BIOLOGICAL-CONTROL OF DAMPING-OFF OF CUCUMBER CAUSED BY PYTHIUM-ULTIMUM WITH A ROOT-COLONIZATION-DEFICIENT STRAIN OF ESCHERICHIA-COLI

Suppression of damping-off of cucumber and root colonization was compa red for strain 501R3 of Enterobacter cloacae and strain S17R1 of Esche richia coli, a rifampicin-resistant mutant of E. coli S17-1. Both stra ins 501R3 and S17R1 provided effective biocontrol of pre-emergence and post-emergence damping-off of cucumber caused by Pythium ultimum when applied as seed treatments containing approximately 10(7) cfu per see d. Plant stands from seeds treated with 501R3 or with S17R1 were 93% a nd 92%, respectively (similar at P less than or equal to 0.05), when s own in soil-less mix infested with P. ultimum. The plant stand with th e pathogen control was only 30% - significantly less (P less than or e qual to 0.05) than stands from the 501R3 and S17R1 treatments. Cucumbe r was only briefly susceptible to damping-off caused by P. ultimum sin ce seeds pregerminated for 29 h or longer before sowing in P. ultimum- infested soil-less mix yielded plant stands similar to the healthy con trol treatment. Plant stands from non-pregerminated seeds were signifi cantly less (P less than or equal to 0.05) than pregerminated seed tre atments. E. coli strain S17R1 was not an effective colonizer of cucumb er rhizosphere. When applied as seed treatments containing approximate ly 10(4) or 10(8) cfu per seed, populations of strain S17R1 were signi ficantly smaller (P less than or equal to 0.05) than with correspondin g treatments of strain 501R3 at 24, 48, 72 and 96 h after sowing in so il-less mix. Populations of strain S17R1 increased only 65-fold over 9 6 h when applied at 10(4) cfu per seed. In contrast, populations of st rain 501R3 increased 24 000-fold over the same period when applied at 10(4) cfu per seed. Populations of strain S17R1 were limited to the up per 1 cm of cucumber root at 7 days and decreased to approximately 10( 1) cfu/g of total root system over 42 days. Strain 501R3 was capable o f becoming widely distributed along cucumber roots and was detected at populations of approximately 10(8) cfu/g of total root system at 42 d ays. Results from this study indicate that E. coli is capable of suppr essing damping-off of cucumber caused by P. ultimum. In addition, exte nsive root colonization by E. coli strain S17R1 was not required in th is biocontrol interaction because of the brief period of susceptibilit y of cucumber to this disease.