USDA, ARS, Wenatchee, Washington, USA

Background and objectives
Poor growth of apple on sites previously cropped to the same species has been termed apple replant disease. Previous research has implicated numerous biotic and abiotic elements in the aetiology of apple replant disease; however, in Washington state a fungal complex composed of species of Cylindrocarpon, Phytophthora, Pythium and Rhizoctonia was consistently shown to be the primary cause of disease development. Specific changes in soil microbial community structure in response to prolonged exposure to apple roots, typically 15-20 ;years, are likely responsible for the poor growth of trees established on a replant site. The temporal nature of transformation of the microbial community has not been documented, although anecdotal evidence suggests that it may occur within a few years of orchard establishment. The objectives of this study were to document the composition of the soil microflora in adjacent blocks within the WVC orchard (East Wenatchee, WA, USA) that had been in production for 0-4 years (first to fifth leaf), and to correlate attributes of these communities with the relative growth of apple transplants in these soils.

Results and conclusions
The fungal community associated with the roots of apple transplants grown in previously uncultivated orchard soil was dominated by saprophytes including species of Mucor, Penicillium and Ulocladium. The relative recovery of fungal saprophytes from roots of apple transplants grown in orchard soil declined as age of the orchard block increased, and was associated with a concomitant increase in the frequency of isolation of Rhizoctonia, Pythium and Phytophthora spp. R. ;solani was not recovered from transplants grown in uncultivated soil or soil from the first leaf block, but accounted for 35% of fungal isolates recovered from transplants grown in soil from the fifth leaf block.

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.