Poster Session: Detection and Diagnosis-Bacteria

331-P

Optimization of copper resistance testing methods for foliar bacterial pathogens of tomato.
S. BOST (1), J. Mixon (1), B. Ownley (2), K. Gwinn (2), C. Sams (2)
(1) University of Tennessee, Nashville, TN, U.S.A.; (2) University of Tennessee, Knoxville, TN, U.S.A.

Copper is a primary control product for bacterial spot (Xanthomonas spp.), bacterial speck (Pseudomonas syringae pv. tomato) and bacterial canker (Clavibacter michiganensis subsp. michiganensis) of tomato. There is a need for a technique that would accurately measure the level of resistance to copper in foliar bacterial pathogens of tomato in a protocol that would be practical for processing large numbers of samples. A protocol was developed that included colony growth measurement on solid medium amended with CuSO₄*5H₂O (200 µg/ml), expressed as a percent of that on a non-amended control. Parameters were modified to produce copper sensitivity reactions that approximated in-vivo disease-control levels. Factors causing greatest variability in cell resistance to copper were medium composition, medium pH, and age of prepared medium. Freshly-prepared sucrose peptone agar adjusted to pH 6.8 (Xanthomonas spp. and C. michiganensis subsp. michiganensis) or 6.2 (P. syringae pv. tomato), provided copper sensitivity ratings that corresponded well with bioassay ratings. Because of its known low-complexing activity, 2-(N-morpholino) ethanesulfonic acid (20 mM) was used to counteract the depressing effect of copper on medium pH.