EVALUATION OF A PLATING ASSAY FOR XANTHOMONAS-CAMPESTRIS PV CAMPESTRIS

A plating assay for detecting Xanthomonas campestris pv. campestris in crucifer seeds was evaluated. The results showed no significant effect of a centrifugation step prior to plating compared to an assay without a centrifugation step with respect to logarithms of colony-forming units (log cfu) of X. c. pv. campestris per ml. The addition of Tween 20, Benlate (benomyl) and Daconil (chlorothalonil) to saline (0.85% NaCl) generally resulted in less log cfu of X. c. pv. campestris per ml. The extraction methods 2.5 hours shaking at room temperature (21 +/- 2-degrees-C) and 1.5 hours soaking in a refrigerator (4-6-degrees-C) generally gave more log cfu of X. c. pv. campestris per ml than 5 minutes shaking the seeds at room temperature. However, 2.5 hours shaking or 1.5 hours soaking of seed lots did not result in more positive (= infected) seed lots than 5 minutes shaking. No significant differences were found between the plating media NSCA and NSCAA for any of the extraction methods with respect to log cfu of X. c. pv. campestris, the number of positive seed lots, or the number of positive subsamples per seed lot. The FS medium was compared to NSCA and NSCAA in one experiment using 5 minutes shaking the seed lots in saline at room temperature. No significant differences were found between NSCA, NSCAA and FS medium with respect to the number of positive seed lots. Also no differences were found between testing seed lots in five subsamples of 2,000 seeds or one sample of 10,000 seeds with regard to the log cfu of X. c. pv. campestris per ml and the number of positive seed lots. A general scheme for routine testing in the Netherlands is presented.