The effects of various fungi, bacteria and different compost extracts on foliar infection of potato (Solarium tuberosum) with Phytophthora infestans were tested in detached leaf bioassays in 2001-2003. Application of microorganism inocula and compost extracts as well as copper oxychloride to excised leaves resulted in different degrees of blight control. In general, suppression of blight lesion growth was not improved by applying the antagonists before the leaves were inoculated with Phytophthora spores. There was some evidence that extracts made from different compost feed stocks of different ages suppressed leaflet infection with blight. However, results were very limited and inconsistent. Moreover, the effects were much smaller than where copper oxychloride was used. Improved efficacy of acceptable alternatives to copper fungicides especially in organic farming is required. No promising effective alternative to the use of copper fungicides to reduce late blight infection in organic potato production systems was identified in the experiments.

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