ECOLOGICAL INTERACTION OF A BIOCONTROL PSEUDOMONAS-FLUORESCENS STRAINPRODUCING 2,4-DIACETYLPHLOROGLUCINOL WITH THE SOFT-ROT POTATO PATHOGEN ERWINIA-CAROTOVORA SUBSP ATROSEPTICA

Erwinia carotovora subspecies atroseptica is the agent of soft rot of potato and causes important crop damage in Europe. Synthetic 2,4-diace tylphloroglucinol (DAPG) inhibited the growth of E. carotovora subsp. atroseptica under in vitro conditions and Pseudomonas fluorescens F113 , which produces DAPG, was studied for biocontrol of a carotovora subs p. atroseptica. Wild-type F113 (or the spontaneous rifampicin-resistan t mutant F113Rif) inhibited growth of E., carotovora subsp. atroseptic a on solid medium, displayed bactericidal activity towards the pathoge n in liquid medium, and prevented Erwinia-mediated rotting of wounded potato tuber under in vitro conditions. F113Rif reduced the population size of E. carotovora subsp. atroseptica in soil and on potato tuber dice in competition experiments carried out with unplanted soil and so il planted with diced potato tubers, respectively. Go-inoculation of p otato tuber seeds with F113Rif and E. carotovora subsp. atroseptica re duced Erwinia contamination of the seed tubers compared with single in oculation with the pathogen. F113G22 is a Tn5::lacZY-induced DAPG-nega tive biosynthetic derivative of F113 and showed no antibiosis towards E. carotovora subsp. atroseptica in vitro. In contrast to F113Rif, F11 3G22 did not inhibit Erwinia-mediated rotting of wounded potato tuber in vitro, did not influence survival off. carotovora subsp. atroseptic a in unplanted soil or soil planted with potato tuber dice and did not reduce Erwinia contamination of potato seed tubers. F113G22(pCU203) i s a complemented derivative with restored DAPG-producing ability. F113 G22(pCU203) had similar effects against E. carotovora subsp. atrosepti ca as F113 (or F113Rif) under in vitro conditions and in soil microcos ms. The results indicate that P. fluorescens F113 is a promising bioco ntrol agent against the potato soft rot agent E. carotovora subsp. atr oseptica and suggest that the pseudomonad's ability to produce DAPG is a key factor in its inhibition of the pathogen.