Comparison of Immunofluorescence and 2 Assays for Detection of Xanthomonas-campestris Pv Undulosa in Seeds of Small Grains

Duveiller, E. Bragard, Claude [UCL]

A monoclonal antibody specific for Xanthomonas campestris pathovars in the translucens group, including X. c. pv. undulosa, the causal agent of bacterial leaf streak or black chaff in wheat and triticale, was used in immunofluorescence and a dot-immunobinding assay to detect pathogens in aqueous seed extracts. These techniques were compared to dilution plating of seed wash water as potential routine seed-indexing methods. Twenty-six cereal seed lots were harvested from plants with different levels of natural disease incidence grown in three locations in Mexico. Among the 26 lots analyzed, pathogen-free seed came from clean fields principally in Ciudad Obregon, where conditions are less favorable for its development. Pathogen populations detected with dilution plate ranged from 1.3 X 10(4) to 5.3 X 10(6) cfu/g of air-dry weight. Populations were recovered from seed lots that had been stored for 3 yr. Some discrepancies in pathogen detection occurred among the different methods used. Seed lots with high pathogen populations were consistently identified with all three methods. Differences in detection thresholds were found with seed wash water as compared to reference strain from pure culture. Apparently a substantial number of dead cells were present in wash water from some seed lots. Immunofluorescence was more sensitive than dot-immunobinding assay and produced more reproducible results. It is proposed as a standard for indexing germ plasm. Because of the high variation in data, direct counts of fluorescent bacterial cells under UV microscopy will probably not be necessary for identifying infected seed. It is recommended that immunofluorescence-positive seed lots should not be considered for sowing in areas favorable for disease development.