Effect of growth culture physiological state, metabolites, and formulation on the viability, phytotoxicity, and efficacy of the take-all biocontrol agent Pseudomonas fluorescens 2-79 stored encapsulated on wheat seeds

Strain 2-79 is a biocontrol agent of take-all, an important root disease of wheat caused by Gaeumannomyces graminis var. tritici. In the rhizosphere, strain 2-79 produces the antibiotic phenazine-1-carboxylic acid as the primary means of disease suppression. Barriers to the commercial use of phenazine-producing pseudomonads, such as strain 2-79 include the lack of liquid-culture and formulation technologies needed to optimize cost-effective mass production and application. For instance, there is little published research concerning the impact of growth culture physiological state and associated metabolites on the biocontrol qualities of the cells harvested and formulated in seed coatings, i.e., efficacy, phytotoxicity, and storage survival. To enable exploration of these issues, cells of strain 2-79 in various physiological states were obtained by harvesting fermentors at 24-h intervals after inoculation. Cells formulated in 0.5% methylcellulose suspended in either water (MW) or metabolite-bearing, spent culture broth (MSB) were applied as wheat-seed coatings, air dried, and stored at 4 degrees C. Younger cells (24-48 h) had twice the drying survival rate but only half of the storage life demonstrated by older cells (72-96 h) (P less than or equal to 0.05). Cell populations surviving drying were 3.5 times higher in MW than in MSB formulations and they remained viable up to 3 times longer (P less than or equal to 0.05). This effect of formulation on viability was attributable to the culture nutrients but not the metabolites present in the spent broth. Disease suppression in bacterized seed treatments was significant (P less than or equal to 0.05) relative to unbacterized controls and averaged 9.1%, but did not vary significantly (P greater than or equal to 0.24) with culture age, encapsulation medium, or storage time. Relative seedling height improvement increased with relative disease suppression (P = 0.003) and significantly decreased with lengthening storage time (P = 0.004). This latter decline in plant growth promotion coincided with the deterioration of biocontrol agent viability during storage. Seed batches inoculated with cells in both MW and MSB encapsulations suffered significant germination losses due to phytotoxic metabolites. The extent of loss was an interactive result of encapsulation medium and storage time (P greater than or equal to 0.01), and the rate of loss was much higher for seeds with MSB than with MW coatings, i.e. 54% compared to 11% loss after 6 months storage.