Suppression of Sclerotinia sclerotiorum apothecial formation by the soil bacterium Serratia plymuthica: identification of a chlorinated macrolide as one of the causal agents

A selection of soil bacteria was screened for their ability to interfere wi th carpogenic germination of Sclerotinia sclerotiorum. Nine out of 300 bact erial isolates were found to significantly suppress apothecial formation. O ne of these isolates, identified as a strain of Serratia plymuthica, was hi ghly effective in inducing complete suppression of apothecial formation at high concentrations, and also strongly inhibited the germination of ascospo res as well as hyphal growth of S. sclerotiorum. A bioassay-guided purifica tion procedure starting with the cell-free supernatant of the bacterial cul ture led to the identification of a chlorinated macrolide as an active comp ound able to induce the observed inhibition. Spectroscopic data showed the compound to be identical to haterumalide A. The data presented show the abi lity of this compound to inhibit apothecial formation and ascospore germina tion. Other possible mechanisms involved in inhibition of apothecial format ion and mycelial and hyphal growth of S. sclerotiorum by the same isolate a re discussed. The relevance of our observations to natural systems will be the subject of further research. (C) 2001 Elsevier Science Ltd. All rights reserved.