Management of Resident Microbial Antagonists to Enhance Apple Growth in Replant Soils

Mark Mazzola

Agroecosystems possess a wealth of microbiological resources with potential to be exploited for the control of soil-borne plant diseases. This is evidenced by the existence of soils that are naturally suppressive to certain plant diseases, the first of which was recognized over 100 years ago. The ability to enhance activity of disease suppressive resident soil microbial communities could serve as a key element in an integrated approach to management of soil-borne diseases. However, effective use of this biological asset is dependent upon identification and management of specific microbial communities with the ability to suppress plant pathogens.

Studies in this laboratory have documented shifts in microbial populations of a former wheat field soil in response to planting apple. The microbial community resident prior to orchard establishment supported optimal growth of apple, but the microbial community that developed in response to the apple rhizosphere was capable of inciting replant disease and reducing apple growth. All elements of the fungal complex that incites replant disease in Washington, including Cylindrocarpon destructans, Phytophthora cactorum, Pythium spp. and Rhizoctonia solani, were recovered from apple seedlings grown in soils collected from orchard blocks that had been established for three years or longer. Significant changes in composition of bacterial communities also were observed with increasing duration of orchard production. These included dramatic reductions in relative recovery of Burkholderia cepacia and transformation of the fluorescent pseudomonad population from one dominated by Pseudomonas putida to a population comprised almost exclusively of Pseudomonas fluorescens bv. III and Pseudomonas syringae.

Interestingly, the microbial community from non-replant soil suppressed Rhizoctonia root rot caused by an introduced isolate of Rhizoctonia solani AG 5, but soil from the same site that had been in apple production for three or more years was conducive to disease development. The majority of P. putida isolates from this site suppress in vitro growth of each element of the fungal complex that incites replant disease, and provide biological control of Rhizoctonia root rot of apple. In contrast, isolates of P. fluorescens bv. III do not exhibit fungistatic activity against any of the target fungi.

Based on the disease suppressive capability of the microbial community resident to the former ‘wheat-field’ soil, studies were conducted to determine the feasibility of a ‘phyto-remediation’ approach to managing microbial communities for the control of replant disease. In greenhouse experiments, cultivating multiple cycles of wheat in orchard replant soils significantly improved growth of apple seedlings subsequently planted into these soils. Enhanced seedling growth was directly associated with a reduction in root infection by species of Pythium and Rhizoctonia. Lesion nematode populations were well below damage threshold populations in orchard soils, but prior wheat cultivation resulted in a further reduction in soil and root populations of this plant parasitic nematode. Although the relative increase in apple seedling growth was consistent across multiple replant soils, the magnitude of the growth response varied among three wheat cultivars grown prior to planting apple.

Subsequent studies have demonstrated that not all wheat cultivars (11 tested) are capable of eliciting this response, and that prior cultivation of replant soils with other grasses, including annual rye grass, does not suppress the target fungal pathogens or enhance apple growth. Preliminary studies suggest that the ability of a specific wheat cultivar to improve subsequent growth of apple in replant soils is determined by the capacity of root exudates to stimulate activity of specific microbial communities. As indicated above, P. putida typically dominates fluorescent pseudomonad populations in soils that support optimal growth of apple. Wheat cultivation of replant soils dramatically increased populations of P. putida and suppressed populations of P. fluorescens bv. III. Wheat cultivars, such as ‘Penawawa’, that enhance growth of apple in replant soils produce root exudates that sustain growth of the Rhizoctonia-suppressive bacterium P. putida strain 2C8 when used as a sole carbon source in minimal media. In contrast, wheat cultivars such as ‘Edwall’ that produce root exudates which are incapable of sustaining growth of strain 2C8 do not enhance subsequent growth of apple in replant orchard soils. Although correlative, these data support the hypothesis that the fluorescent pseudomonad community has an important role in disease suppression and enhanced growth of apple observed in response to prior cultivation of replant soils with specific wheat cultivars.

This research was supported in part by funding from the USDA Integrated Pest Management Grants Program and the Washington Tree Fruit Research Commission.

Mark Mazzola
USDA-ARS Tree Fruit Research Lab
1104 N. Western Ave.
Wenatchee, WA 98801
Tel: (509) 664-2280
Mazzola@tfrl.ars.usda.gov

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