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Alternaria brown spot (ABS) of mandarins, caused by a pathotype of the fungus Alternaria alternata, is a serious disease both in humid and semi-arid citrus-growing regions of the world. The pathogen affects leaves and fruit of susceptible mandarin cultivars such as 'Fortune', 'Nova', 'Minneola', and 'Murcott'. The epidemiology of ABS was studied mainly in humid areas in Florida but, due to climatic differences, this information cannot be extrapolated to semi-arid regions like Spain. Field studies were conducted in 2011 in the experimental orchards at IVIA, Valencia. Dynamics of the airborne Alternaria conidia were monitored weekly by a spore trap, and the percentage of pathogenic isolates was determined periodically using the selective medium, ARSA, and pathogenicity tests. The presence of inoculum on affected leaves, shoots, leaf litter, and weeds was also determined. Infection periods were monitored weekly by exposing trap or indicator plants of 'Fortune' and 'Nova' mandarins. Environmental variables were recorded by an automated meteorological station. Although Alternaria conidia were detected through the experimental period, only 5% of all the isolates were pathogenic. As affected leaves and shoots were determined to be the main source of inoculum, the survival of the fungus in the leaf litter was higher than previously reported in Florida. Pathogenic isolates were detected also in weeds, but only at very low levels. Infections in trap plants occurred mainly in May-June and September-October. A significant positive correlation was detected between disease incidence and rainfall.