Improved Sample Preparation for PCR-Based Assays in the Detection of Xanthomonads Causing Bacterial Leaf Spot of Tomato

Aims: To develop a sampling procedure for PCR-based screening of bacterial leaf spot (BLS)-causing xanthomonads without DNA extraction from infected tomato plants.

Place and Duration of Study: University of Copenhagen, Denmark and Sokoine University of Agriculture, Morogoro, Tanzania between July 2008 and November 2010.

Methodology: Flinders Technology Associates (FTA®) plant cards and Chromatography paper or Whatman® paper strips (WPS) were spotted with bacterial suspensions from 24-h-old cultures from reference strains of BLS-causing xanthomonads, or sap obtained by grinding or hand maceration of plant tissue, were used as templates in PCR reactions or isolation of live bacterial cells on Nutrient agar (NA) media. Samples were tested by PCR with Xan 7 genus/-specific Xanthomonas primers or in multiplex with 26S rRNA primers. Isolation of bacteria was done by streaking aliquots of 75 µl of a suspension from a disc (2-mm-punch by Harris Micro Punch®) in triplicate, removed from each of the FTA plant card and WPS onto NA media.

Results: The FTA plant card spotted with pure cultures of reference strains of xanthomonads and sap from grinding or direct maceration of plant tissue resulted in more clear PCR bands (402 bp) and (594 bp of rRNA gene in multiplex) than the WPS samples. Sensitivity of detection by the FTA paper-based PCR was ≈ 5.0 x 102, while that of the WPS was > 1.0 x 103 CFU/ml. The WPS (but not the FTA) was proved to be useful for saving living bacteria cells for up to one week of storage at ambient temperatures.

Conclusion: Both FTA plant card and WPS can be used for PCR detection of BLS-causing xanthomonads in tomato. However, the FTA plant card is recommended as it produced clearer PCR products than WPS. WPS is recommended for experiments requiring isolation of live bacterial cells on NA media.