Management of resident plant growth-promoting rhizobacteria with the cropping system: a review of experience in the US Pacific Northwest

In view of the inconsistent performance of single or mixtures of plant growth-promoting rhizobacteria (PGPR) strains formulated for commercial use, and the high cost of regulatory approval for either a proprietary strain intended for disease control or a crop plant transformed to express a disease-suppressive or other growth-promoting PGPR trait, management of resident PGPR with the cropping system remains the most practical and affordable strategy available for use of these beneficial rhizosphere microorganisms in agriculture. A cropping system is defined as the integration of management (agricultural) practices and plant genotypes (species and varieties) to produce crops for particular end-uses and environmental benefits. The build-up in response to monoculture cereals of specific genotypes of Pseudomonas fluorescens with ability to inhibit Gaeumannomyces graminis var. tritici by production of 2,4-diacetylphoroglucinol (DAPG), accounting for take-all decline in the US Pacific Northwest, illustrates what is possible but apparently not unique globally. Other crops or cropping systems enrich for populations of the same or other genotypes of DAPG-producing P. fluorescens or, possibly and logically, genotypes with ability to produce one or more of the five other antibiotic or antibiotic-like substances inhibitory to other soilborne plant pathogens. In the U.S Pacific Northwest, maintenance of threshold populations of resident PGPR inhibitory to G. graminis var. tritici is the centerpiece of an integrated system used by growers to augment take-all decline while also limiting damage caused by pythium and rhizoctonia root rot and fusarium root and crown rot in the direct-seed (no-till) cereal-intensive cropping systems while growing varieties of these cereals (winter and spring wheat, barley and triticale) fully susceptible to all four root diseases.