Acidovorax avenae subsp. citrulli is a gram-negative, rod-shaped, bacterium with average dimensions of 0.5 x 1.7 µm, and is motile by a single polar flagellum. The bacterium is strictly aerobic, positive for oxidase activity and grows at 41° C (106° F) but not 4° C (39° F). A. avenae subsp. citrulli grows on many general bacterial growth media and produces smooth, round, cream-colored, nonfluorescent colonies after 48 hr on King’s medium B (Figure 21). Using physiological characteristics , the pathogen was initially classified as a member of the group III pseudomonads and identified as Pseudomonas pseudoalcaligenes subsp. citrulli. Based on nucleic acid hybridization data, analysis of whole-cell protein and carbon substrate utilization profiles, the pathogen was reclassified as A. avenae subsp. citrulli.