Detection of Acidovorax valerianellae in corn-salad seeds, seed transmission of the pathogen and disease development in the field

Acidovorax valerianellae , the causal agent of bacterial black spot of corn salad and responsible for severe economic losses to this vegetable in France, was successfully transmitted to corn-salad plants by artificially inoculated seeds in glasshouse and field experiments. In the field experiments, climatic data recorded under plastic tunnels indicated that increasing temperature and relative humidity increased symptom development. To investigate the possible contamination of commercial seedlots of corn salad, a seed test was developed consisting of soaking batches of seeds (five batches each of 5000, 1000, 500 and 100 seeds) overnight at 4°C in distilled sterile water, followed by dilution-plating of seed extracts on TSAV (tryptic soya agar for A. valerianellae ) semiselective medium. Suspected colonies were identified by biochemical and pathogenicity tests or, within 24 h, using antibodies specific to A. valerianellae . Acidovorax valerianellae was detected in three lots. Seed infection levels ranged from 0·10 to 0·89% of contaminated seeds and a single seed carried up to 1800 A. valerianellae colony-forming units.