Biological Control of Damping‐off of Cucumber caused by Pythium ultimum with a Root‐Colonization‐Deficient strain of Escherichia coli

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Abstract
Suppression of damping‐off of cucumber and root colonization was compared for strain 501R3 of Enterobacter cloacae and strain S17R1 of Escherichia coli, a rifampicinresistant mutant of E. coli S17‐1. Both strains 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 107 cfu per seed. Plant stands from seeds treated with 501R3 or with S17R1 were 93% and 92%, respectively (similar at P ≥ 0.05), when sown in soil‐less mix infested with P. ultimum. The plant stand with the pathogen control was only 30%—significantly less (P ≥ 0.05) than stands from the 501R3 and S17R1 treatments. Cucumber was only briefly susceptible to damping‐off caused by P. ultimum since seeds pregerminated for 29 h or longer before sowing in P. ultimum‐infested soil‐less mix yielded plant stands similar to the healthy control treatment. Plant stands from nonpregerminated seeds were significantly less (P ≥ 0.05) than pregerminated seed treatments. E. coli strain S17R1 was not an effective colonizer of cucumber rhizosphere. When applied as seed treatments containing approximately 104 or 108 cfu per seed, populations of strain S17R1 were significantly smaller (P ≥ 0.05) than with corresponding treatments of strain 501R3 at 24, 48, 72 and 96 h after sowing in soil‐less mix. Populations of strain S17R1 increased only 65‐fold over 96 h when applied at 104 cfu per seed. In contrast, populations of strain 501R3 increased 24000‐fold over the same period when applied at 104 cfu per seed. Populations of strain S17R1 were limited to the upper 1 cm of cucumber root at 7 days and decreased to approximately 101 cfu/g of total root system over 42 days. Strain 501R3 was capable of becoming widely distributed along cucumber roots and was detected at populations of approximately 108 cfu/g of total root system at 42 days. Results from this study indicate that E. coli is capable of suppressing damping‐off of cucumber caused by P. ultimum. In addition, extensive root colonization by E. coli strain S17R1 was not required in this biocontrol interaction because of the brief period of susceptibility of cucumber to this disease.