Selective Isolation ofXanthomonas campestrispv.campestrisfrom Crucifer Seeds

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Abstract
Saprophytic and antagonistic bacteria associated with crucifer [Brassica spp.] seeds often interfere with the isolation of X. campestris pv. campestris [Xcc] on general plating media such as nutrient starch cycloheximide agar (NSCA). The number of colonies of Xcc recovered from various crucifer seed lots often depends on the number of saprophytic and antagonistic bacteria present on NSCA plates. Up to 50% fewer colonies of Xcc were recovered when antagonistic bacteria were present in the seed washings. Adding nitrofurantoin and vancomycin, respectively, at 10 and 0.5 .mu.g ml to NSCA (=NSCAA) and 2 and 0.1 .mu.g ml to basal starch cycloheximide agar (=BSCAA) significantly reduced the development of saprophytic and antagonistic bacteria recovered from washings of crucifer seeds. On NSCAA, 20 strains of Xcc had plating efficiencies of 0.87 or more compared to 1.00 for NSCA. On BSCAA, 15 strains had plating efficiencies of 0.85 or greater, whereas those of 5 additional strains ranged 0.54-0.72. On NSCAA or BSCAA, Xcc was recovered from undiluted or 1:10 dilutions of seed washings, whereas on NSCA, dilutions of 1:100 were needed to reduce saprophytic and/or antagonistic bacteria. Twelve of 102 commercial seed lots assayed during a 2 mo. period on NSCA. NSCAA and BSCAA yielded Xcc. In 6 of these seed lots Xcc was not detected by using NSCA alone. Xcc was detected in 4 lots on all 3 media, in 4 lots on BSCAA alone, and in 1 lot on NSCAA. Because of a zero tolerance for black rot and the natural variation in the bacterial flora of crucifer seeds, all 3 media should be used for assaying seeds for Xcc.

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