First Report of Infection of Honeydew with Acidovorax avenae subsp. citrulli

Honeydew fruits (Cucumis melo var. inodorus Jacq. 'Morning Ice,' 'Honeybrew,' and 'Green Flesh') with circular, 3- to 10-mm-diameter lesions that did not extend into the flesh of the fruit were collected from a field in Frio County, TX, in October 1996. Lesions were either water-soaked or had a scabby center with a water-soaked edge. Non-fluorescent, gram-negative bacteria were consistently isolated from lesions. Eleven representative bacterial strains had 0.835 to 0.950 similarity to Acidovorax avenae subsp. citrulli, using Biolog GN Microplates and the MicroLog data base release 3.50 (Biolog, Hayward, CA). Strains were tested for pathogenicity on watermelon (C. lanatus (Thunb.) Matsum. & Nakai 'Royal Sweet') and honeydew seedlings (cv. Morning Ice) by daubing suspensions (approximately 10⁸ CFU/ml) of bacteria onto cotyledons of 1-week-old transplants. Water soaking, followed by necrosis, occurred after 3 to 5 days. A. avenae subsp. citrulli, the causal agent of bacterial fruit blotch of watermelon, was reisolated from lesions. Detached fruit of honeydew (various cultivars) and watermelon (cv. Emperor) were surface disinfested with 70% ethanol, injected sub-epidermally with bacterial suspensions in blemish-free areas, and incubated at 24°C. Water-soaked lesions developed on fruit 10 to 14 days later and the bacteria were reisolated from inoculated fruit. Fruit inoculated with sterile water did not develop symptoms. Female flowers of honeydew plants (cv. Honeybrew) were inoculated at the time of pollination with a suspension of bacteria daubed on the fruit surface. Pollinated fruits were sealed in a moist chamber for 48 h. Circular, water-soaked spots developed 10 to 14 days later. A. avenae subsp. citrulli was reisolated from these lesions. The affected 44-ha field utilized center pivot irrigation and the incidence of diseased fruit exceeded 50%. The source of the pathogen is unknown. Bacterial fruit blotch was identified in watermelon grown under overhead irrigation 0.8 km from this honeydew field 9 weeks earlier. In past years, bacterial fruit blotch has occurred in watermelon fields in Frio County (1). Honeydew from other fields in the vicinity, including one that was also sprinkler irrigated, did not show these symptoms. This is the first report of A. avenae subsp. citrulli causing a disease of honeydew fruit. Reference: M. C. Black et al. Plant Dis. 78:831, 1994.