Cuba.

Tomato is one of the crops of greater distribution in the world. Early blight, caused by Alternaria solani Sor., is among the most important disease in the reduction of yields and commercial values of production. Resistant cultivars are an alternative to control the disease; therefore the variability of the pathogen population should be investigated. In the present work, the cultural, pathogenic and genetic information of 109 isolates of A. solani is analyzed. The cultural characteristics of the isolates showed a high variability and they were not related with the geographical origin, aggressiveness or genetic polymorphism of the isolates; which makes them not adequate for these species population studies. It was shown for first time, by histological studies, that epidermal tomato cells react the same way to the infection with conidia or mycelia of A. solani; therefore, verify the validity of mycelia as alternative inocula in studies of this plant-pathogen interaction. The previous result, together with the possibility of differentiating genotypes resistant and susceptible to the pathogen, and the identification of highly aggressive isolates allowed to elaborate a methodology of screening (“in vitro') resistant genotypes. Fungus isolates were classified according to the necrotic leaf area in: lower aggressiveness (10%), middle aggressiveness (11-40%) and highly aggressiveness (41%). Differences among the aggressiveness of the isolates were corroborated by other indicators: incubation period, duration of the generation and number of necrotic spots per leaf and plant. The population of the molecular variance analysis of tomato and potato showed a high genetic distance (¦µ=0,30) and highly significant (p¡Ü0,0001). These differences were corroborated when analysing the differential aggressiveness among isolates from both crops, in tomato. Both, genetic and pathogenic results, are the first combined evidences to support the hypothesis of specialization of this pathogen. Random association of the cultural, pathogenic and genetic characteristics, and the geographical origin of A. solani isolates make unjustified divisions of the Cuban population of the pathogen based on these properties. The evidence of specialization and the random distribution of variability in the pathogen population have practical implications for the breeding programs and in the management of tomato early blight. These are the first results in Cuba, where a population analysis is carried out, combining three of the most important aspects that characterize variability of a fungal plant pathogen.