Acidovorax avenae subsp. citrulli newly reported to cause bacterial fruit blotch of watermelon in Greece

In July 2005 and September 2006, samples of mature fruits of F1 hybrid watermelon (Citrullus lanatus) cv. Obla were received from the areas of Chryssoupoli (Macedonia, northern Greece) and Vagia (central Greece), respectively. Fruits had small, irregular, water-soaked lesions and brown cracks on their surface, brown discolouration and water-soaked areas in the rind underneath the lesions, and watery flesh rot (Figs. 1, 2). Disease incidence was reported as severe in both areas, according to the sample information sheets sent by local agronomists. Bacterial isolates recovered on nutrient agar (NA) from the affected fruits were Gram-negative, oxidase positive, non-fluorescent on King’s medium B, pathogenic to inoculated watermelon fruits and to seedlings of watermelon, melon, cucumber and pumpkin; isolates induced tobacco hypersensitivity and formed characteristic white colonies on nutrient agar. Based on these data, the isolates were identified as Acidovorax avenae subsp. citrulli (Aac).

In May 2008, young grafted watermelon plants (F1 hybrid cv. Byblos) received from the area of Varda (Peloponnese, southern Greece) showed brown, angular, necrotic spots or larger lesions on leaves (Figs. 3, 4). This outbreak was reported by the agronomist in charge of the crop to have affected about 50% of a plot of 12,000 plants. The bacterial isolates recovered on NA from the affected plants showed the above properties and also growth at 41°C, no starch hydrolysis, oxidative glucose metabolism and utilisation of D-galactose, D-glucose, L-arabinose, but not adonitol, arginine or sucrose, as sole carbon source. An immunofluorescence test with Aac-specific polyclonal antiserum (LOEWE, Germany) and sequencing (COGENICS, UK)of the products of PCR with two sets of Aac-specific primers, viz. BX-L1F/BX-R5F and BX-L1F/BX-S-R2R (Bahar et al., 2008), or the set 63f/1389r (Osborn et al., 2000) amplifying part of the 16S rDNA region, verified the isolates as Aac.In Rep-, Eric- or Box-PCR, the isolates from plants and Aac reference strains produced similar banding patterns. Koch’s postulates were fulfilled on seedlings and fruits of the above mentioned cucurbits with all isolates from plants. One such isolate was deposited in the Benaki Phytopathological Institute Culture Collection as BPIC2124. This is the first report confirming Aac naturally infecting watermelon plants and fruits in Greece.