Detection of Acidovorax valerianellae, the causing agent of bacterial leaf spots in corn salad [Valerianella locusta (L.) Laterr.], in corn salad seeds

Aim:  The black leaf spot disease on corn salad caused by the bacterium Acidovorax valerianellae has been observed in Europe for several years and causes economic losses in corn salad cropping. Contaminated seeds or infested soil are considered as the major infection sources. The use of healthy seed material is the only way to prevent disease outbreaks. Therefore, a sensitive diagnostic method for seed testing should be developed. Methods and Results:  Using a triple antibody sandwich ELISA with a high-specific monoclonal antibody, a quick and reliable detection method for contamination of seed lots with the pathogen was developed. This method allowed to detect contaminated seed lots as well as contamination with A. valerianellae in single seeds. Furthermore, the occurrence and distribution of the pathogen could be shown in symptomatic corn salad leaves and in naturally infested seeds by transmission electron microscopy and immunogold labelling for the first time. Conclusion:  Our results confirm the seed transmission of this corn salad disease. Pathogen load and distribution vary between positively tested seed lots. Significance and Impact of the study:  With this method, not only routine testing of seed material to eliminate contaminated seed lots from production is possible but also the control of sanitation procedures to reduce contamination.