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First Demonstration of Koch's Postulates for Lasiodiplodia theobromae Fruit Spot on Eggplant (Solanum melongena)

During October 2004, diseased eggplant fruit from a commercial farm in Colquitt County, Georgia, developed circular, tan, water-soaked lesions. Gray, septate mycelia quickly covered the fruit. Diseased fruit became shriveled, spongy, and mummified. Disease incidence in the field was approximately 1%. Lasiodiplodia theobromae (Pat.) Griffon & Maubl. (synonym Botryodiplodia theobromae Pat.) (2) was isolated from the margins of lesions and cultured on acidified potato dextrose agar. The fungus produced grayish colonies with aerial hyphae and black ostiolate pycnidia massed into stroma. Mature elliptical conidia (25.8 × 15.6 μm) were brown, had a single septation, and longitudinal striations. Isolates obtained from peanut and pecan were included in the pathogenicity tests. Mature fruit cv. Nightshade were surface disinfested for 30 s in 70% ethanol, followed by 60 s in 0.5% sodium hypochlorite, rinsed twice in sterile distilled water, and allowed to dry. Inoculations were made by placing an agar plug containing L. theobromae mycelial side down on the surface of the fruit or wounding with a sterile toothpick containing mycelium of the fungus. Fruit similarly inoculated with agar plugs or sterile toothpicks served as controls. There were a total of three replicates. Fruit were placed in plastic containers lined with moistened paper towels. Containers were placed in a dew chamber and incubated (28°C, relative humidity >95%) for 3 days, and then evaluated. Symptoms identical to those observed on naturally infected fruit developed on inoculated fruit. Controls remained disease free. L. theobromae was reisolated from all symptomatic tissue, satisfying Koch's postulates. Disease damage on wounded fruit was twice that of nonwounded fruit. However, seven of nine inoculations with agar plugs containing L. theobromae resulted in infection. Lesion lengths from wound inoculations were 9.8, 7.3, and 5.2 cm for isolates from peanut, pecan, and eggplant, respectively. Generally, L. theobromae is considered a facultative wound pathogen or a secondary invader (3). However, this study suggests that direct infection can occur. Although fruit spot has been reported previously on eggplant (1), to our knowledge, this is the first report verifying L. theobromae as the causal agent.

References: (1) S. A. Alfieri et al. Index of Plant Diseases in Florida. Fla. Dep. Agric. Consum. Serv. Bull. 11, 1984. (2) H. L. Barnett and B. B. Hunter. Illustrated Guide of Imperfect Fungi. 4th ed. The American Phytopathological Society St. Paul, MN, 1998. (3) P. M. Phipps and D. M. Porter. Plant Dis. 82:1205, 1998.