Natural Infection of Citronmelon with Acidovorax avenae subsp. citrulli

Abstract

Citronmelon fruits (Citrullus lanatus var. citroides (Bailey) Mansf.) with lesions were collected from a cowpea field in Frio County, TX, in July 1997. The lesions were circular, necrotic, or water-soaked, approximately 3 mm in diameter, and did not extend into the flesh of the fruit. Nonfluorescent, gram-negative bacteria were consistently isolated from lesions. Six representative strains were identified as Acidovorax avenae subsp. citrulli (Aac), using Biolog GN MicroPlates and the MicroLog data base release 3.50 (0.533 to 0.813 similarity). Aac causes leaf and fruit lesions (bacterial fruit blotch, BFB) on watermelon (C. lanatus (Thunb.) Matsum. & Nakai). Strains were tested for pathogenicity on watermelon seedlings (cv. Royal Sweet) by daubing bacterial suspensions (approximately 108 CFU/ml) onto cotyledons of 1-week-old seedlings. Water soaking of cotyledons, followed by necrosis and seedling death, occurred within 5 days. These symptoms were indistinguishable from those caused by watermelon strains of Aac. Bacteria were reisolated from symptomatic seedlings. The source of the infection is not known. Watermelons had been grown in this field in 1996, but no BFB symptoms were observed. Citron fruit infected with Aac were found in nearby watermelon fields where BFB was present; the closest field was 50 m from the cowpea field. These observations suggest that citronmelon, a common weed in south Texas, has the potential to perpetuate Aac. This is the first documentation of a naturally occurring infection of citronmelon with Aac.