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First Report of a Wilt and Stem Rot of Muskmelon and Watermelon Transplants Incited by Pseudomonas cichorii in Serbia

In 1989, a wilt and stem rot of muskmelon (Cucumis melo L.) was reported on seedlings grown under plastic in northern Serbia (1). In 1998, a similar disease of watermelon (Citrullus vulgaris L.) transplants occurred in central Serbia, resulting in losses estimated at 20% of plants grown in the nursery. Initial symptoms appeared as water-soaked lesions on the stem and lower surfaces of cotyledons and first true leaves. Necrosis started from leaf water-soaked areas and spread over the entire plant. Many stems softened and toppled. Wilting was also associated with the latter stages of the disease. Six representative strains isolated from water-soaked stem lesions on muskmelon in 1989 (three strains) and on watermelon in 1998 (three strains) were characterized. All strains were gram negative, strictly aerobic, motile, rod-shaped, and fluorescent on King's B medium, but negative for levan production, soft rot of potato, and arginine dihydrolase activity. Furthermore, strains were oxidase positive and induced a strong hypersensitive reaction in tobacco leaves. All of these reactions are characteristic of Pseudomonas cichorii (2). All six strains were confirmed to be P. cichorii based on analysis of cell-wall fatty acid profiles using the Microbial Identification System (MIDI, Newark, DE). Similarity indices ranged from 0.87 to 0.97. Pathogenicity of the P. cichorii strains was confirmed by prick inoculation of the hypocotyl of 2-week-old plants of domestic muskmelon cv. Sezam and watermelon cv. Rosa. Six plants of each cultivar were pricked with a dissecting needle dipped into a bacterial suspension (10⁸ CFU/ml) prepared from each of the six strains and placed in a humidity chamber at 26°C for 48 h. Water-soaking occurred on all plants around the inoculation site after 24 h. The upper part of the plants lost turgor and wilted. Eventually, the hypocotyl and leaf veins became water-soaked, and the stem tissue softened, resulting in toppling of plants. The symptoms were identical to those described on muskmelon in 1989 and observed on watermelon in 1998. No symptoms developed on control plants. The bacterium was reisolated from the inoculated plants and shown to be identical to the original strains, confirming that P. cichorii was responsible for the disease of muskmelon and watermelon seedlings in 1989 and 1998, respectively.

References: (1) M. Arsenijevic and V. Stojsin. Contemp. Agric. 9:487, 1989. (2) R. A. Lelliott et al. J. Appl. Bacteriol. 29:470, 1966.