PHYTO-MANAGEMENT OF MICROBIAL COMMUNITY STRUCTURE TO ENHANCE GROWTH OF APPLE IN REPLANT SOILS

摘要

Replant disease of apple is the primary biological impediment to the establishment of an economically viable orchard on sites previously cultivated to this crop. Control of replant disease has typically relied on pre-plant application of broad-spectrum soil fumigants. However, due to environmental and health concerns, the long-term availability of these chemicals is in doubt. Our studies defining etiology of apple replant disease in Washington have enabled evaluation of specific biologically-based systems for control of the causal fungal complex that incites this disease. Under controlled greenhouse conditions, cultivation of replant soils with wheat prior to planting apple, substantially reduced root infection or infestation by Pythium spp., Rhizoctonia spp. and Pratylenchus penetrans, resulting in enhanced seedling growth. This response occurred in a wheat cultivar-specific manner and was not induced by other grasses including annual ryegrass. "Growth-enhancing" but not "growth-neutral" wheat cultivars induced a consistent alteration in certain components of the saprophytic microbial community, including composition of the fluorescent pseudomonad population in replant soils; Pseudomonas fluorescens bv. III dominated replant soils but Pseudomonas putida was the primary species after wheat cultivation of the same soils. The majority of P. putida, but not P. fluorescens bv. III, isolates from these soils exhibit in vitro inhibition toward all elements of the fungal complex inciting replant disease. Likewise, root exudates from growth-enhancing wheat cultivars could serve as a sole carbon source for growth of the biocontrol strain P. putida 2C8 but this was not the case for exudates from growth-neutral wheat cultivars. These findings strongly suggest that alterations in the fluorescent pseudomonad community, in part, contribute to the suppression of apple replant that is observed in response to prior cultivation of replant soils with wheat.