Search

Lack of technologies to produce and deliver effective biological control agents (BCAs) is a major barrier to their commercialisation. A myriad of variables associated with BCA cultivation, formulation, drying, storage and reconstitution processes complicate agent quality maximisation. An efficient assay using a 96-well microplate format to allow an integrated approach to optimising these process variables is presented. The assay involves growing the BCA of interest in flasks or fermentors, formulating cells harvested from growth cultures, delivering microlitre droplets of formulated cells to microplate wells, air-drying droplets, storing plates, reconstituting dried cells and monitoring the rate of cell growth to a specified yield using a plate-reading spectrophotometer. Spectrophotometer assessments of cell activity were significantly correlated with microdilution plate viable cell enumeration. Relevant variables (culture harvest age, cultivation and formulation ingredients, storage atmosphere and temperature) were tested with each step of the assay process to view their individual and combined impact on resultant microbial activity. The utility of this method to evaluate many treatments was demonstrated on seven strains ofPseudomonas fluorescensandEnterobacter cloacaeknown to suppress fungal diseases of wheat and potatoes. [ABSTRACT FROM AUTHOR]

Introduction of US-8 genotypes of Phytophthora infestans has coincided with an increase in severity of potato late blight in North America. As alternatives to chemical fungicides, 18 bacterial strains patented as biological control agents (BCA) of both sprouting and Fusarium dry rot were cultivated in three liquid media and screened in wounded potato bioassays for their ability to suppress late blight incited by P. infestans (US-8, mating type A2). Washed or unwashed stationary-phase bacteria were mixed with fungal zoospores to inoculate potato wounds with 5µL containing ∼108 bacterial CFU/mL and 2×104 zoospore count/mL. Disease suppressiveness was evaluated after tubers were stored a week at 15°C, 90% relative humidity. One-fifth of the 108 BCA treatments screened, reduced late blight by 25-60%, including among other strains Pseudomonas fluorescens S22:T:04 (showing most consistency), P22:Y:05, S11:P:12 and Enterobacter cloacae S11:T:07. Small-scale pilot testing of these four strains, alone and in combination, was conducted under conditions simulating a commercial application. Suspensions of 4×104 P. infestans sporangia/mL were sprayed at a rate of 1.6 mL followed by 0.8 mL of bacteria treatment at ∼5×109 CFU/mL per each of 90 unwounded potatoes. Three replicate boxes per treatment (30 tubers per box) were randomized in storage and maintained 4 weeks at 7.2°C, 95% relative humidity. All BCA treatments significantly reduced disease; and unwashed bacteria outperformed those washed free of culture broth. Disease suppression ranged from 35% up to 86% the first test year and from 35 to 91% the second year. Highest overall performance rankings significantly above the control were achieved by the following strains in culture broth: four-strain mix > P. fluorescens S22:T:04> P. fluorescens S11:P:12. Combined with previous demonstrations of dry rot and sprout suppression, the consistent late blight control by these strains and strain mixtures suggests the commercial feasibility of a single treatment for broad spectrum suppression of post-harvest potato diseases and sprouting. [ABSTRACT FROM AUTHOR]