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First Report of Acidovorax avenae subsp. citrulli Infecting Edible Seed Watermelon (Citrullus lanatus var. lanatus) in China

Edible seed watermelon, Citrullus lanatus var. lanatus, is a type of watermelon from which only the seeds are consumed. Between 2003 and 2005, a bacterial disease was discovered on this type of watermelon in several regions of Xinjiang Province in western China, with a diseased area of 40,000 ha. The average disease incidence on fruit and seedlings was approximately 30 and 20%, respectively. Symptoms were most noticeable on fruit. Lesions on the fruit rind were first noticeable as small pinpoint water-soaked areas. As the lesions rapidly enlarged, black, starshaped cracks formed in the rind, and the rind along with the flesh around the spots hardened. If no secondary invaders or saprophytes entered through the cracks, the inside flesh often disintegrated into dry stiff cavities. The fruit with necrotic spots became markedly malformed. The pathogen also infected seedlings. First symptoms on the seedling appeared as dark, water-soaked lesions on the underside and edge of the cotyledons. As the cotyledon expanded, the lesions become necrotic and often extended along the length of the midrib and eventually merged into large, black, withered areas. To determine the causal agent of the disease, 54 bacterial strains were isolated from cotyledons, fruits, or seeds of edible seed watermelon. By spray inoculation, these strains were inoculated onto edible seed watermelon cv. Xinzigua1 at the two-true-leaf stage with bacterial concentrations at 10⁷ CFU/ml. All strains were strongly virulent to cv. Xinzigua1, which is planted in large areas in Xinjiang, by forming many water-soaked spots on cotyledons and true leaves. There were no symptoms in the control that was sprayed with sterile deionized water. Through biological and biochemical characteristics, including Gram reaction and tests for catalase, oxidase, and oxygen requirement, all strains were identified as Acidovorax avenae subsp. citrulli. To verify pathogen identification, specific polymerase chain reaction was also carried out. One set of 16S primers, WFB1/WFB2 for A. avenae subsp. citrulli, was used (1). A single unique band of approximately 360 bp was amplified for all strains tested. To our knowledge, this is the first report of A. avenae subsp. citrulli infecting edible seed watermelon.

Reference: (1) R. R. Walcott and R. D. Gitaitis. Plant Dis.84:470, 2000.