CHANGES IN MICROBIAL COMMUNITY STRUCTURE IN RESPONSE TO APPLE ROOTS: IMPACT ON DISEASE-SUPPRESSIVE POTENTIAL AND DEVELOPMENT OF APPLE REPLANT DISEASE
USDA, ARS, Wenatchee, Washington, USA
Background and objectives
Results and conclusions
In uncultivated soil, Burkholderia cepacia and Bacillus megaterium dominated the bacterial community isolated from the rhizosphere of apple transplants and bulk soil, respectively. As orchard block age increased, populations of these species in their respective environments declined precipitously, with the rhizosphere bacterial community composed of a progressively more diverse group of species and the bulk soil bacterial community dominated by Pseudomonas fluorescens. Similar shifts in bacterial and fungal communities could be induced by planting uncultivated soil from this orchard with successive cycles of apple. In both cases, populations of plant parasitic nematodes remained low, and populations of Pratylenchus penetrans were less than 10/g soil.
The biomass of apple transplants grown in orchard soils exhibited a general decline as age of the orchard block increased. Likewise, the ability of the resident soil microflora to suppress an introduced pathogen diminished with increasing orchard block age. Growth of apple transplants was not affected by introduction of R. ;solani into uncultivated soil or soil from the first or second leaf block. However, growth of transplants in soil from the third, fourth or fifth leaf block infested with R. ;solani was suppressed significantly relative to that obtained in the corresponding uninfested soil. The inability of the resident microflora to suppress R. ;solani was associated with the changes in microbial populations cited above, as well as a dramatic reduction in the population of 2,4-diacetylphloroglucinol-producing fluorescent pseudomonads (from 54% to 9% of total fluorescent pseudomonad population).
These studies demonstrate that a rapid transformation of bacterial and fungal communities occurs in response to planting soil to apple. This is characterized by a shift in the fungal community from one that is dominated by saprophytic fungi to a population dominated by species capable of causing root rot of apple. Similarly, there was a rapid alteration in the resident soil microflora from a community that suppressed Rhizoctonia root rot to one that was conducive to disease development. In addition, this study suggests that a microflora capable of inducing apple replant disease may develop shortly after orchard establishment (2-3 ;years), rather than being confined to sites previously occupied by very old fruit trees.