In Vivo and in Vitro Interactions between Pseudomonas fluorescens and Pythium ultimum in the Suppression of Damping-off in Tomato Seedlings

Pseudomonas fluorescens 5.014 and its mutant 5-2/4, which is capable of 2,4-diacetylphloroglucinol production, were evaluated for biological control of Pythium ultimum on tomato seedlings. Bacterization of tomato seeds with these strains reduced the development of P. ultimum-induced damping-off expressed as plant weight, disease index, and fungal colonization. The protection against Pythium-induced damping-off did not differ significantly between the strains in comparisons of fresh weight of tomato seedlings. However, strain 5-2/4 protected the seedlings to a higher extent in comparisons of disease index. The time of introduction of the pathogen and the amount of fungal inoculum were important for disease development. Strain 5.014 inhibited P. ultimum to a larger extent than the mutant 5-2/4 on Kingʹs medium B, a medium that promotes siderophore production. HPLC analysis showed that strain 5-2/4 produced a significantly lower amount of siderophores. The mutant was also observed to produce a lower amount of extracellular protease in vitro compared to strain 5.014. On yeast–malt medium, mutant strain 5-2/4 inhibited the growth of P. ultimum to a larger extent than 5.014 and produced a high amount of the antibiotic 2,4-diacetylphloroglucinol. P. ultimum exposed to 2,4-diacetylphloroglucinol in vitro formed disrupted mycelium. A significant increase of glutathione, a reducing tripeptide important for cellular protection, was noted in P. ultimum after exposure to the antibiotic compound (10 μg/ml).