Location: Fruit and Nut Research
Title: A comparison of the bioassay test and culture to detect Xanthomonas citri subsp. citri Authors
Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 6, 2013
Publication Date: N/A
Interpretive Summary: Citrus canker (caused by the bacterium Xanthomonas citri subsp. citri, Xcc) causes serious losses in citrus in tropical and subtropical citrus production regions. Detecting Xcc is critical for regulation purposes, research and disease management. This study compared culture to bioassay for detecting bacteria of citrus canker. Washes of lesions from fruit, leaves and shoots using a culture-based method for detection were compared to a “gold-standard” bioassay of injection-infiltrated leaves of susceptible ‘Duncan’ grapefruit. There was good agreement in detection of active lesions identified by both methods. False negatives were low (0.9% to 6.5% of lesions). False positives ranged from 4.3 to 21.4%. The greatest proportion of false positives was obtained when colony forming units of Xcc was =102 bacteria/mm2/min. A Receiver Operator Characteristic analysis of these data demonstrated good to excellent accuracy of culture in detecting Xcc (Area Under the Curve = 0.80-0.98). Compared to bioassay, culture can be a reliable way to detect Xcc in lesion samples. These data confirm the usefulness of culture as a reliable method to detect active citrus canker lesions in samples of grapefruit both for scientific and regulatory purposes.
Technical Abstract: Citrus canker (caused by Xanthomonas citri subsp. citri [Xcc]) can cause yield loss of susceptible citrus and result in trade restrictions of fresh fruit. For both regulatory purposes and epidemiological studies, accurate detection and quantification of viable inoculum is critical. Two accepted methods used to detect and quantify Xcc are injection-infiltration bioassay and culture, but these two methods have not been directly compared using field-obtained samples. The two methods were compared using washates of lesions were taken from fruit, leaves and shoots in a commercial orchard in Florida in 2009-10 and 2010-11, with bioassay being the assumed standard. Despite some misclassifications, true positives (sensitivity) and true negatives (specificity) were the dominant classes using culture. False positives for lesions from shoots ranged from 13.1 to 21.4% in 2009-10 and 2010-11, respectively, and false positives for lesions from fruit and leaves ranged from 4.3to 15.7%, in the two seasons, respectively. The false positive rate for culture compared to injection-infiltration bioassay was highest (0.16 to 0.55), due to more frequent recovery of Xcc by culture at =102 colony forming units (CFU) Xcc per ml. The false negative rate was consistently lower (0.02-0.21), confirming that in only a few cases did culture fail to detect Xcc when it was present. Area under the curve for receiver operator characteristic analysis ranged from 0.80-0.97, confirming that culture provided an accurate diagnosis in most cases. There was a higher frequency of lesions from shoots with a CFU =102 Xcc compared to lesions from fruit or leaves, making culture more effective at detecting these. The data demonstrate that culture is a reliable way to detect and quantify Xcc compared to injection-infiltration bioassay, particularly when the CFU is =102 Xcc per ml.