Bacterial inoculum from a previous crop affects fungal disease development on subsequent nonhost crops.

•  Winter wheat (Triticum aestivum) and winter barley (Hordeum vulgare) were grown on soil contaminated with different levels of a marked bacterial potato pathogen Erwinia carotovora ssp. atroseptica (Eca) from a previous blackleg-infected potato crop, to determine whether the presence of this pathogen could affect disease development on wheat and barley. •  The diseases 'septoria leaf blotch' and 'powdery mildew' on wheat (caused by Septoria tritici and Blumeria graminis f. sp. tritici, respectively) and 'scald' or 'rhynchosporium' on barley (caused by Rhynchosporium secalis) were assessed over two growing seasons, and the presence of the marked Eca strain monitored. •  Wheat diseases were greater on the areas with high levels of Eca contamination than on areas with low-level contamination. Rhynchosporium on barley was not affected overall, although it decreased on the high-level contamination areas early in the season. The Eca strain was detected on the upper foliage of both wheat and barley. •  Increased S. tritici symptoms in the field may be due to 'synergistic' interactions between the Eca and the fungal mycelium, as reported in previously published laboratory experiments. The potential importance of such fungal-bacterial interactions is discussed.