<i>Brenneria salicis</i>, the bacterium causing watermark disease in willow, resides as an endophyte in wood

Summary Brenneria salicis has been studied in willow wood only in relation to watermark disease. In this pathogenic condition, the bacterium occurs at high concentrations. Pathogenicity of B. salicis is still uncontrollable and the disease unpredictable because the plant–bacteria interaction is not understood. Thanks to molecular techniques B. salicis can be detected at low concentrations, which are found in most non‐pathogenic interactions. Brenneria salicis was identified and traced with a new specific three‐primer polymerase chain reaction and its identity and relative concentration in biological samples confirmed through denaturing gradient gel electrophoresis profiling. Brenneria salicis was found in symptomless willows sampled randomly in Flanders agricultural areas, in young nursery willows, and also in poplar ( Populus ) and alder ( Alnus ). It harboured the nitrogenase reductase gene NifH and promoted growth and chlorophyll in willow. Inoculated luminescence‐marked B. salicis circulated through the whole plant without inducing disease and exuded at the leaf margins. Other willow endophytes identified were Rahnella , Sphingomonas and Methylobacterium . In conclusion, because endophytic B. salicis is generally observed in willow, disease must not be dependent on infection. Leaf‐to‐leaf spread is proposed as an important mechanism for spread of B. salicis .