Alternaria brown spot (ABS), caused by the 'tangerine' pathotype of Alternaria alternata, (Fr.) Keissl. is a serious disease in several mandarin cultivars such as 'Fortune', 'Nova' and other hybrids. Although the disease is widespread in the Mediterranean Basin, the epidemiology of the disease in semiarid conditions is unknown. Epidemiological studies were conducted during two consecutive years in Spain. The presence of inoculum was studies on leaves, twigs and fruit on affected trees, as well as weeds and leaf litter was determined. The dynamics of airborne conidia of Alternaria were studied with a spore trap and by exposing plates of the semi-selective medium ARSA. The effect of environmental factors in the incidence of ABS was studied through exposure of trap plants.Conidia of Alternaria were continuously detected throughout the period of study and pathogenic isolates were obtained in all sampling dates. However, the number of pathogenic isolates was not correlated with the total captured. Affected leaves and shoots were considered the main source of inoculum, with 15-76% of pathogenic isolates. Pathogenic isolates were also found in leaf litter and weeds of the genus Sonchus, but in very low proportion. The pathogen survived in fallen immature leaves for up to 76 days, coinciding with complete leaf litter decomposition. In contrast to humid areas, infections occurred mainly in spring and autumn, on weeks with rainfall ≥2.5 mm and average temperature ≥12.5°C. Based on quantile regression, the amount of rainfall, number of rain days, and total wetness duration were considered important factors increasing ABS incidence during infection periods.The infection efficiency of the pathogen was studied by inoculating plants of 'Fortune' and 'Nova' under different combinations of temperature (8-35°C) and wetness durations (8-48 hours). Data obtained from the infection efficiency study were fitted to generalised additive models (GAMs) and a generic infection model. Disease incidence significantly increased with leaf wetness duration over 16 hours. Disease incidence decreased with temperatures below 12°C or above 27°C. No infections were observed at 35ºC. The incidence of ABS did not Abstract increase after a dry period of one hour in 'Fortune' and two hours in 'Nova'. Post-infection temperature had a significant effect in the incubation period of the ABS pathogen, but maximum symptom expression was observed after no more than 30 hours in 'Fortune' and about 60 hours in 'Nova'. With GAMs models, disease incidence at low temperature was higher in 'Fortune' than in 'Nova'.The two infection models obtained, the Alter-Rater model developed in Florida and a simple rule system (SRS) based on temperature and rainfall were evaluated using independent infection data obtained from trap plants of 'Fortune' and 'Nova' exposed weekly in three orchards during 2011 to 2013 (n = 266). Model evaluation was performed using ROC curve analysis. Considering that the high sensitivity of the models is critical for the control of ABS, the partia...