Saturday, 15 July 2006

Impact of Long-Term Orchard Ground-Cover Management Practices on Apple Seedling Growth in an Orchard Soil.

Angelika Rumberger, Dept for Crop and Soil Science, Cornell Univ, 726 Bradfield Hall, Ithaca, NY 14853, Ian Merwin, Dept for Horticulture, 120 Plant Science Building, Cornell University, Ithaca, NY 14853, and Janice E. Thies, Dept of Crop and Soil Sciences, Cornell Univ, Ithaca, NY 14853.

Apple Replant Disease (ARD) symptoms are often found to be more severe, when new trees are planted directly into the old tree row positions of the previous orchard, rather than into previously grass-covered drive lanes. In recent experiments we found that not only is replanted tree growth decreased in the old rows compared to the grass lanes, but soil in the old row position was lower in soil organic matter content and likely contained herbicide residues from the previous orchard as observed by poor grass establishment in the old row position. Soil microbial community composition also differed between old tree rows and grass lanes. These observations raised the question of whether orchard Ground-cover Management systems (GMSs) used in old orchards could exacerbate (or alleviate) ARD severity in newly planted orchards. We conducted a bioassay to evaluate the effects of GMSs on apple seedling growth in soil sampled from tree rows that had received four different GMSs for the past 13 years and from the grass lanes between the tree rows. The GMSs were: (i) pre-emergence herbicide, a mix of glyphosate, norflurazon and diuron 2.0, 3.5 and 2.5 kg active ingredient (a.i.) ha-1, applied each May; (ii) post-emergence herbicide, glyphosate applied at 2 kg a.i. ha-1 in May and July; (iii) mulch, a 15 cm layer of hardwood bark mulch applied every other year; and (iv) mowed grass, a mixed turfgrass of Festuca rubra and 25 other herbaceous groundcovers. Soils were either used without treatment or pasteurized to reduce soil-borne pathogens. Thirteen years of herbicide application in pre- and post-emergence herbicide treatments had significantly lowered the soil organic matter content in the tree rows compared to that in the adjacent grass-covered drive lanes. Whereas, mulch applications doubled the initial soil organic matter content compared with other GMSs. Soil microbial community composition was assessed with PCR-DGGE. The soil microbial community composition of soil sampled from the grass lanes and the mowed grass treatment was similar for bacteria, fungi and oomycetes. The bacterial community composition of soil from the pre-emergence herbicide treatment differed from that of all other GMSs and the grass lane soil. The fungal community composition in both herbicide treatments (i) and (ii) were similar to each other, but somewhat different from grass-covered soil. The fungal community of the mulch treatment was different from all other treatments. The oomycete community composition of the post-emergence herbicide, mulch and mowed grass treatments and the adjacent grass lane soil was similar, however we were not able to amplify oomycetes DNA from pre-emergence herbicide treated soil. This might indicate that the pre-emergence herbicide treatment had a suppressive effect on soil-borne oomycetes. Seedling growth in all GMSs was similar to the growth in grass lane soil. There were also no differences in seedling growth between the pasteurized and non-pasteurized soils, which might indicate that the pathogen pressure was low at the studied site. However, seedling growth was significantly greater in the pre-emergence herbicide treatment than in the post-emergence herbicide or mulch treatments. GMSs of the previous orchard can significantly affect the growth of newly planted seedlings. GMSs that change soil organic matter content and alter soil microbial communities may affect the severity of ARD.

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