Selection of biocontrol agents of phytopathogenic oomycetes to be used in protection strategies in combination to low doses of copper in organic farming

Although the European Union is aiming at banning copper fungicides in organic farming, an alternative with the same efficacy as copper is not yet available and the control of phytopathogenic oomycetes is still based mainly on the application of this metal. Thus the combination of low doses of copper with a biocontrol agent could be a short term strategy. Copper is a large spectrum fungicide and bactericide, so the combination with microbial biocontrol agents could be very difficult. Up to now the selection of new biocontrol agents did not take in account the possibility to be combined or alternated in strategies with reduced doses of copper and no studies exist regarding this approach. On this basis, we selected a bacterial strain, Lysobacter capsici AZ78, which is able to survive in presence of copper ions and effectively control Plasmopara viticola and Phytophthora infestans. Trials carried out under controlled conditions showed that the combination of L. capsici AZ78 and low doses of copper-based fungicides provided a better control of both the oomycetes than the two active ingredients used solo. Following the sequencing and annotation of the genome of L. capsici AZ78, we hypothesize that the copper resistance relies on the presence of genes encoding for copper efflux system as the copper exporting PIB-type ATPases (ctpA) and the copper resistant operon that contains the genes encoding for the multicopper oxidase (copA) and copper resistance protein B (copB). Interestingly, mining the genome of L. capsici AZ78 also showed the presence of several genes involved in the resistance to other heavy metals as arsenic (arsenate reductase), cadmium, cobalt and zinc (Resistance-nodulation-cell division multidrug transporters). Overall, the results achieved in this work suggest that L. capsici AZ78 could be a suitable candidate for developing a new biofungicide that can be combined with low doses of copper in organic farming. Furthermore, the resistance to other heavy metals of L. capsici AZ78 may be the basis for other type of uses, for example as biofungicide in heavy metal contaminated soils.