The immunological agglutitation method, using either latex particles or Staphylococcus aureus conjugated with anti-Aac-IgG, was tested for diagnosing bacterial fruit blotch of watermelon caused by Acidovorax avenae subsp. citrulli (Aac). In either case, when the suspensions of various plant pathogenic bacteria were tested, only Aac yielded a strong positive reaction. When sap prepared from sections including lesions was tested, both methods produced positive reactions with all sections from artificially inoculated watermelon plants. Moreover, positive reactions were observed with lesions older than 28 days. These methods are very simple, and only about 5min/sample is needed to reach a diagnosis.