The watermelon fruit blotch organism (Acidovorax avenae subsp. citrulli) was confirmed as the cause of a bacterial disease of rockmelon seedlings in January 1996. Further outbreaks occurred in commercial nurseries during 1996–98. An associated field disease was not observed in rockmelon and honeydew crops until May 1998 when wet conditions led to severe leaf spotting and fruit infection in many crops in the Burdekin district of North Queensland. Isolates of A. avenae subsp. citrulli originating from these outbreaks were considerably more pathogenic to rockmelon plants than isolates originating from watermelon crops in South Queensland. They were also less pathogenic to the weed host Cucumis myriocarpus and could constitute new strains of the fruit blotch organism. Tests showed the disease was readily seed transmissible from naturally infected rockmelon and honeydew fruit for at least 3 months after seed extraction.

In March 1999, a foliar bacterial disease was observed in a commercial crop of cucumber (Cucumis sativus L.) cv. Jetset in Gumlu in northern Queensland, Australia. Initial symptoms consisted of angular, chlorotic, water-soaked lesions that later dried to necrotic areas of light brown, dead tissue. White bacterial ooze was commonly found on the undersides of young water-soaked lesions. Lesions were delimited by veins and distributed uniformly over leaf surfaces, and more than 20% of the crop was affected. No symptoms were observed on plant stems or fruits. Bacterial streaming from the edges of freshly cut young lesions was clearly visible in a droplet of water under ×100 magnification in the laboratory. Isolations were made from young lesions on King's medium B (1). A slow-growing, white, gram-negative, nonfluorescent bacterium was consistently isolated. Three isolates of the bacterium were identified, using the Biolog software program (Biolog, Hayward CA), and in each instance, the bacterium was confirmed as Acidovorax avenae subsp. citrulli, with a similarity of >0.80. Koch's postulates were completed with 8-day-old glasshouse-grown cucumber (cv. Jetset) seedlings. Seedlings were misted until runoff with a bacterial suspension of 3 × 108 CFU/ml and enclosed in plastic bags for ≈30 h at 22°C. Water-soaked lesions were observed on cucumber cotyledons 4 days after inoculation. This is the first report of A. avenae subsp. citrulli as a pathogen of cucumber. Reference: (1) E. O. King et al. J. Lab. Clin. Med. 44:301, 1954.

In March 2001, a foliar bacterial disease was observed on gramma seedlings (Cucurbita moschata L.) cv. Ken Special Hybrid 864 in a commercial nursery in Bowen, north Queensland, Australia. Symptoms included chlorosis of cotyledons and angular, water-soaked lesions from the tips of the cotyledons to the petioles. Brown, angular, water-soaked lesions that were delimited by the leaf veins were also present on newly emerged true leaves. Streaming of bacterial cells from the edges of cut lesions was seen in a droplet of water with ×100 magnification. Isolations attempted on King's medium B consistently yielded a slow-growing, cream to white, gram-negative bacterium. Bacterium was identified as Acidovorax avenae subsp. citrulli based on carbon source utilization profiles (Biolog, Hayward CA) and polymerase chain reaction (PCR) using a primer pair based on the 16S-23S internal transcribed spacer region. When tested in rep-PCR with the BoxA1R primer (2), the isolate produced a banding pattern similar to other Australian A. avenae subsp. citrulli isolates previously shown to be pathogenic to rockmelon (1). Koch's postulates were completed with 20 2-week-old glasshouse-grown gramma (cv. Ken Special Hybrid 864) seedlings. Seedlings were misted until runoff with a 3 × 108 CFU/ml bacterial suspension and enclosed in plastic bags for 48 h at 23°C. Water-soaked lesions developed on cotyledons of all seedlings 6 days after inoculation, and bacterium was reisolated from symptomatic tissue. To our knowledge, this is the first report of A. avenae subsp. citrulli as a pathogen of C. moschata References: (1) R. G. O'Brien and H. L. Martin. Aust. J. Exp. Agric. 39:479, 1999 (2) J. Versalovic et al. Methods Mol. Cell Biol. 5:25, 1994.

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