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Alternaria Leaf Spot, Twig Blight, and Fruit Rot of Highbush Blueberry in Argentina

Disease symptoms have been observed since October 1997 on highbush blueberry (Vaccinium corymbosum L.) cvs. Georgia Gem, O'Neal, and Sharpblue cultivated in Buenos Aires. Lesions were observed on recently planted, as well as mature plants, in commercial fields. Circular-to-irregular, light brown-to-gray leaf spots with brownish red borders, initially 3 to 7 mm in diameter, enlarged and coalesced. Blight developed on twigs. Reddish circular spots appeared on stems, developing small cankers. Dark sunken lesions were observed on attached ripening berries. During December 2002, postharvest fruit rot was noted. Small pieces of diseased leaves, twigs, stems, and fruits were surface sterilized with 0.2% NaOCl, plated on 2% potato dextrose agar (pH 7), and incubated at 20 ± 3°C. Symptomatic fruits were placed in plastic trays in humid chambers. In all cases, olive mycelium developed after 3 days with septate hyphae and abundant ovoid and obclavate muriformly septate conidia. The isolate obtained from diseased leaves of cv. O'Neal was used to test pathogenicity on micropropagated potted plants of 20-cm height and ripe fruits contained in plastic trays. Both plants and fruits belonged to cv. O'Neal. A suspension of 2 × 10⁵ conidia per ml was sprayed on needle-punctured young stems, fully expanded leaves, and mature fruits. Plants and trays were covered with polyethylene bags and kept at 24 ± 3°C under fluorescent light (12-hr photoperiod). The bags were removed after 72 hr. Symptoms appeared after 3 days on fruits and 8 days on leaves and stems. Controls remained symptomless. The inoculated pathogen was recovered from diseased organs and identified as Alternaria tenuissima (Kunze:Fr.) Wiltshire (1). To our knowledge, this is the first report on the presence of A. tenuissima affecting blueberry crops in Argentina.

Reference: (1) E. G. Simmons. Mycotaxon 70:325, 1999.