Transformation of soil microbial community structure and Rhizoctonia-suppressive potential in response to apple roots

Changes in the composition of soil microbial communities and relative disea se-suppressive ability of resident microflora in response to apple cultivat ion were assessed in orchard sails from a site possessing trees established for 1 to 5 years. The fungal community from roots of apple seedlings grown in noncultivated orchard soil was dominated by isolates from genera common ly considered saprophytic. Plant-pathogenic fungi in the genera Phytophthor a, Pythium, and Rhizoctonia constituted an increasing proportion of the fun gal community isolated from seedling roots with increasing orchard block ag e. Bacillus megaterium and Burkholderia cepacia dominated the bacterial com munities recovered from noncultivated soil and the rhizosphere of apple see dlings grown in orchard soil, respectively. Populations of the two bacteria in their respective habitats declined dramatically with increasing orchard block age. Lesion nematode populations did not differ among soil and root samples from orchard blocks of different ages. Similar changes in microbial communities were observed in response to planting noncultivated orchard so il to five successive cycles of 'Gala' apple seedlings. Pasteurization of s oil had no effect on apple growth in noncultivated soil but significantly e nhanced apple growth in third-year orchard block soil. Seedlings grown in p asteurized soil from the third-year orchard block were equal in size to tho se grown in noncultivated soil, demonstrating that suppression of plant gro wth resulted from changes in the composition of the soil microbial communit y. Rhizoctonia solani anastomosis group 5 (AG 5) had no effect on growth of apple trees in noncultivated soil but significantly reduced the growth of apple trees in soil from third-year orchard soil. Changes in the ability of the resident soil microflora to suppress R. solani AG 5 were associated wi th reductions in the relative populations of Burkholderia cepacia and Pseud omonas putida in the rhizosphere of apple.