Application of Beneficial Microorganisms to Seeds during Drum Priming

Five microbial plant growth promoters or biocontrol agents (Pseudomonas fluorescens CHA0, Pseudomonas sp. AB842, Bacillus subtilis MBI600, Trichoderma harzianum T22 and T. virens G20) were assessed for ability to proliferate on seeds of carrot, parsnip and leek. In small-scale priming systems, both pseudomonads and MBI600 (when applied as cells) at levels between 10(5) and 10(6) cfu g-1 seed were able to colonise all seeds at the end of priming (240 h) despite initial poor recovery after addition of the cells in some cases. Pf CHA0 was a particularly aggressive seed coloniser often comprising the total pseudomonad population at the end of priming. Drying the seed after priming resulted in <1 log10 cfu g-1 seed loss for the pseudomonads but greater losses for MBI600 on carrot and leek seed. Application of spores of MBI600 resulted in little loss in cfu g-1 seed following addition of the cells and these levels were maintained throughout the priming period and after drying back. Both T22 and G20 were recovered from carrot and parsnip at the end of priming and in general reflected survival of the inoculum rather than proliferation. T22 and G20 could not be recovered from leek seed following priming. Comparisons were made between proliferation and survival in large-scale drum priming with post-priming application of microorganisms. Pf CHA0 proliferated on carrot and parsnip seed during drum priming and survived the drying back process whereas there was no recovery when applied as a post-priming treatment. In contrast, MBI600 could not be recovered when applied during priming as cells, whereas recovery was good when applied post-priming as spores. T22 spores could be applied in either manner. Post-priming application of metalaxyl and thiabendazole had variable effects on microorganism recovery. The significance of the application of microorganisms to seeds during priming is discussed.