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Pathogen Biology
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.
 Figure 21 |
Originally A. avenae subsp. citrulli was reported to cause watermelon seedling lesions and blight, but no fruit rot. Additionally, the original strain recovered from plant introductions at the Georgia (GA) Agricultural Experiment Station did not induce a hypersensitive response (HR) on tobacco leaves. In contrast, strains recovered from the first BFB outbreaks in commercial watermelon fields in 1989 were HR-positive (5) and caused severe fruit rot. This discrepancy led to confusion about the identity of the causal agent of BFB. However, based on fatty acid analysis, DNA fingerprinting data (Figure 22), and carbon substrate utilization, it was shown that at least two distinct sub-groups of the pathogen existed . In general, group II strains are highly aggressive on watermelon , mildly aggressive on other cucurbits, and include strains that caused the original BFB outbreaks in commercial fields. In contrast, group I strains are mildly to moderately aggressive on a wide range of cucurbit hosts and include the strains that caused watermelon seedling blight on plant introductions at the GA Agricultural Experiment Station. This strain might have mutated to reduced aggressiveness, or it might represent a genetically distinct subpopulation. At present there is still much that is unknown about the biology of A. avenae subsp. citrulli.
 Figure 22 |