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A Leaf Spot Disease of Highbush Blueberry Caused by Alternaria tenuissima. R. D. Milholland, Associate Professor of Plant Pathology, North Carolina State University, Raleigh 27607; Phytopathology 63:1395-1397. Accepted for publication 30 April 1973. DOI: 10.1094/Phyto-63-1395.

Cultures of Alternaria tenuissima isolated from blueberry leaf lesions and decayed fruit were pathogenic on highbush blueberry leaves. Symptoms appear as circular to irregular-shaped brown lesions, 1- to 5-mm in diam, surrounded by a red border. Lesions continued to enlarge under high humidity; whereas, those developing after removing the plant from a saturated atmosphere remained as small red flecks. Optimum temperature for fungal growth in vitro was 28 C; whereas, the optimum temperature for disease development was 20 C. More spores were produced by the cultures grown at 25 C under 2,583 lux than in the dark. All blueberry cultivars were susceptible to A. tenuissima; ‘Wolcott’ was the most susceptible and ‘Angola’ the least susceptible. Spore concentration was also a major factor in disease development. A three-fold increase in leaf lesions was obtained with a concentration of 10⁶ spores/ml as compared with 10⁵ spores/ml.