Conclusions
After screening 85 PAC strains for inhibitory effect against P. plurivora and performing subsequent plate assays using the culture filtrates of eight strains, one strain of interest was identified. This strain, P. europaea 3_136_1, produced at least four compounds, which are reported for the first time from this genus and species. Two of these, sclerin and sclerotinin A, inhibited P. citricola s.s. in the low mM range. These compounds have not been reported as antimicrobial despite many studies on their biological effects. Previous researchers have shown that sclerin and sclerotinin A and related compounds are phytotoxic in vivo ( Pedras & Ahiahonu 2004). As noted above, one hypothesis for the mode of action for DSE was improvement in root health ( Schulz 2006). Our data show that they are toxic to oomycetes of the P. citricola s.l. species complex. This study provides a mechanistic basis to support the idea that some PAC isolates protect their host against some oomycete root pathogens ( Tellenbach & Sieber 2012). The selection scheme used to detect and characterize these metabolites was first to detect the variety of interaction types between PAC and P. plurivora, quantifying the strength of interaction afterward, and finally, targeting metabolites and assaying their antagonistic potential. Therefore, these results justify the need for further testing using PAC isolates to investigate the variability in the production of these metabolites and to inoculate conifer trees with isolate P. europaea 3_136_1 and other PAC strains to potentially reduce the damage resulting from Phytophthora in a more natural environment.