Cherry leaf spot disease caused by Alternaria spp. has been reported in China (1) but never in Greece. The fungus Alternaria alternata (Fr.:Fr.) Keissler (CBS 119115) was isolated on acidified potato dextrose agar (2.5 ml of 85% lactic acid per liter of nutrient medium) from the leaves of cherry cv. A1/5 (hybrid of Prunus avium (L.) L.) at the Pomology Institute Naoussa, Greece in 2005. Microscopic morphology showed acropetal chains of multicellular conidia (dictyoconidia) produced sympodially from simple, sometimes branched, short or elongate conidiophores. Conidia are obclavate, obpyriform, sometimes ovoid or ellipsoidal, often with a short conical or cylindrical beak, pale brown, smooth-walled or verrucose. The percentage of diseased leaves was approximately 30 to 40%. Lesions first appeared on leaves in late spring as small, round, blackish spots and gradually enlarged to 2 to 5 mm in diameter with a brownish purple border. Some spots turned grayish brown, but most underwent a secondary enlargement and become irregular and much darker, acquiring a frogeye appearance. Similar symptoms were also observed on the leaves of cherry hybrids A1/3 and A5 established in the same cherry orchard. Zhu and Chang (1) reported that infected leaves of cherry trees by Alternaria seraci showed holes and resulted in early leaf fall. Symptoms were reproduced in the laboratory by spraying 2-year-old plants (20 plants) at the stage of fullleaf development, planted in pots, of the same cherry hybrid with conidia of the fungus. Aqueous suspension was adjusted to 3 × 10-6 conidia per ml (approximately 100 ml per plant) collected from an isolation of the fungus. Control plants were sprayed with distilled water without conidia. Plants were then placed in a growth chamber at 25°C for 3 months. After the incubation period, some leaves of the inoculated plants developed similar to those previously observed. No disease symptoms were observed in control leaves. Koch's postulates were satisfied after reisolating the fungus. To our knowledge, this is the first report of the occurrence Alternaria leaf spot on cherry trees in Greece.
Reference: (1) J. Zhu and Y. Chang. China Fruits 3:9, 2004.
