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First Report of Alternaria Brown Spot of Citrus in Spain

In 1998, a new disease of Fortune mandarin trees was detected in orchards in the eastern province of Valencia. This is one of the most important late-maturing cultivars grown in Spain. Symptoms were typical of Alternaria brown spot of citrus (2). Young leaves showed brown necrotic and irregular blighted areas with characteristic yellow halos. The necrosis had a tendency to follow the veins. On fruits, symptoms included light brown, slightly depressed spots to circular and dark brown areas on the external surface. Infected young fruits and leaves often fell and the mature fruits were unmarketable due to lesions, resulting in important economic losses. Isolations on potato dextrose agar supplemented with 0.5 mg/ml of streptomycin sulfate (PDAS) from affected leaves and fruits consistently yielded Alternaria alternata (Fr.:Fr.) Keissl., which was identified based on conidial morphological characteristics. Pathogenicity tests were conducted using 15 isolates from fruit and leaves by inoculating detached immature Fortune leaves with a sterile water suspension of 5 × 10⁵ conidia per ml. Drops of this suspension (40 μl each) were placed on the lower surfaces of each leaflet using four leaves per isolate. Leaves were incubated in a moist chamber in the dark at 27°C (1). After 48 h, most of these isolates caused necrotic lesions on the leaves similar to those observed in the field, and the fungus was reisolated, confirming Koch's postulates. In 1999, the fungus spread to other citrus-growing areas, and to date the disease has been detected affecting Fortune and Nova mandarins and Minneola tangelo. This is the first report of Alternaria brown spot of citrus in Spain.

References: (1) K. Kohmoto et al. Phytopathology 81:719, 1991. (2) J. O. Whiteside. Plant Dis. Rep. 60:326, 1976.